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Ecosystem ecology meets adaptive management: Food web response to a controlled flood on the Colorado River, Glen Canyon
Ecological Applications
Abstract
Large dams have been constructed on rivers to meet human demands for water, electricity, navigation, and recreation. As a consequence, flow and temperature regimes have been altered, strongly affecting river food webs and ecosystem processes. Experimental high-flow dam releases, i.e., controlled floods, have been implemented on the Colorado River, U.S.A., in an effort to reestablish pulsed flood events, redistribute sediments, improve conditions for native fishes, and increase understanding of how dam operations affect physical and biological processes. We quantified secondary production and organic matter flows in the food web below Glen Canyon dam for two years prior and one year after an experimental controlled flood in March 2008. Invertebrate biomass and secondary production declined significantly following the flood (total biomass, 55% decline; total production, 56% decline), with most of the decline driven by reductions in two nonnative invertebrate taxa, Potamopyrgus antipodarum and Gammarus lacustris. Diatoms dominated the trophic basis of invertebrate production before and after the controlled flood, and the largest organic matter flows were from diatoms to the three most productive invertebrate taxa (P. antipodarum, G. lacustris, and Tubificida). In contrast to invertebrates, production of rainbow trout (Oncorhynchus mykiss) increased substantially (194%) following the flood, despite the large decline in total secondary production of the invertebrate assemblage. This counterintuitive result is reconciled by a post-flood increase in production and drift concentrations of select invertebrate prey (i.e., Chironomidae and Simuliidae) that supported a large proportion of trout production but had relatively low secondary production. In addition, interaction strengths, measured as species impact values, were strongest between rainbow trout and these two taxa before and after the flood, demonstrating that the dominant consumer-resource interactions were not necessarily congruent with the dominant organic matter flows. Our study illustrates the value of detailed food web analysis for elucidating pathways by which dam management may alter production and strengths of species interactions in river food webs. We suggest that controlled floods may increase production of nonnative rainbow trout, and this information can be used to help guide future dam management decisions.
... However, like many other large rivers, relative allochthonous detrital inputs are low due to river width, and river regulation has likely further decreased standing stocks by blocking upstream sources and reducing peak floods (annual input = 11.1 g ash-free dry mass [AFDM] m −2 year −1 below Glen Canyon Dam; Kennedy & Ralston, 2012). Food webs immediately downstream from Glen Canyon Dam are fueled by instream primary production due to high water clarity and low detrital inputs (Cross et al., 2011). Aquatic primary producers are dominated by filamentous algae (e.g., Cladophora glomerata, Ulothrix zonata, and Spirogyra spp.), macrophytes (e.g., Potamogeton pectinatus), aquatic moss (Fontinalis spp.), stonewort (Chara contraria), and epiphytic diatoms (Hardwick et al., 1992;Wellard Kelly et al., 2013). ...
... Aquatic primary producers are dominated by filamentous algae (e.g., Cladophora glomerata, Ulothrix zonata, and Spirogyra spp.), macrophytes (e.g., Potamogeton pectinatus), aquatic moss (Fontinalis spp.), stonewort (Chara contraria), and epiphytic diatoms (Hardwick et al., 1992;Wellard Kelly et al., 2013). Invertebrate assemblages are depauperate and composed primarily of chironomid midges, New Zealand mudsnails (Potamopyrgus antipodarum), amphipods (Gammarus lacustris), and oligochaete worms (Cross et al., 2011). ...
... Furthermore, our results highlight the potential for favorable invasive species management outcomes (i.e., saltcedar control) via the herbivory of a biological control agent to alter the quality of resource subsidies and potentially produce unintended responses in adjacent ecosystems. Although our study focused on the breakdown of allochthonous OM, we recognize that many large-river tailwaters, including our study area downstream of Glen Canyon Dam, are autotrophic (Chowanski et al., 2020;Cross et al., 2011;Davis et al., 2012), and detrital inputs and utilization are likely low. Nonetheless, allochthonous OM may provide a key donor-controlled "slow energy pathway," which could be important for buffering and stabilizing food web dynamics in the tailwater (Moore et al., 2004). ...
Predicting how increasing temperatures interact with other global change drivers to influence the structure and dynamics of Earth's ecosystems is a primary challenge in ecology. Our study made use of multiple simultaneous “natural experiments” to examine how rapid warming, declining nutrients, invasive consumers, and riparian invasive species management interact to influence leaf decomposition in a large and regulated river. Specifically, we compared the breakdown of cottonwood ( Populus fremontii ), willow ( Salix exigua ), and saltcedar ( Tamarix sp.) leaf litter in 2022 to a previous experiment from 1998 that occurred under much cooler water temperatures, and had higher water phosphorus concentrations, low numbers of invasive New Zealand mudsnails ( Potamopyrgus antipodarum ), and unaltered litter chemistry from the herbivory of saltcedar leaf beetles ( Diorhabda carinulata ). We found that the effects of up to 10°C warmer temperatures on leaf decomposition were mediated by the establishment and management of invasive species and declining water nutrient concentrations arising from upstream reservoir lowering. Such interactions led to accelerated breakdown of saltcedar, but relatively minor effects of warming on the rate of cottonwood and willow decomposition. Additionally, our results demonstrate the potential for favorable invasive species management outcomes in the terrestrial environment to produce unintended responses in adjacent freshwater ecosystems. As temperatures continue to rise, it is critical that future studies consider how warming interacts with multiple stressors and environmental factors to influence processes such as decomposition in freshwater ecosystems.
... Subsequently, the modifications imposed by this snail at lower trophic levels endanger predators. With the native benthic community suppressed by populations of P. antipodarum, the diet of fishes shifts from nutritious to less digestible prey, jeopardising their fitness and survival (Rakauskas et al. 2016;Butkus and Rakauskas 2020;Vinson and Baker 2008;Cross et al. 2011). In summary, the introduction of the New Zealand mud snail can alter the structure and functioning of newly invaded ecosystems (Richards et al. 2001;Alonso and Castro-Díez 2012b;Moore et al. 2012;Spyra et al. 2015). ...
... (Kerans et al. 2005;Spyra et al. 2015). The loss of such species is known to reduce water quality (Cross et al. 2011;Sandvik et al. 2022). A reduction of native macroinvertebrates could also cause predators to suffer population decline, due to a reduction of prey availability. ...
... A reduction of native macroinvertebrates could also cause predators to suffer population decline, due to a reduction of prey availability. In the case of native fish, the replacement of native prey by this introduced snail has been seen to compromise food intake (Vinson and Baker 2008;Cross et al. 2011;Butkus and Rakauskas 2020). Madeira's only freshwater native fish, the European eel (Anguilla anguilla), has not been studied in the context of mud snail invasion and the food web. ...
... In contrast, stable flow regimes under regulation can result in the build-up of periphyton and benthic organic matter, favoring taxa with more cosmopolitan life history traits and ecological requirements (Poff et al., 2007), and increasing the density of generalists Robinson & Uehlinger, 2008). In other regulated rivers, stable flows and increased primary productivity have been shown to alter the relative importance of organic resources, shifting the ecosystem's trophic base towards autotrophy (as in the Colorado River, Cross et al., 2011). Associated increases in the secondary production of invasive, generalist taxa can result in the local extinction of species (Bunn & Arthington, 2002;Poff & Zimmerman, 2010), generation of trophic dead-ends (Cross et al., 2010;Vinson & Baker, 2008), constraints on food-web attributes and interactions (McHugh et al., 2010;Sabo et al., 2009;Wootton et al., 1996), and changes in energy pathways and trophic linkages . ...
... Environmental flows are commonly designed to reintroduce selected components of the original flow regime to achieve specific ecomorphological targets (Gillespie et al., 2015;Konrad et al., 2011;Tonkin et al., 2021;Yarnell et al., 2015), such as the simulation of seasonal floods by implementing periodic experimental floods (Konrad et al., 2011). Although high implementation costs can limit the application of experimental floods (see Andrews & Pizzi, 2000;Scheurer & Molinari, 2003), several studies on systems such as the Colorado (Melis, 2011) and Spöl have demonstrated the utility of experimental flood programs to restore ecological dynamics from flow disturbance (Cross et al., 2011Korman et al., 2011;Robinson et al., 2018). In particular, the application of a functional perspective involving, for example, trophic interactions, can explain observed counterintuitive patterns, such as the rainbow troutchironomid-simuliid trophic nexus on the Colorado River (Cross et al., 2011). ...
... Although high implementation costs can limit the application of experimental floods (see Andrews & Pizzi, 2000;Scheurer & Molinari, 2003), several studies on systems such as the Colorado (Melis, 2011) and Spöl have demonstrated the utility of experimental flood programs to restore ecological dynamics from flow disturbance (Cross et al., 2011Korman et al., 2011;Robinson et al., 2018). In particular, the application of a functional perspective involving, for example, trophic interactions, can explain observed counterintuitive patterns, such as the rainbow troutchironomid-simuliid trophic nexus on the Colorado River (Cross et al., 2011). However, few studies have looked at flow restoration from a functional/trophic point of view (e.g., Cross et al., 2011;Korman et al., 2011;Sabo et al., 2018;Weisberg & Burton, 1993). ...
Flow regulation of montane and alpine headwater streams can fundamentally alter food web structure and energy flows through changes in productivity, resource availability, and community assembly. Dam flow-release schemes can be used to mitigate the environmental impacts of flow regulation via environmental flows, which can increase discharge variability and other ecologically important hydrological properties. In particular, managed floods can reintroduce disturbance to the system and stimulate the reactivation of physical habitat dynamics. However, how managed floods might restore ecosystem processes is virtually unknown. In this study, we examined patterns in potential energy fluxes before, during and after a long-term experimental flood program on the river Spöl, a regulated alpine River in southeast Switzerland. We used benthic samples collected during long-term monitoring and stable isotope analysis (δ 13 C and δ 15 N) of macroinver-tebrates and their potential food sources to reconstruct secondary production, and potential energy fluxes, over a 20-year study period. The experimental floods did not alter the relative importance of basal resources but resulted in a considerable decline in secondary production, which remained low after the discontinuation of the floods. Our data suggest that a lack of recolonization by mosses following the discontinuation of the experimental flood program on the river Spöl may have driven patterns in energy fluxes by limiting macroinvertebrates using mosses for habitat. The effects of environmental flows on energy flows in this system thus depend on flood disturbance and the environmental context following the discontinuation of floods.
... Food web processes may therefore have a strong bearing on the success or failure of these actions, which involve significant economic investment (Bernhardt et al., 2005;Roni et al., 2008). For example, restoration activities focused on augmenting habitat quantity may have diminishing returns if productivity of a target population is also limited by food availability (Cross et al., 2011;Naman et al., 2018). ...
... Regardless of how feeding links are defined, basic food web description provides important information about the trophic roles of focal species and the architecture of feeding relationships. It is the basis for many other food web perspectives (see below) and is also useful in itself as a diagnostic tool if food webs are described repeatedly over environmental gradients or dynamics (e.g., Cross et al., 2011;Morley et al., 2020). ...
... This approach combines estimates of consumer diets, energy assimilation efficiencies, and population-level production to construct "quantitative flow food webs," whereby food web linkages are scaled to the magnitude of organic matter flowing along a given pathway (Benke, 2018). These quantitative flow food webs can be used to determine which linkages are most important for fueling consumer production over a given time interval (typically seasonally or annually) and can be linked across trophic levels to quantify energy flows from basal resources (periphyton and terrestrial litter) up to fish (Cross et al., 2011). This approach has been used to examine the energy flow pathways that support both aquatic invertebrate (Benke & Wallace, 1980;Hall et al., 2001;Nelson et al., 2020), and fish production (Bellmore et al., 2013;Marcarelli et al., 2020) in rivers, and to derive numerous management-relevant metrics such as food limitation of fishes, top-down control on invertebrate prey populations, interspecific competition for food, and carrying capacity (Box 1; Cross et al., 2011;Bellmore et al., 2013). ...
Food web analyses offer useful insights into understanding how species interactions, trophic relationships, and energy flow underpin important demographic parameters of fish populations such as survival, growth, and reproduction. However, the vast amount of food web literature and the diversity of approaches can be a deterrent to fisheries practitioners engaged in on‐the‐ground research, monitoring, or restoration. Incorporation of food web perspectives into contemporary fisheries management and conservation is especially rare in riverine systems, where approaches often focus more on the influence of physical habitat and water temperature on fish populations. In this review, we first discuss the importance of food webs in the context of several common fisheries management issues, including assessing carrying capacity, evaluating the effects of habitat change, examining species introductions or extinctions, considering bioaccumulation of toxins, and predicting the effects of climate change and other anthropogenic stressors on riverine fishes. We then examine several relevant perspectives: basic food web description, metabolic models, trophic basis of production, mass‐abundance network approaches, ecological stoichiometry, and mathematical modeling. Finally, we highlight several existing and emerging methodologies including diet and prey surveys, eDNA, stable isotopes, fatty acids, and community and network analysis. Although our emphasis and most examples are focused on salmonids in riverine environments, the concepts are easily generalizable to other freshwater fish taxa and ecosystems.
This article is categorized under: Water and Life > Nature of Freshwater Ecosystems
Water and Life > Conservation, Management, and Awareness
Water and Life > Methods
... Such flows may be designed for specific species or more broadly to attempt to restore ecosystem processes. The response of stream fishes and biota to designer flows is sometimes difficult to predict and outcomes may be counter to expectations (Cross et al. 2011;Korman et al. 2011;Avery et al. 2015) and highly dependent on their timing (Yackulic et al. 2022a). ...
... Limited data were also available to assess whether flooding occurring outside of fall season would stimulate similar upstream movements. Only a single springtimed experimental flood event (2008; Cross et al. 2011) and single equalization flow (2011) occurred during our 19 year study period--both of which occurred prior to increases in brown trout abundance. A limitation of our analyses was that in no years during the brown trout expansion did a Fall HFE occur independently of the weir. ...
River ecosystems have been altered by flow regulation and species introductions. Regulated flow regimes often include releases designed to benefit certain species or restore ecosystem processes, and invasive species suppression programs may include efforts to restrict access to spawning habitat. The impacts of these management interventions are often uncertain. Here, we assess hypotheses regarding introduced brown trout (Salmo trutta) movement in a regulated river. We model mark–recapture data in a multistate framework to assess whether movement was affected by the operation of a tributary weir (restricting access to spawning habitat), experimental releases of fall-timed high flow experiments (Fall HFEs), or simply increased during the fall, spawning season. Our results suggest that the presence of the weir led to reduced tributary homing and the release of Fall HFEs stimulated upstream movement and straying. Both effects are of a similar magnitude, however the Fall HFE effect is more certain. Our results suggest the expansion of an invasive species was stimulated by management interventions, and demonstrate the potential for unanticipated outcomes of restoration in highly altered river ecosystems.
... Dams regulate nearly half of the world's river volume (1), with unintended consequences for aquatic ecosystems (2,3). Flows in regulated rivers are often designed to meet the demand or maximize the profitability of hydropower, which currently provides approximately 16% of the energy to the global power grid (4). ...
... This is a much greater distance than is typically considered within a dam's footprint. Nonetheless, the large spatial footprint of dam management on riverine GPP reported here fits with other broad scale geomorphic (34), biogeochemical (35), hydrological (36), and ecological effects (3,9) that extend far downstream from the dam. ...
Aquatic primary production is the foundation of many river food webs. Dams change the physical template of rivers, often driving food webs toward greater reliance on aquatic primary production. Nonetheless, the effects of regulated flow regimes on primary production are poorly understood. Load following is a common dam flow management strategy that involves sub-daily changes in water releases proportional to fluctuations in electrical power demand. This flow regime causes an artificial tide, wetting and drying channel margins and altering river depth and water clarity, all processes that are likely to affect primary production. In collaboration with dam operators, we designed an experimental flow regime whose goal was to mitigate negative effects of load following on ecosystem processes. The experimental flow contrasted steady-low flows on weekends with load following flows on weekdays. Here, we quantify the effect of this experimental flow on springtime gross primary production (GPP) 90-to-425 km downstream of Glen Canyon Dam on the Colorado River, AZ, USA. GPP during steady-low flows was 41% higher than during load following flows, mostly owing to non-linear reductions in sediment-driven turbidity. The experimental flow increased weekly GPP even after controlling for variation in weekly mean discharge, demonstrating a negative effect of load following on GPP. We estimate that this environmental flow increased springtime carbon fixation by 0.27 g C m–2 d–1, which is ecologically meaningful considering median C fixation in 356 U.S. rivers of 0.44 g C m–2 d–1 and the fact that native fish populations in this river are food-limited.
... Au-delà de l'échelle populationnelle et mono-spécifique, la gestion adaptative peut s'appliquer à des échelles biologiques plus larges telles que les communautés ou les écosystèmes. Le programme de gestion adaptative du barrage du Glen Canyon sur le fleuve Colorado (États-Unis) est un des exemples les plus connus d'utilisation de cette approche pour la gestion d'un écosystème (Cross et al., 2011). La régulation fluviale (via la construction de barrages) représente une altération généralisée du paysage et conduit à des changements dans le régime d'écoulement fluvial, de température et d'apport de sédiments. ...
... Au barrage du Glen Canyon, des crues expérimentales avaient été mises en place avec des objectifs de restauration des écosystèmes, de redistribution des sédiments des affluents pour la formation de bancs de sable à des fins récréatives, et de maintien de la truite arcen-ciel (Oncorhynchusmykiss).Cependant, il existait de nombreuses incertitudes liées aux potentiels bénéfices ou nuisances de ces crues sur la structuration des réseaux trophiques fluviaux. Une étude d'impact en aval du barrage a permis de montrer que les crues favorisaient le risque d'invasion des écosystèmes jusqu'ici tamponnés par la disponibilité des ressources alimentaires (Cross et al., 2011). Ces résultats sont apparus comme ...
Publié dans Faune Sauvage.
La gestion adaptative est un processus de gestion qui évolue
avec l’état et les connaissances du système naturel considéré,
par exemple une population. Elle est conduite de manière à
développer explicitement les connaissances du système, pour
répondre au mieux à sa dynamique et aux objectifs de gestion
établis et régulièrement révisés. Si les applications concrètes
sont encore peu développées, leur mise en oeuvre est possible
dans des contextes variés. Cet article en livre quelques exemples.
... Practitioners need information about how environmental change and management interventions affect food available to fish to make robust management decisions. This is underscored by strategies such as the Glen Canyon Dam Adaptive Management Program on the Colorado River that explicitly supported incorporation of food production and availability into management decisions, which was supported by large-scale experiments to probe uncertainty with respect to the foodscape responses and fish population linkages (including salmonids) (Cross et al., 2011;Deemer et al., 2022). Although the role of physical habitat in driving salmonid abundance is sufficiently well understood to meaningfully influence habitat management (Jorgensen et al., 2021), the equally important role of prey availability is not at the same level of empirical maturity to support informed management interventions. ...
Riverine fishes face many challenges including habitat degradation and climate change, which alter the productivity of the riverscapes in which fish live, reproduce, and feed. Understanding the watershed portfolio of foraging and growth opportunities that sustain productive and resilient fish populations is important for prioritizing conservation and restoration. However, the spatiotemporal distribution and availability of fish food are poorly understood relative to other factors such as abiotic habitat quantity and quality (e.g., water temperature). In this paper, we build on the concept of “foodscapes,” and describe three components of food for fish, including abundance, accessibility, and quality. We then discuss methodological advances to help address three key questions: (1) Why is food availability hard to estimate? (2) What are the consequences of uncertainty in food availability estimates? and (3) What approaches are available or emerging for quantifying food available to fish? To address the first question, we characterize data acquisition and analytical challenges; for the second, we demonstrate the importance of evaluating and communicating potential consequences of uncertainty; and for the third, we posit opportunities for future work. Collectively, we highlight the need for greater appreciation of the role food plays in stream fish conservation, especially given its critical influence on responses to warming temperatures.
This article is categorized under: Water and Life > Nature of Freshwater Ecosystems
Water and Life > Conservation, Management, and Awareness
Water and Life > Methods
... Another option would be to include more controlled flood releases consisting of the maximum amount of water that can safely be passed through the dam (up to 1274 m 3 /s) during the historic spring snowmelt period. This option may have social benefits by building sand deposits that serve as campsites before or during the primary recreational boating season; cultural benefits by increasing the supply of river-sourced windblown sand that can help preserve archaeological sites in the river corridor by keeping them buried with a protective cover of sediment; as well as biological benefits for organisms adapted to spring and summer flooding (Cross et al., 2011;Mahoney & Rood, 1998;. ...
River flow patterns are primary drivers of lotic ecosystems, and hundreds of metrics have been developed to quantify flow attributes. Although existing metrics have been a powerful tool in designing environmental flows, they are often developed with specific resources in mind and are rarely directly comparable with each other (i.e., units are often different). Here, we focus on natural flows as the resource of interest and develop z-score metrics that measure the naturalness of regulated flows, incorporating natural means and interannual variation. These "eZ metrics" summarize whole year, subdaily, and functional flow patterns as standard deviations from natural such that their values are directly comparable. We illustrate their utility with a case study from the Colorado River downstream of Glen Canyon Dam in Arizona, USA. We calculated metrics for 1964-2022, spanning >5 decades of changing water policy , hydropower generation, and flow experimentation. We evaluate four options for estimating natural baseline flows. Across metrics, we found that subdaily stage variation deviated the most from baseline. Flows to satisfy regional water policy and power demands altered metrics more than designer flows (which target specific resource outcomes), and years with low water releases were closest to natural. Most of the designer flows have not made flow patterns more natural, due to incorrect seasonal timing, small magnitude, or short duration. By explicitly considering inter-annual variability and quantifying how regulated flows differ from natural using standard deviations, these metrics can inform management when the goal is to restore a natural flow regime.
... and animal material = 0.70 (95% PI: 0.56-0.84; Benke & Wallace, 1980, 1997Cross et al., 2007Cross et al., , 2011Welch, 1968). Variability in NPE was incorporated by resampling values from an assumed beta distribution with median NPE = 0.45 (95% PI = 0.40-0.50). ...
Warming temperatures are altering communities and trophic networks across Earth's ecosystems. While the overall influence of warming on food webs is often context‐dependent, increasing temperatures are predicted to change communities in two fundamental ways: (1) by reducing average body size and (2) by increasing individual metabolic rates. These warming‐induced changes have the potential to influence the distribution of food web fluxes, food web stability, and the relative importance of deterministic and stochastic ecological processes shaping community assembly. Here, we quantified patterns and the relative distribution of organic matter fluxes through stream food webs spanning a broad natural temperature gradient (5–27°C). We then related these patterns to species and community trait distributions of mean body size and population biomass turnover (P:B) within and across streams. We predicted that (1) communities in warmer streams would exhibit smaller body size and higher P:B and (2) organic matter fluxes within warmer communities would increasingly skew toward smaller, higher P:B populations. Across the temperature gradient, warmer communities were characterized by smaller body size (~9% per °C) and higher P:B (~7% faster turnover per °C) populations on average. Additionally, organic matter fluxes within warmer streams were increasingly skewed toward higher P:B populations, demonstrating that warming can restructure organic matter fluxes in both an absolute and relative sense. With warming, the relative distribution of organic matter fluxes was decreasingly likely to arise through the random sorting of species, suggesting stronger selection for traits driving high turnover with increasing temperature. Our study suggests that a warming world will favor energy fluxes through “smaller and faster” populations, and that these changes may be more predictable than previously thought.
... River flow out of the upper two reservoirs has been regulated since the 1960's (Bestgen & Crist, 2000;Hundley, 2009), with water release from the dams following adaptive management plans to enhance ecological health of riparian systems (Cross et al., 2011;Hall et al., 2015;Pulwarty & Melis, 2001). Examples include spring flood releases from the Flaming Gorge Dam to match natural floods from downstream tributaries to activate floodplains to augment fish hatch habitat in the 425-mile section between Flaming Gorge and the slack water deposits at Lake Powell. ...
Aim
Emerging research shows how bioindicators, specifically bats, can serve as a means for monitoring conservation and management of riparian corridors for multiple taxonomic groups. To track changes in the composition or abundance of bioindicator species, researchers must attain a baseline in species presence and relative activity. We examined the spatial and temporal patterns of bat community composition and activity along a 1000‐mile river corridor to determine species diversity trends by latitude and habitat.
Location
Colorado River Basin.
Methods
Here we describe the results from an acoustic bat survey conducted opportunistically on the 2019 Sesquicentennial Colorado River Exploring Expedition. This broad, 1000‐mile survey provides a baseline for species distributions over a large geographic range.
Results
In total, we collected 63 nights of acoustic data over 70 days and recorded over 59,000 files equating to 45,363 caLL files (≥2 pulses). 18,490 (41% of caLL files) were identified as species (n = 19 bat species). We applied non‐metric multidimensional scaling to characterize spatiotemporal patterns of activity between species, as well as compared bat activity among river features and local environmental conditions (i.e., temperature and time since sunset) using an information theoretic approach.
Conclusion
Species composition varied by physiographic region and adjacent river habitat, thus providing a quantifiable measure of determining habitat quality along this major river system and providing baseline information for using bats as bioindicators of habitat quality.
... Designed experimental floods (also named controlled, artificial, and managed floods; Gillespie et al., 2015) have increasingly been used throughout the world to enhance river ecosystem integrity in heavily modified rivers. Releases of experimental floods as part of environmental flow (e-flow) programs have been conducted in a number of river systems around the world, including the United States (Cross et al., 2011;Patten et al., 2001;Schmidt et al., 2001;Shafroth et al., 2010), Switzerland Robinson, 2012;Robinson et al., 2018), France , Spain (Magdaleno, 2017), New Zealand (Lessard et al., 2013) and Australia (Coleman & Williams, 2017). These releases predominantly aimed to improve the geomorphological and ecological conditions of the rivers by promoting sediment transport within the river (Scheurer & Molinari, 2003). ...
Releasing experimental floods as part of environmental flow programs aims to restore river beds by moving and restoring sediments to improve hydromorphological conditions of the river. However, it remains a challenge to understand how flood release characteristics affect channel morphology, sediment transport, and hydrodynamics. In this study, field surveys and a 2D hydro‐morphodynamic and sediment transport numerical model were used to determine how differences in flood magnitude and falling limb alter hydrogeomorphic conditions within a 4 km reach of the lower Spöl River. The model was constrained by drone flight‐derived high‐resolution digital elevation models and two field‐measured flood releases. The highest flood magnitude of 40 m ³ /s resulted in 2,700 m ³ of total sediment transport, 2,000 m ³ of net total volumetric change and 16 900 m ² more wetted area after the flood. The same flood, simulated with an increase in falling limb slope, resulted in a decrease in the duration of full sediment mobility and a corresponding reduction of 8% in net total volumetric change and 5.3% in the total wetted area. Contrastingly, the lowest flood magnitude of 25 m ³ /s produced 130% lower total sediment transport, 105% lower net total volumetric changes and 10% less wetted area after the flood. Overall, we show that hydro‐morphodynamic modelling of river erosion and deposition combined with spatially rich topographic datasets are extremely useful in forming designed environmental flood scenarios to optimise sediment transport and thus hydrogeomorphic changes to set environmental flows. We contend that scenario modelling is necessary to help water managers optimise the amount of water allocated to environmental flows and to simultaneously restore and maintain riverine dynamics in heavily modified rivers.
... As consumers at intermediate trophic levels, stream-dwelling macroinvertebrates in streams are influenced by bottom-up, as well as top-down, forces 93 . Thus, simplification of macroinvertebrate functional diversity may affect ecosystem functioning and services 94 , and modify species interactions, energy and material flows, and food-web stability 95,96 . In particular, functional redundancy also showed a significant decrease near road crossings, which could reduce the ability of the community to overcome new anthropogenic disturbances. ...
Functional diversity is regarded as a key concept in understanding the link between ecosystem function and biodiversity, and is therefore widely investigated in relation to human-induced impacts. However, information on how the intersection of roads and streams (hereafter road crossings, representing a widespread habitat transformation in relation to human development), influences the functional diversity of stream-dwelling macroinvertebrates is still missing. The general aim of our study was to provide a comprehensible picture on the impacts of road crossing structures on multiple facets of the functional diversity of stream-dwelling macroinvertebrates. In addition, we also investigated changes in trait structure. Our research showed that road crossing structures had negative impacts on functional richness and dispersion; i.e., functional diversification. However, we found no significant impact on functional divergence and evenness components. We found a decrease in functional redundancy at road crossing structures. This indicates a reduced ability of the community to recover from disturbances. Finally, we found that road crossings drive stream habitat and hydrological changes in parallel with modification of the trait composition of stream-dwelling macroinvertebrate assemblages. All these results suggest that road crossings cause notable changes in the functional diversity of stream-dwelling macroinvertebrate assemblages.
... The reason comes from the fact that food webs comprise the scaffold of energy and nutrients fluxes through communities (Woodward et al., 2005) and that the relevance of food webs for biodiversity conservation programs is intrinsic: the species could not exist without trophic networks. The implementation of food web studies remains challenging even in epigean ecosystems, given the multifaceted implications human activities have on communities and their trophic interactions (Cross et al., 2011). However, the recent advances made in the understanding of the aquatic subterranean food webs (Malard et al., 2023) provide fertile ground for future studies in this field of ecology. ...
... Luu and von Melding [60] claim that for small-to-medium-scale HPFs, in general, environmental impact assessments often fail to address thoroughly issues such as forest loss/degradation, damage to aquatic ecosystems, exacerbation of environmental hazards, and social impacts including displacement and disruption of livelihood activities such as fisheries, aquaculture, and near-river agriculture (cf. [34,45,49,50,61,62]). To frame our questioning the sustainability of hydropower in Vietnam, we review some of these issues with reference to local contexts in the following subsections. ...
Vietnam has one of the most intensively energy-exploited riverscapes in Asia with at least 720 hydropower facilities of various capacities currently in operation or in some stage of construction. These facilities represent about 26 GW of installed capacity. This degree of domestic exploitation is often overshadowed by the geopolitically contested manipulation of the waters of the international Mekong River. In contrast, the utilization of Vietnam’s hydropower resources has unfolded gradually and largely unremarked for more than half a century. This perspective argues that the harnessing of rivers and streams for electricity generation is the result of not only the country’s abundant hydrologic resources, but also its history, culture, and (geo)politics. The paper traces the processes that have produced this high level of river exploitation, its ambiguous history, and the uncertain future of hydropower in Vietnam in the context of sustainability. Further, the renewed interest in dam-building in recent years is part of a “theater of decarbonization” that masks the operation of powerful domestic and international lobbies with an interest in “heavy engineering” projects that will do little to meet the nation’s rapidly growing electricity needs but will likely incur detrimental ecological and sociological impacts. The paper ends by positing that rather than forging ahead with the construction of additional small hydropower facilities, a more ecologically and socially equitable policy could instead critically examine the sustainability of existing capabilities, resolve the factors limiting the development of other renewable sources of energy, and face the fundamental challenge of curbing energy use.
... Plots show mean monthly gross primary productivity, mean monthly water temperature, proportion of days per month with turbidity greater than 50 Formazin Nephelometric Units (FNU), and mean monthly flow with months with high-flow experiments marked in dashed blue lines for each of our four defined reaches in the Grand Canyon Colorado River across the duration of our study. efitted from the disturbance in the long term (Cross et al. 2011). Whether similar responses should be expected during the fall (when ecosystem productivity is declining) or in fur-ther downriver river segments (in which a high proportion of primary producers and secondary consumers are of high quality) is unknown. ...
Individual growth can exert strong control on population dynamics but is constrained by resource acquisition rates. Difficulty in accurately quantifying resource availability over large spatial extents and at high temporal frequencies often limits attempts to understand the extent to which resources limit individual growth. Daily estimates of stream metabolism, including gross primary productivity (GPP), are increasingly available but have not, to our knowledge, been linked to fish growth. Here we examine how environmental variables such as GPP, water temperature, turbidity, and high-flow releases from a dam are linked to spatiotemporal variation in the growth of flannelmouth sucker (Catostomus latipinnis) in the Colorado River within the Grand Canyon. We fit state-space growth models to 6 years of mark–recapture data collected in four river reaches spanning 300 river kilometers. Consistent with past research in this system, we find that all four environmental variables influence growth in length of a native primary consumer fish. GPP and temperature have a positive influence on growth, while turbidity and high-flow events have a negative influence. Water temperature is the dominant driver of spatiotemporal variation in growth, while the link between high-frequency GPP and fish growth is relatively novel. Fish growth is likely to be linked to stream metabolism in other systems where overall productivity, not the quality of primary producers, limits the food webs that support fish growth.
... This approach has now been employed in multiple rivers for different purposes. For example, it has been used to quantify the food web (from basal foods to fishes) in the Colorado River to understand the impacts of nonnative species, assist in management strategies (Cross et al., 2011), determine longitudinal impacts of dams (Cross et al., 2013), and quantify flows of mercury and its fate (Walters et al., 2020). The approach has also been used in developing a detailed web in a river-floodplain mosaic of the Methow River, Washington, emphasizing how quantitative food webs can be applied in fisheries management and restoration (Bellmore et al., 2013). ...
... Another example is the Colorado system, which has been affected by a prolonged drought impeding any future dam releases until reservoir levels return to pre-drought levels (Korman et al., 2023). Here, managers expect a serious ecosystem transition in the fisheries resulting from the drought in relation to changes (substantial increases) in river temperatures that likely will transfer through food webs and alter ecosystem functioning (Cross et al., 2011;Kennedy et al., 2016). Both systems, Spöl and Colorado, can be viewed as experiments in disturbance ecology and especially the resilience of regulated rivers to novel disturbance (e.g., drought) and perturbations (e.g., human-related extreme events). ...
Artificial high flows attempt to simulate natural flood pulses in flow-regulated rivers with the intent to improve their ecological integrity. The long-term use of such high flow events have shown beneficial ecological effects on various rivers globally. However, such responses are often non-linear and characterized by underlying feedback mechanisms among ecosystem components. The question arises as to what happens when such high flow releases are disrupted or even discontinued. Here, we used the long-term (22 years) monitoring dataset from the river Spöl to examine whether discontinuation (2016-2021) of the flood program (annual artificial high flows from 2000 to 2016) resulted in the ecological degradation of the river. We used monitoring data of physico-chemistry, periphyton, benthic organic matter, macroinvertebrates and fish (brown trout, Salmo trutta fario L.) in the analysis. The flood program had no long-term effect on water physico-chemistry with most parameters showing typical variations associated with season and inter-annual weather patterns. The floods were effective at mobilizing bed sediments that reduced periphyton biomass and benthic organic matter following each flood. Increases in periphyton biomass and benthic organic matter occurred between floods, but both parameters showed no significant increase with discontinuation of the flood program. Floods reduced macroinvertebrate densities, but with density increases occurring between floods. The pulsed disturbances, and the progressive change in the habitat template, resulted in shifts in community assembly by reducing densities of Gammarus fossarum, a dominant crustacean, which allowed other taxa to colonize the system. Macroinvertebrate densities remained low after discontinuation of the floods, although G. fossarum densities have increased substantially while other taxa, especially some stoneflies, remained low in abundance. Notably, community assembly returned to a pre-flood composition with discontinuation of the floods. The abundance of brown trout increased substantially during the flood program but returned to low pre-flood numbers with discontinuation of the floods. We conclude that the flood program was beneficial to the ecology of the river Spöl and discontinuation of the floods resulted in degradation of the system after a relatively short lag period. However, the system showed high resilience to an earlier perturbation, a sediment spill in 2013, suggesting a rapid positive response by biota with resumption of the flood program.
... Species interaction networks have a wide range of practical applications, such as evaluating ecosystem response to human-altered landscapes, guiding future management decisions (Cross et al., 2011) or exploring how communities may respond to global warming (Gorman et al., 2019). Conservation and ecosystem management efforts aimed at regulating species abundances can, for example, use the information provided by an interaction network to prioritise which species to conserve or eradicate based on their role in the community (Cirtwill et al., 2018). ...
Network theory allows us to understand complex systems by evaluating how their constituent elements interact with one another. Such networks are built from matrices which describe the effect of each element on all others. Quantifying the strength of these interactions from empirical data can be difficult, however, because the number of potential interactions increases nonlinearly as more elements are included in the system, and not all interactions may be empirically observable when some elements are rare.
We present a novel modelling framework which uses measures of species performance in the presence of varying densities of their potential interaction partners to estimate the strength of pairwise interactions in diverse horizontal systems.
Our method allows us to directly estimate pairwise effects when they are statistically identifiable and to approximate pairwise effects when they would otherwise be statistically unidentifiable. The resulting interaction matrices can include positive and negative effects, the effect of a species on itself, and allows for non‐symmetrical interactions.
We show how to link the parameters inferred by our framework to a population dynamics model to make inferences about the effect of interactions on community dynamics and diversity.
The advantages of these features are illustrated with a case study on an annual wildflower community of 22 focal and 52 neighbouring species, and a discussion of potential applications of this framework extending well beyond plant community ecology.
... Invertebrate secondary production (g ash-free dry mass, [AFDM] m −2 year −1 ), hereafter 'secondary production,' was estimated for each taxon by habitat type and reach using the size-frequency method, instantaneous growth rate method or by multiplying bootstrapped annual biomass estimates by published production/biomass ratios (Benke and Huryn 2017). For estimates produced using the size-frequency method, we used Q 10 temperature-corrected CPIs (cohort production interval) to account for differences in development time due to the strong influence of Fort Peck Dam on water temperature regimes in the Missouri River (Cross et al. 2011). For all methods, we used bootstrapping to estimate medians and 2.5% and 97.5% quantiles of annual secondary production (Benke and Huryn 2017). ...
Understanding how the Earth's surface (i.e. ‘nature's stage') influences connections between biodiversity and ecosystem function (BEF) is a central objective in ecology. Despite recent calls to examine these connections at multiple trophic levels and at more complex and realistic scales, little is known about how landscape structure shapes BEF relationships among animal communities in nature. We coupled high‐resolution habitat mapping with extensive field sampling to quantify connections among the geophysical habitat templet, invertebrate assemblages and secondary production in two large North American riverscapes. Patterns of sediment size governed invertebrate assemblage structure, with particularly strong effects on composition, richness and taxonomic and functional diversity. These relationships propagated to drive positive relationships between biodiversity and secondary production that were modified by scale, context‐dependencies and anthropogenic modification. Finally, leveraging spatially‐explicit descriptions of geophysical and biological properties, we uncovered distinct and nested spatial scales of biodiversity and secondary production, and suggest that multiple geophysical processes simultaneously influence these patterns at different scales. Together, our findings advance our understanding of relationships between the physical templet and patterns of BEF, and help to predict how perturbations to the Earth's surface may propagate to influence biodiversity and energy flux through food webs.
... August 2019) and every 2 weeks (4 sampling intervals) after carcass treatment started (12 August-1 October 2019). To quantitatively sample benthic invertebrates, we used a Scuba diver-operated suction sampler constructed from an electric bilge pump mounted on a plastic cutting board with a 500-μm mesh collection net (Cross et al., 2011). ...
Invasive species can have negative effects on native biodiversity and ecosystem function, and suppression is often required to minimize the effects. However, management actions to suppress invasive species may cause negative, unintended effects on non‐target taxa. Across the United States, lake trout (Salvelinus namaycush) are invasive in many freshwater ecosystems, reducing native fish abundance and diversity through predation and competition. In an integrated pest management approach, lake trout embryos in Yellowstone Lake, Wyoming, are suppressed by depositing lake trout carcasses onto spawning sites; the carcasses reduce dissolved oxygen concentrations as they decay, causing embryo mortality. We conducted a field experiment during one ice‐free season at four sites in Yellowstone Lake to investigate the non‐target effects of carcass treatment on benthic invertebrates, which could have consequences for native fish diets. While overall invertebrate density and biomass did not respond to carcass treatment, Chironomidae midges and Sphaeriidae fingernail clams decreased in abundance. Carcass treatment altered invertebrate community structure based on density, but not biomass. Carcass treatment to suppress invasive fish embryos has spatially localized, non‐target effects on some benthic invertebrate taxa. Given the small spatial extent of carcass treatment within the lake, we conclude it is unlikely that carcass treatment will alter food availability for native fishes.
... Streamflows expand, contract, link, and disconnect habitats; and periodically reset food webs via bed-scouring floods or dewatering during drought (Power et al., 1988;Trush et al., 2000). This hydrologic variability drives spatial and temporal responses of hydraulic habitat (depth, velocity, and shear stress) and shifts in energy flow, biomass, and biotic interactions in river food webs (Cross et al., 2011;Power & Dietrich, 2002). For foraging predators in streams, the interaction of hydraulic habitat with prey biomass and availability determines their growth potential-the potential energy available for growth under specific contexts of prey availability and energetic costs (Fausch, 1984;Hughes & Dill, 1990). ...
The growth of any organism depends on habitat conditions, food availability, and their seasonal interactions. Yet in the vast literature on Pacific salmon (Oncorhynchus), the seasonal interaction between habitat conditions and food availability has received relatively little attention. We examined juvenile Oncorhynchus mykiss rearing, physical habitat, and resource phenologies in two Mediterranean coastal streams—one perennial, cool, and shaded and the other intermittent, seasonally warm, and sunny. We used a bioenergetic model to investigate the timing and magnitude of growth potential for drift‐foraging O. mykiss during the spring and summer in both systems. Growth potential peaked at least 2 months earlier in the intermittent stream than in the perennial stream. By early summer (June), growth potential had declined in the intermittent stream, whereas growth rates were peaking in the perennial stream. However, the mid‐July lipid content of juvenile O. mykiss in the intermittent stream was nearly twice that of fish in the perennial stream. By late summer (August), foraging profitability declined in both streams, as abiotic conditions in the intermittent stream approached lethal. In contrast, the perennial stream maintained suitable abiotic conditions even though the growth rate was low. We suggest that the divergent resource phenologies and seasonal mortality risks experienced by anadromous O. mykiss rearing in these streams could drive diversification of traits governing size, age, and timing of outmigration.
... Due to the low level of replication, we assessed significant differences in MSP between individual streams by comparing 95% confidence intervals (CI), with non-overlapping CIs being considered different at the 0.05 level (Cross et al. 2011). We tested for differences in P/B ratios between land-use types and all four streams with generalized linear mixed models (GLMM) (Bates et al. 2014) using the "glmer" function from the package "lme4". ...
Agricultural land‐use affects the environmental and biological characteristics of stream ecosystems through multiple pathways including nutrient and pesticide contamination, riparian clear‐cutting and hydromorphological degradation. These changes in the abiotic environment can have a direct effect on the productivity of macroinvertebrate communities through environmental filtering and via altered resource conditions encompassing a shift from allochthonous to autochthonous primary production and changes in elemental stoichiometry and food quality. Additionally, macroinvertebrate productivity can be affected indirectly via biological mechanisms, such as changes in species interactions, richness, competition, and predation. We studied the effects of agriculture on structural and functional descriptors of macroinvertebrate communities by assessing environmental characteristics and macroinvertebrate secondary production (MSP), biomass and density in two forested and two agricultural streams and investigated underlying biotic mechanisms. On average, MSP was 1.6–3.6, biomass 2.8–6.2 and density 5–13 times higher in agricultural than in forested streams. This pattern was associated with higher nutrient concentrations, standing crops of riparian herbaceous vegetation, suspended particulate organic carbon, quantity and quality of epilithic biofilms and chlorophyll‐a concentrations in seston and biofilm of the agricultural streams. Species richness and evenness were significantly lower in agricultural than in forested streams. A negative relationship between MSP and species richness and evenness indicated that density compensation and trait dominance were the prevalent mechanisms facilitating higher MSP in agricultural streams.
Our findings suggest that the loss of riparian canopy and excess nutrient conditions are the major environmental drivers contributing to homogenization of ecological niches and dominance of highly productive non‐insect generalist species. This study highlights the importance of an ecosystem approach to understanding how complex aggregate stressors affect the regulation of consumer–resource interactions. There is an urgent need to preserve or restore natural riparian vegetation, fostering habitat and resource diversity and limiting nutrient contamination to stream ecosystems.
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... Invertebrate physiology, resource consumption, reproduction, and phenology all respond to environmental stressors (Shah et al., 2020), but relatively little is known about emerging responses of consumer-resource interactions to these stressors. Although patterns of resource preference and dietary analysis have been central components of freshwater conceptual frameworks, including the river continuum concept, ecological stoichiometry, and food web analysis (Benke & Wallace, 1980;Cross et al., 2005;Vannote et al., 1980), limited information regarding diet composition of key taxa has made specific conservation actions that target invertebrates and the food resources that they require under global change challenging (Amarasekare, 2015;Cross et al., 2011). ...
Abstract Many aquatic invertebrates are declining or facing extinction from stressors that compromise physiology, resource consumption, reproduction, and phenology. However, the influence of these common stressors specifically on consumer–resource interactions for aquatic invertebrate consumers is only beginning to be understood. We conducted a field study to investigate Pteronarcys californica (i.e., the “giant salmonfly”), a large‐bodied insect that is ecologically and culturally significant to rivers throughout the western United States. We sampled gut contents and polyunsaturated fatty acid composition of salmonflies to compare resource consumption across river (Madison or Gallatin, Montana), sex (male or female), and habitat (rock or woody debris). We found that allochthonous detritus comprised the majority of salmonfly diets in the Gallatin and Madison Rivers, making up 68% of the gut contents on average, followed by amorphous detritus, diatoms, and filamentous algae. Diets showed little variation across river, sex, or length. Minor differences in diets were detected by habitat type, with a higher proportion of diatoms in the diets of salmonflies collected from rocky habitat compared to woody debris. Fatty acid composition generally supported the results of gut content analysis but highlighted the importance of primary producers. The presence of eicosapentaenoic acid (20:5n‐3) and alpha linolenic acid (18:3n‐3) indicated consumption of diatoms and filamentous green algae, respectively. Our research underscores the importance of a healthy riparian zone that provides allochthonous detritus for invertebrate nutrition as well as the role of algae as an important source of fatty acids.
... To evaluate if engineering structures affect the efficiency at which consumers control their resources, we calculated interaction strength between macroinvertebrates and their resources as species impact (SI) (Cross et al., 2011;Wootton, 1997): ...
Many large rivers used for navigation have lost their hydromorphological heterogeneity, which has led to the widespread loss of native biodiversity and the concurrent establishment of non-native communities. While the effects on biodiversity are well-described, we know little about how the loss of natural habitats and the restructuring of communities cumulate into effects on riverine food webs. We constructed binary and ingestion webs for benthic macroinvertebrates and their resources in the Elbe River (Germany) and compared if food chain length, food web complexity, robustness, ingestion rates, and consumer-resource interaction strength differ among three shoreline engineering practices. Food webs at profoundly altered shorelines were significantly less complex and had significantly shorter food chains than the food web at the semi-natural shoreline. However, food web robustness to a simulated loss of species was comparable at all shorelines. Total ingestion rates were up to eight times lower at highly altered shorelines due to significantly lower ingestion rates by native species. Predator–prey interaction strength was comparable among shorelines due to higher shares of non-native predators, indicating that non-native predators can be functionally equivalent to native predators. We attributed the observed food web differences to the absence of complex habitats at profoundly altered shorelines and the accompanied absence of specialized consumers. Our study provides empirical evidence that hydromorphological modifications reduce the efficiency of food webs to control organic matter dynamics and may ultimately affect the provisioning of riverine ecosystem services.
... Experimental floods can be used as management actions to enhance geomorphological/ecological conditions below dams and serve as large-scale flow experiments to improve the understanding of geomorphological/ecological response mechanisms to such disturbances (Konrad et al., 2011). Prime examples of long-term experimental flood applications are those of the Colorado (Cross et al., 2011) and Spöl River . These studies provide a foundation on the topic, showing that ecosystem responses to extreme flow manipulations depend on the time-frame, status of abiotic components, and the adaptation of organisms to flood disturbance. ...
Rivers regulated by dams display several ecosystem alterations due to modified flow and sediment regimes. Downstream from a dam, ecosystem degradation occurs because of reduced disturbance, mostly derived from limitations on flow variability and sediment supply. In the last decade, most flow restoration/dam impact mitigation was oriented towards the development of environmental flows. Flow variability (and consequent disturbance) can be reintroduced by releasing artificial high flows (experimental floods). Flow-sediment interactions during experimental floods represent strong ecosystem drivers, influencing nutrient dynamics, and metabolic and functional properties. In river networks, sediment and water inputs from tributaries generate points of discontinuity that can drive major changes in environmental conditions, affecting habitat structure and determining functional differences between upstream and downstream. However, despite the relevance for management, flow/sediment relations during environmental flows − and more importantly during experimental floods − remain poorly understood, mostly due to the lack of empirical evidence. In this study, we examined how a major tributary (source of water and sediments) modified the physical habitat template of a regulated river, thereby influencing ecological and geomorphological responses to experimental floods. Methods combined high-resolution drone mapping techniques with a wide range of biological samples collected in field surveys before, during, and after experimental floods in an alpine river. Data were used to quantify changes in relevant functional and structural ecosystem properties, relating ecological responses to geomorphological dynamics. Results highlight the importance of tributaries in restoring ecosystem properties lost after damming, enhancing the resilience of the system. In addition, we observed that disturbance legacy played a fundamental role in determining ecological conditions of a river prior to experimental floods, thus confirming that considering flow variability and sediment availability is crucial in adaptive dam management and environmental flows design.
... River assemblages in metropolitan areas commonly shift towards dominance by taxa that can tolerate the new environment and local extinction of those that cannot (Hobbs et al., 2006). In many cases, nonnative taxa that fill vacant niches or lack effective predators thrive and dominate biomass and production (Cross et al., 2011;Wang et al., 2018a). Changes to the physical regime may also modify the river's energetic basis, with a trend towards algal production and detritus subsidy as the dominant fuel for food webs (Cross et al., 2013;Colléter et al., 2015). ...
Tropical and subtropical rivers are being subjected to multiple stressors from human disturbance (e.g., water pollution and habitat degradation). Understanding the relationship between environmental conditions and the river ecosystem is important for improving river management. We built 14 Ecopath models composed of 28 functional groups (trophic levels [TLs] of 1.0-3.8) along a subtropical urban river to explore the influence of environmental changes on system attributes. From headwaters to downstream, the model outputs showed that the transfer efficiency (TE), energy flow parameters, and ecosystem theory indices exhibited significant (P < 0.05) differences across a longitudinal gradient of disturbance, indicating heterogeneous attributes of local river segments. The high TE values of TLs I, II, and III separated the upper, middle, and lower reaches, respectively, which could be attributed to the shift in dominant consumption flows from upstream 'periphyton - aquatic insects - insectivorous fish' to midstream 'detritus - shrimp - crustaceavorous fish' and to downstream 'phytoplankton - filter-feeding invertebrates/fish'. Structural equation modelling was used to test the causal relationships among environmental variables and demonstrated that abiotic factors directly influenced biomass composition and indirectly influenced trophic networks. Water quality, including dissolved oxygen and flow velocity; habitat characteristics, such as riffles, cobble-gravel substrate, and seasonal floodplain; and biological indicators, including the relative contributions (%) of decapods, insectivorous fish, and insect scrapers to biomass composition, had significant (P < 0.05) positive impacts on system maturity (evaluated by omnivory, connectance, and cycling indices). In the future, it will be possible to evaluate the health of river ecosystems by monitoring representative environmental factors, which could be a cost-effective approach to system-level improvement.
... Rainbow trout are abundant and dominant throughout a large segment (>100 km) of the Colorado River, encapsulating gradients in both physical (i.e., temperature, turbidity, etc.) and biological conditions (i.e., population densities, prey availability, etc.). Importantly, rainbow trout condition, growth, and reproduction are strongly and positively related to variation in prey availability (Cross et al. 2011;Yard et al. 2016;Korman et al. 2017Korman et al. , 2021, which indicates that foraging decisions by rainbow trout are strongly linked to individual fitness. ...
Drift-feeding fish are typically considered size-selective predators. Yet, few studies have explicitly tested which aspect of prey “size” best explains size selection by drift-foraging fish. Here, we develop a Bayesian discrete choice model to evaluate how attributes of both prey and predator simultaneously influence size-selective foraging. We apply the model to a large dataset of paired invertebrate drift (n = 784) and rainbow trout (Oncorhynchus mykiss) diets (n = 1028). We characterized prey “size” using six metrics (length, width, area, hemispherical area, volume, mass) and used pseudo-R² to determine which metric best explained observed prey selection across seven taxa. We found that rainbow trout are positively size-selective, they are selecting prey based upon differences in prey width, and size-selectivity increases with fish length. Rainbow trout demonstrated strong selection for the adult and pupae stages of aquatic insects relative to their larval stages. Our study provides strong empirical evidence for size-selective foraging in rainbow trout and demonstrates prey selection is based primarily upon width, not length or area as has been widely reported.
... Stable isotope tissue collection. We collected pectoral fin tissues from the distal end of the fin for isotope analysis from fishes captured in the river upstream and river downstream habitats in March (13-31), April (1-2), June (8)(9)(10), and September (22)(23)(24)(25)(26)(27)(28) in 2018 using raft electrofishing (ETS Electrofishing Systems, Madison, Wisconsin), and from reservoir fishes in April and May 2019 captured in trammel nets (45.2 m long × 1.2 m tall with 25 mm internal bar mesh and 305 mm outer bar mesh). Fin tissues typically have an isotopic half-life of several months for both δ 13 C and δ 15 N 57-59 . ...
Instream barriers can constrain dispersal of nonnative fishes, creating opportunities to test their impact on native communities above and below these barriers. Deposition of sediments in a river inflow to Lake Powell, USA resulted in creation of a large waterfall prohibiting upstream movement of fishes from the reservoir allowing us to evaluate the trophic niche of fishes above and below this barrier. We expected niche overlap among native and nonnative species would increase in local assemblages downstream of the barrier where nonnative fish diversity and abundance were higher. Fishes upstream of the barrier had more distinct isotopic niches and species exhibited a wider range in δ15N relative to downstream. In the reservoir, species were more constrained in δ15N and differed more in δ13C, representing a shorter, wider food web. Differences in energetic pathways and resource availability among habitats likely contributed to differences in isotopic niches. Endangered Razorback Sucker (Xyrauchen texanus) aggregate at some reservoir inflows in the Colorado River basin, and this is where we found the highest niche overlap among species. Whether isotopic niche overlap among adult native and nonnative species has negative consequences is unclear, because data on resource availability and use are lacking; however, these observations do indicate the potential for competition. Still, the impacts of diet overlap among trophic generalists, such as Razorback Sucker, are likely low, particularly in habitats with diverse and abundant food bases such as river-reservoir inflows.
... Rainbow Trout Oncorhynchus mykiss is one of the most widespread coldwater fishes and is often found in dam tailwaters. In the highly regulated Colorado River, simulated flood events have been shown to alter Rainbow Trout prey availability (Cross et al. 2011), and high-flow events contribute to large recruitment events of Rainbow Trout that lead to short-term population increases that can be unsustainable due to food limitations (Korman et al. 2017). The reduced growth rates in the Korman et al. (2017) study affected multiple life stages and processes, which created negative feedback loops that regulated the abundance and biomass of this population. ...
The construction and operation of dams represents one of the most significant anthropogenic impacts to the aquatic environment of freshwater ecosystems and includes changes in flow, temperature, water chemistry, sedimentation, and nutrient delivery. Despite the substantial changes caused by dams, we have a limited understanding of how dams influence important rate functions of fish, including growth rates. This study measured the growth rates of Rainbow Trout Oncorhynchus mykiss from successive captures of individually marked fish over seven annual increments within four river sections downstream of Libby Dam on the Kootenai River, Montana. We modeled the influence of hydropowerrelated environmental variables on Rainbow Trout length and weight growth rates using linear mixed-effects models. The top models predicting annual length and weight growth rates contained measures of water chemistry (ratio of total N to total P [N:P]) during the growing season, winter substrate coverage by the diatom Didymosphenia geminata, and an interactive term between winter D.geminata coverage and fish size at tagging. Winter D.geminata coverage and N:P were negatively correlated with annual growth rates, but the interactive term indicates that the influence of winter D.geminata coverage disproportionally affects smaller fish more than larger fish. We hypothesize that N:P and D.geminata are influencing Rainbow Trout growth rates through lower-trophic-level impacts. Top Rainbow Trout length and weight growth models explained 94.6% and 92.2%, respectively, of the annual variability in growth rates, of which 87.7% and 76.2%, respectively, were attributable to fixed effects. An experimental nutrient addition study and robust trophic monitoring efforts in the Kootenai River downstream of Libby Dam would be an effective means of independent corroboration of these study results. If successful, nutrient addition may be an effective management strategy to improve annual Rainbow Trout growth rates, mitigating for the nutrient retention occurring in the large reservoir upstream of Libby Dam.
... For many fish species, aquatic macroinvertebrates are a primary source of energy (Behnke 2010). Understanding how specific environmental changes alter the flow of in-stream energy to fish can thus be of great importance to conservation and management efforts (Cross et al. 2011;Bellmore et al. 2012;Scholl et al. 2019). ...
Stream habitat changes affecting primary consumers often indirectly impact secondary consumers such as fishes. Blooms of the benthic algae Didymosphenia geminata (Didymo) are known to affect stream macroinvertebrates, but the potential indirect trophic impacts on fish consumers are poorly understood. In streams of the Kootenai River basin, we quantified the diet, condition, and growth rate of species of trout, char, and sculpin. In 2018, macroinvertebrate taxa composition was different between a stream with Didymo and a stream without, but trout diets, energy demand, and growth rates were similar. Trout abundance was higher in the stream with Didymo, but the amount of drifting invertebrates was higher in the stream without. In 2019, we surveyed 28 streams with a gradient of coverage. Didymo abundance was correlated only with the percentage of aquatic invertebrates in trout diets and was not related to diets of char or sculpin or condition of any species. Thus, we found no evidence for a trophic link between Didymo blooms and the condition or growth of trout, char, or sculpin in mountainous headwater streams.
... Non-biting midges in the family Chironomidae are distributed worldwide and often contribute substantially to the energetics of aquatic ecosystems and surrounding landscapes through their rapid growth rates, short life cycles, and high densities (Merritt et al. 2008, Cross et al. 2011. Chironomids (Fig. 2C) emerge year-round on the Green River, though most adults are found in spring and summer months. ...
Aquatic insects exhibit complex life cycles that include egg, larval, adult, and, in some instances, pupal stages. Disturbances at any of these life stages can affect overall population dynamics. Yet, efforts to understand the effects of disturbances, such as hydrologic alterations, overwhelmingly focus on the larval life stage of aquatic insects. We evaluated the potential for load-following flows associated with hydroelectric power production to act as a population bottleneck for aquatic insects via reductions in the availability and temporal persistence of optimal oviposition habitats. Specifically, we quantified the oviposition habitat selectivity of Baetis spp. (Baetidae), Brachycentrus occidentalis (Brachycentridae), Chironomidae (Diptera), and Hydropsyche occidentalis (Hydropsychidae) downstream of Flaming Gorge Dam, Utah, USA. We found that all taxa except H. occidentalis preferentially laid eggs on large emergent substrates located along the river edge. Peak discharge associated with load-following flows substantially reduced the number of emergent substrates available for oviposition, and daily low flows exposed eggs in these habitats to desiccation and drying. When subjected to experimental drying, both Baetis and H. occidentalis eggs experienced nearly 100% mortality after 2 h, whereas most B. occidentalis remained viable after 8 h. Our paired field and experimental results are consistent with the hypothesis that load-following flows from hydroelectric dams produce a population bottleneck for aquatic insects by short circuiting recruitment processes. Environmental flows that seek to improve the health of tailwater aquatic insect populations would benefit from consideration of habitat requirements for all life stages of aquatic insects.
... Such flushing operations can be designed to restore sediment dynamics and associated ecological functions in residual flow reaches (Cross et al., 2011;Kondolf and Wilcock, 1996;Konrad et al., 2011;Loire et al., 2019;Poff et al., 1997;Poff and Schmidt, 2016). An example of this is the extensive long-term study of artificial flood releases at the Spöl river, Switzerland, which has been conducted since 2000 and has changed the diversity of macroinvertebrates (Robinson, 2012;Robinson et al., 2018;Robinson and Uehlinger, 2008). ...
River reaches downstream of dams with constant residual discharge often lack sediment supply and periodic high flows due to dam sediment retention and flow regulation, respectively. To test a novel multi‐deposit methodology for defining environmental flows, activating dynamics of river morphology downstream of dams, a flood was released from Rossens Dam in Switzerland. This event was combined for the first time with a multi‐deposit configuration of sediment replenishment consisting of four artificial deposits allocated as alternate bars along the river banks as a restoration measure. To validate the sediment transport behaviour observed in laboratory tests, stones were equipped with RFID PIT tags, a fix‐antenna was installed at the river bed and a mobile antenna was used to allow the investigation of erosion, transport and deposition of replenished sediments. The extension of the erosion period was determined for the tracked stones and average transport velocities were found on the order of 10‐3 m/s. To estimate the erosion efficiency of the flood, defined as the eroded tagged stones compared to the released water volume, the hydrograph was divided in different periods: rising limb, constant peak discharge, decreasing limb. During the rising limb of the flood, which lasted for 20% of the total flood duration, more than 40% of the PIT tags were transported. The defined erosion efficiency is a measure to support the hydrograph design of artificial flood releases at dams. The deposition of tagged stones resulted in a repeating cluster formation as expected from previous laboratory experiments, creating an increase in hydraulic habitat diversity. The analogy of results from field and laboratory experiments confirmed the robustness of the multi‐deposit sediment replenishment method. Combined with the gained knowledge of the erosion efficiency, results could spark further application and research of multi‐deposit sediment replenishment techniques as a habitat oriented river restoration measure.
... The accumulation of organismal biomass is also a cumulative process, where taxa-specific life history characteristics including feeding guild, mobility, life-span, and quiescence allow the assimilation of energy across multiple spatial and temporal scales (local to regional, days to years; Bowler & Benton, 2005;Dingle & Drake, 2007). Furthermore, changes in community composition dictate energy flow paths through food webs (Cross et al., 2011;Power, Parker, & Dietrich, 2008). Thus, identifying the sources of energy supporting community productivity requires the simultaneous accounting of the spatial and temporal variability inherent to both energy and community dynamics. ...
• Accurately accounting for flows of energy through food webs is challenging because of the spatial and temporal variability associated with energy production and consumption. Wet–dry tropical rivers have a highly seasonal discharge regime where wet season flows allow access to energy sources (inundated wetlands) that are not available during the dry season when aquatic consumers are confined to disconnected waterholes.
• We combined measures of fish community biomass with previously published feeding guild specific stable isotope analyses to explore how opposing wet‐ and dry‐season habitat templates influence spatial and temporal trends in the sources of energy supporting fish biomass throughout a river network in the wet–dry tropics of northern Australia.
• Accounting for the relative contribution of each feeding guild to fish community biomass was a critical component of our analyses, as a single feeding guild (invertivore/piscivore) influenced spatial and temporal patterns in the sources of energy supporting overall fish biomass. During the early dry season, the reliance of fish communities on autochthonous sources of energy (periphyton) decreased from the upper to lower reaches of the river network, which correlates with increasing floodplain area and wet season inundation times. These patterns disappeared by the late dry season as fish in both upper and lower reaches became increasingly reliant on autochthonous sources produced within waterholes over the course of the dry season, indicating that the large wet‐season gains in fish biomass are maintained through the dry season by energy produced within waterhole refuges.
• Collectively these results indicate that a combination of autochthonous and allochthonous sources of energy work in unison to support fish community biomass throughout the Mitchell River catchment and that access to these sources of energy is dictated by seasonal patterns in discharge interacting with spatial variability in river geomorphology (channel geometry and floodplain area).
• Many rivers are experiencing decreased flows due to water resource development and more frequent and severe droughts. Thus, we suggest our study provides insight into how changes in discharge regime could influence food web energetics throughout river networks.
... Relatively high growth rates are likely the result of warmer seasonal temperatures compared to the Colorado River (see temperature data in Voichick and Wright 2007), as well as a more diverse and abundant food supply in these tributaries compared to the Colorado River. Macroinvertebrate data collected from Havasu, Shinumo, and Bright Angel creeks show a relatively high proportion and density of quality food items (i.e., mayflies, stoneflies, caddisflies; Trammell et al. 2012, Whiting et al. 2014, Spurgeon et al. 2015a) that are generally lacking in the Colorado River (Cross et al. 2011, Kennedy et al. 2016). ...
This 2019 annual report describes humpback chub translocations and associated activities implemented by the Native Fish Ecology and Conservation Program, Grand Canyon National Park, and funded by the U. S. Bureau of Reclamation. The activities described herein contribute toward the implementation of conservation measures included in the U. S. Fish and Wildlife Service Biological Opinion on the operations of Glen Canyon Dam.
Food webs vary in space and time. The structure and spatial arrangement of food webs are theorized to mediate temporal dynamics of energy flow, but empirical corroboration in intermediate‐scale landscapes is scarce. River‐floodplain landscapes encompass a mosaic of aquatic habitat patches and food webs, supporting a variety of aquatic consumers of conservation concern. How the structure and productivity of these patch‐scale food webs change through time, and how floodplain restoration influences their dynamics, are unevaluated. We measured productivity and food‐web dynamics across a mosaic of main‐channel and side‐channel habitats of the Methow River, WA, USA, during two study years (2009–2010; 2015–2016) and examined how food webs that sustained juvenile anadromous salmonids responded to habitat manipulation. By quantifying temporal variation in secondary production and organic matter flow across nontreated river‐floodplain habitats and comparing that variation to a side channel treated with engineered logjams, we jointly confronted spatial food‐web theory and assessed whether food‐web dynamics in the treated side channel exceeded natural variation exhibited in nontreated habitats. We observed that organic matter flow through the more complex, main‐channel food web was similar between study years, whereas organic matter flow through the simpler, side‐channel food webs changed up to ~4‐fold. In the side channel treated with engineered logjams, production of benthic invertebrates and juvenile salmonids increased between study years by 2× and 4×, respectively; however, these changes did not surpass the temporal variation observed in untreated habitats. For instance, juvenile salmonid production rose 17‐fold in one untreated side‐channel habitat, and natural aggregation of large wood in another coincided with a shift to community and food‐web dominance by juvenile salmonids. Our findings suggest that interannual dynamism in material flux across floodplain habitat mosaics is interrelated with patchiness in food‐web complexity and may overshadow the ecological responses to localized river restoration. Although this dynamism may inhibit detection of the ecological effects of river restoration, it may also act to stabilize aquatic ecosystems and buffer salmon and other species of conservation concern in the long term. As such, natural, landscape‐level patchiness and dynamism in food webs should be integrated into conceptual foundations of process‐based, river restoration.
Limiting resources are often unevenly distributed among competing consumers, as larger, more aggressive, fast growing, early emerging, or rapidly reproducing species disproportionately exploit food or habitat. In such cases, asymmetric competition can occur (i.e., when limited resources are divided up unequally among individuals or species), resulting in stronger negative effects for some species and comparatively weaker effects for others. We hypothesize that the accessibility of food web pathways influences asymmetric competition among stream fishes. To test this hypothesis, we studied the interactions of invasive brown trout (Salmo trutta) with native Rio Grande sucker (Catostomus plebeius or Pantosteus plebeius) and Rio Grande chub (Gila pandora) in mountain streams of New Mexico, USA, from 2020 to 2021. Our study found support for the hypothesis that the accessibility of food web pathways drives asymmetric competition in mountain stream fishes, as native chub and sucker grew at slower rates than invasive brown trout across all observed body sizes. Our conclusion is evidenced by (1) strong diet overlap driven by benthic macroinvertebrates that (2) were important in fueling the growth of native fishes but were also (3) disproportionately exploited by trout. Because trout fed on small aquatic invertebrates, large crayfish, other fish, and inputs of terrestrial prey, they could incorporate energy from differing food web pathways, whereas native fishes were mostly limited to small benthic prey (and chubs fed on terrestrial arthropods). As trout, chub, and sucker competed for small benthic prey, trout could also incorporate other sources of food, whereas chub and sucker had fewer options. Although two or more species may compete for one resource, their rates of growth and production are also dependent on, and fueled by, other food web pathways. Our work demonstrates that such food web pathways help explain differences in growth and production between invasive and native fishes. Our findings show that tracking energy flows through the different food web pathways provides a useful and intuitive means of describing the important sources of food (i.e., the trophic basis of production) that fuel growth and production and sheds light on the processes that drive asymmetric competition.
In this report, we compiled, summarized, analyzed, and modeled water temperature and hydrology data collected from the Trinity River, California to meet six objectives:
1) Construct and populate a comprehensive quality-controlled water temperature dataset for analysis and archive it for future use; 2) Evaluate patterns in the relationship between flow and temperature in space and time before the construction of the TRD, pre-ROD, and post-ROD; 3) Evaluate how ROD flows have affected the thermal regime, temperature compliance, and biological objectives on the Trinity River; 4) Evaluate temperature compliance across multiple years (pre-ROD and post-ROD); 5) Update conceptual models on stream temperature dynamics in the Trinity River; 6) Develop management recommendations to provide water temperatures in the Trinity River that more efficiently meet restoration objectives of the TRRP.
Damming of a river can trap and elevate levels of sediment-bound elements and alter food web dynamics in created reservoirs. It follows that dams may alter how elements and other nutrients, like the beneficial omega-3 fatty acids (n-3 FAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are accumulated in fish and thus the chemical composition of species above and below this barrier to migration. This study examined the spatial and species differences in contaminants and nutrients in fish from the Wolastoq | Saint John River (New Brunswick, Canada) in association with a large hydroelectric dam (Mactaquac Generating Station; MQGS), a river which supports both recreational fisheries and subsistence fishing by Indigenous communities. In 2020 and 2021, Smallmouth Bass, Yellow Perch, American Eel, and Striped Bass were collected from locations upstream (reservoir and river) and downstream of the MQGS and analyzed for mercury (Hg) and 30 other trace elements, n-3 FAs, δ15N, and δ13C. Fish from the reservoir were highest in the beneficial elements P, S, and K, while fish from upstream of the reservoir had lower levels of toxic elements, including Hg. The dam appeared to alter food web dynamics, as fish from the reservoir and immediately downstream of the dam had higher δ15N and reservoir fish were depleted in δ13C. DHA and Hg were positively corelated with δ15N, and EPA in Smallmouth Bass was higher in sites where fish had higher δ13C. Overall, this study suggests that the dam altered food web dynamics and the uptake of contaminants and nutrients by fish, and that location and species are important factors when examining the risks and benefits of consuming wild fish from a system impacted by a large dam.
Rehabilitation of large Anthropocene rivers requires engagement of diverse stakeholders across a broad range of sociopolitical boundaries. Competing objectives often constrain options for ecological restoration of large rivers whereas fewer competing objectives may exist in a subset of tributaries. Further, tributaries contribute toward building a “portfolio” of river ecosystem assets through physical and biological processes that may present opportunities to enhance the resilience of large river fishes. Our goal is to review roles of tributaries in enhancing mainstem large river fish populations. We present case histories from two greatly altered and distinct large-river tributary systems that highlight how tributaries contribute four portfolio assets to support large-river fish populations: 1) habitat diversity, 2) connectivity, 3) ecological asynchrony, and 4) density-dependent processes. Finally, we identify future research directions to advance our understanding of tributary roles and inform conservation actions. In the Missouri River United States, we focus on conservation efforts for the state endangered lake sturgeon, which inhabits large rivers and tributaries in the Midwest and Eastern United States. In the Colorado River, Grand Canyon United States, we focus on conservation efforts for recovery of the federally threatened humpback chub. In the Missouri River, habitat diversity focused on physical habitats such as substrate for reproduction, and deep-water habitats for refuge, whereas augmenting habitat diversity for Colorado River fishes focused on managing populations in tributaries with minimally impaired thermal and flow regimes. Connectivity enhancements in the Missouri River focused on increasing habitat accessibility that may require removal of physical structures like low-head dams; whereas in the Colorado River, the lack of connectivity may benefit native fishes as the disconnection provides refuge from non-native fish predation. Hydrologic variability among tributaries was present in both systems, likely underscoring ecological asynchrony. These case studies also described density dependent processes that could influence success of restoration actions. Although actions to restore populations varied by river system, these examples show that these four portfolio assets can help guide restoration activities across a diverse range of mainstem rivers and their tributaries. Using these assets as a guide, we suggest these can be transferable to other large river-tributary systems.
Dams interrupt the longitudinal connectivity of rivers and consequently disrupt the structure and function of downstream communities. The Serial Discontinuity Concept recognizes these disruptions and suggests that the impacts of dams on downstream communities should attenuate with increasing distance from the dam. The impacts of dams on communities immediately downstream are well studied, but less is known about how dams affect longitudinal patterns of community structure and functional attributes. To investigate the impacts of a large hydropower dam on downstream macroinvertebrate assemblage structure and function, we sampled riffle habitats along a 49-km reach on the Green River below Flaming Gorge Dam for 1 year. We collected monthly samples from seven sites, ranging from 0.8 to 49 km below the dam. Macroinvertebrate assemblage composition differed among sites below the dam and exhibited distinct longitudinal patterns. Taxonomic richness increased with increasing distance from the dam, but total macroinvertebrate abundance and annual secondary production decreased with increasing distance from the dam. Furthermore, functional feeding group composition differed among sites and also exhibited longitudinal patterns. Our results contribute to a body of evidence that demonstrates longitudinal effects of dams on downstream macroinvertebrate community structure and function.
Steady low reservoir releases allow downstream aquatic invertebrates (bugs) to lay and hatch eggs and increase production. These releases also reduce revenue from hydropeaking operations, increase costs to hydropower customers, reduce funds to maintain project infrastructure, repay project loans, and exacerbate hydropower production-ecosystem conflicts. This paper has the purpose to (1) quantify tradeoffs between the number of days of bug flows and hydropower revenue, (2) identify ways to reduce costs to hydropower customers, and (3) inform the design of a financial instrument to increase bug production, compensate hydropower customers for costs, and reduce conflict. A linear program identified tradeoffs between hydropower revenue and number of days of steady low releases per month for different contract and market energy prices and monthly release volumes across March to October months when bugs are most productive. We found that bug flows on 8 weekend days per summer month in 2018 from Glen Canyon Dam, Arizona reduced hydropower revenue by 600,000 (August). Shifting bug flow days to Spring/Fall months reduced costs. To reduce conflict, we suggest creating a new financial instrument funded by the Federal Treasury for ~600,000 per month. The instrument can give ecosystem managers more flexibility to choose days for steady low releases that advantage bugs and pay hydropower producers for costs. Next steps are to engage Federal agencies on the benefits and limitations of the proposed instrument and expand to steady high releases that mobilize sediment, build sandbars, and disadvantage non-native, invasive fish populations.
Species invasions are a leading threat to ecosystems globally, but our understanding of interactions among multiple invasive species and their outcomes on ecosystem properties is undeveloped despite their significance to conservation and management. Here we studied a large lake in Minnesota, USA, that experienced a simultaneous surge in invasive zebra mussel and spiny water flea populations. A long-term (2000–2018) dataset offered a rare opportunity to assess whole-ecosystem shifts following the co-invasion. Within two years, the native crustacean zooplankton community declined abruptly in density and productivity (−93% and −91%, respectively). Summer phytoplankton abundance and water clarity remained stable across the time series, an unexpected outcome given the high density of zebra mussels in the lake. Observational data and modeling indicate that removal of native herbivorous zooplankton by the predatory spiny water flea reduced zooplankton grazing pressure enough to compensate new grazing losses due to zebra mussels, resulting in a zero net effect on phytoplankton abundance and water clarity despite a wholesale shift in secondary production from the pelagic to the benthic food web. This study reveals the extent of direct and indirect effects of two aquatic invaders on food-web processes that cancel shifts in water clarity, a highly valued ecosystem service. Observational data and modeling are used to reveal the extent of direct and indirect effects of spiny water flea and zebra mussels on food-web processes that cancel shifts in water clarity.
Flushing flows are deliberate high-flow releases designed to mimic effects of floods in removing fine sediment from downstream aquatic habitats. A special case of environmental flows, flushing flows are intended to act on geomorphic processes to mitigate ecological effects of dams. We review definitions advanced for these flows, and then propose a new term, morphogenic releases, for high flows designed to interact with the channel to produce biophysical changes in channel characteristics. Similarly, ecomorphogenic releases is proposed for those flows intended specifically to improve aquatic and riparian habitats. We then review objectives of such specific environmental flows and potential conflicts with environmental, socio-economic, and dam-operational issues. We also provide some recommendations based on our literature review and experiences to specify the dimensions of morphogenic flows. Experiments to assess effects of morphogenic flows are increasingly part of the process of defining environmental flows. We highlight the challenges to implement morphogenic flows and some alternative actions when such flows cannot be implemented, recognizing that flow is not the only variable of interest downstream of dams to mitigate effects of dams; that mitigation of channel form, sediment supply, water quality, or other factors may also be important. Returning the channel to a pre-regulation state is often not possible, and it is necessary to address issues pragmatically by identifying the most efficient improvement options and what is financially feasible and socially desirable.
Hydropeaking refers to the mode of hydropower dam operation where sub-daily changes in flow are used to vary the generation of electricity in accordance with demand. A typical pattern produces maximum power during the day (i.e., the peak), and minimal power at night. Hydropeaking is considered necessary to stabilize the energy grid since it is the only reliably flexible method of producing electricity besides fossil fuels. With the planned phase-out of traditional coal-fired electricity production across Canada by 2030, and the increased reliance on intermittent wind and solar generation, the flexibility of hydropeaking will assume an increased importance. However, hydropower generation comes with costs; hydropeaking in particular is considered one of the most ecologically harmful modes of operation since downstream biota are subjected to flows that deviate greatly from typical natural flow regime patterns. The ecological effects of hydropeaking have been examined in a growing body of literature, but mitigation options do exist that include dam operational and/or structural modifications. This paper will explore the importance of hydropeaking in the Canadian electricity system, the ecological consequences of flexible hydropower, and mitigation options that could potentially strike a balance between meeting Canadian energy needs and minimizing ecosystem impacts.
Secondary production is an integrative measure of the accumulation of heterotrophic biomass through time and can be a valuable tool to design, implement, and assess restoration initiatives. To highlight applications of secondary production in restoration contexts, we identify recent papers from the literature, use these to make generalizations about how the concept is applied, and examine why it may not be utilized more commonly. We identified 21 papers that empirically quantified secondary production to compare pre/post-restoration or assess restored sites relative to reference ones. Every study was aquatic, suggesting that secondary production is an underutilized tool in terrestrial restoration studies. We discuss various ways that food web perspectives inform restoration secondary production outcomes, such as through shifts in aquatic basal resource pools and alleviation of nutrient limitation, changes which ripple through food webs supporting higher trophic level production. Despite challenges inherent to calculating secondary production, the approach holds much promise—it is a composite metric simultaneously reflecting components of ecosystem structure and dynamics that restoration initiatives target.
Anadromous Pacific salmon are semelparous, and resource subsidies from spawning adults (marine-derived nutrients, or MDN) benefit juvenile salmonids rearing in freshwater. However, it remains unclear how MDN assimilation relates to spawner abundance within a watershed. To address this, we examined seasonal, watershed-scale patterns of MDN assimilation in rearing coho (Oncorhynchus kisutch) and Chinook (O. tshawytscha) salmon and compared it with spawner biomass and landscape features in a western Alaska watershed with contrasting structural complexity in two sub-drainages. Adult salmon biomass density was estimated from escapement and spawner distribution data, and MDN assimilation in juvenile salmon was estimated via stable isotopes. In the North River, MDN assimilation was lowest in early summer, prior to annual spawning migrations, increased after spawning, and peaked in late winter. In the more complex mainstem Unalakleet River, MDN assimilation was higher but varied minimally from summer through fall before increasing in late fall and winter. Summer MDN assimilation, prior to salmon spawning, was primarily a function of habitat complexity, where MDN was highest in sloughs and the more complex mainstem river. After salmon spawned, fall MDN assimilation was a function of adult pink and Chinook salmon biomass as well as MDN assimilation that occurred prior to spawning (that is, summer MDN), but unrelated to total summer biomass (all salmon species biomass combined). Thus, MDN assimilation by juvenile salmon in the fall was a function of species-specific adult spawner abundance but seasonal patterns of MDN assimilation were masked in complex habitat where summer MDN assimilation remained high.
Variations in the flow regime lead to changes in the spatial and temporal distributions of nutrients entering the estuary area from the river, which causes corresponding changes in the trophic structure of the estuarine food web and ultimately affects the entire ecosystem. Therefore, studying the responses of estuarine food webs to changes in upstream water and sediment conditions is of great significance for protecting the biodiversity, stability, and functional integrity of estuarine ecosystems. This study focused on the world-famous sandy river, the Yellow River, and the south and north shores of the Yellow River estuary and selected sample areas with different environmental characteristics. Typical species were sampled and measured for carbon- and nitrogen-stable isotopes in the wet and dry seasons, respectively. Based on community indicators of stable isotopes, the trophic structure patterns of the aquatic ecosystem in the Yellow River estuary at different times and spaces were described. The results showed that the north shore area had greater trophic diversity and lower trophic redundancy than the south shore area. This kind of gap was more obvious in the wet season when water was abundant than in the dry season. The south shore area with lower aquatic productivity had greater reliance to input from terrestrial organic matter than the north shore area. The increase in terrestrial material input during the wet season increased the carbon-stable isotope value of the south shore food web and changed the basic food sources. The north shore food web, which did not depend on a single food source, had more abundant organic matter sources. The trophic diversity increased in the wet season, but the trophic redundancy was reduced. Our research suggests that the trophic structure patterns of estuarine ecosystems vary with the hydrological regime and environmental conditions, especially turbidity, which greatly affect the stability of estuarine ecosystems.
Mercury (Hg) biomagnification in aquatic food webs is a global concern; yet, the ways species traits and interactions mediate these fluxes remain poorly understood. Few pathways dominated Hg flux in the Colorado River despite large spatial differences in food web complexity, and fluxes were mediated by one functional trait, predation resistance. New Zealand mudsnails are predator resistant and a trophic dead end for Hg in food webs we studied. Fishes preferred blackflies, which accounted for 56 to 80% of Hg flux to fishes, even where blackflies were rare. Food web properties, i.e., match/mismatch between insect production and fish consumption, governed amounts of Hg retained in the river versus exported to land. An experimental flood redistributed Hg fluxes in the simplified tailwater food web, but not in complex downstream food webs. Recognizing that species traits, species interactions, and disturbance mediate contaminant exposure can improve risk management of linked aquatic-terrestrial ecosystems.
A century of hydrologic data (1895-1999) and 50 yr of aquatic macroinvertebrate assemblage data (1947-1999) were examined for two tailwater reaches of the Green River downstream from Flaming Gorge Dam in northeastern Utah, USA (40°54′ N, 109°25′ W). One reach was located upstream of an intermittent tributary, and the other downstream. The purpose of the study was to chronicle long-term dynamics and the effect of partial thermal restoration on invertebrate assemblages. The immediate hydrologic effect of the dam was a large decrease in annual maximum daily discharges, water temperatures, and sediment transport. Upstream of the intermittent tributary, macroinvertebrate genera declined from >70 to
Estimation of invertebrate biomass is a critical step in addressing many ecological questions in aquatic environments. Length-dry mass regressions are the most widely used approach for estimating benthic invertebrate biomass because they are faster and more precise than other methods. A compilation and analysis of length-mass regressions using the power model, M (mass) = a L (length)(b), are presented from 30 y of data collected by the authors, primarily from the southeastern USA, along with published regressions from the rest of North America. A total of 442 new and published regressions are presented, mostly for genus or species, based on total body length or other linear measurements. The regressions include 64 families of aquatic insects and 12 families of other invertebrate groups (mostly molluscs and crustaceans). Regressions were obtained for 134 insect genera (155 species) and 153 total invertebrate genera (184 species). Regressions are provided for both body length and head width for some taxa. In some cases, regressions are provided from multiple localities for single taxa. When using body length in the equations, there were no significant differences in the mean value of the exponent b among 8 insect orders or Amphipoda. The mean value of b for insects was 2.79, ranging from only 2.69 to 2.91 among orders. The mean value of b for Decapoda (3.63), however, was significantly higher than all insects orders and amphipods. Mean values of a were not significantly different among the 8 insect orders and Amphipoda, reflecting considerable variability within orders. Reasons for potential differences in b among taxa are explained with hypothetical examples showing how b responds to changes in linear dimensions and specific gravity. When using head width as the linear dimension in the power model, the mean value of b was higher (3.11) than for body length and more variable among orders (2.8-3.3). Values of b for Ephemeroptera (3.3) were significantly higher than those for Odonata, Megaloptera, and Diptera. For those equations in which ash-free dry mass was used, % ash varied considerably among functional feeding groups (3.3-12.4%). Percent ash varied from 4.0% to 8.5% among major insect orders, but was 18.9% for snails (without shells). Family-level regressions also are presented so that they can be used when generic equations are unavailable or when organisms are only identified to the family level. It is our intention that these regressions be used by others in estimating mass from linear dimensions, but potential errors must be recognized.
Dry weight, ash-free weight, protein and energy content of food and faeces of Gammarus pseudolimnaeus Bousfield on different diets were compared. When feeding on elm or maple leaves, approximatately 10% of the dry weight, 14-18% of the protein, and 17-19% of the energy of the ingested material was assimilated. When mycelium of various fungi which commonly decompose leaves in streams was offered, Gammarus assimilated 42.6-75.6% of the dry weight, 73.3-96.4% of the protein, and 67.9-83.2% of the energy of the food ingested. These results and the presumable preference of detritius-feeders for leaf areas with highesphyphal concentrations indicate that the significance of fungal substances in the diet of leaf-eating animals may be considerably higher than suggested by estimates of the average microbial biomass per unit weight of decomposing leaves. /// Сравнивали сухой вес, беззольный вес, содержание белков и калорийность пищи и экскрементов Gammaues pseudolimnaeus Bousfield которых кормили различной пищей. При питании личтвой вяза и клена ассимилровалось примерно 10% сухого веса, 14-18% белков и 17-19% энергии потребленного материала. При кормлении мицелием различных грибов, обычно разлагающих листья в воде, гаммарусы усваивали 42,6-75,6% сухого веса 73,3-96,4% белка и 67,9-83,2% энергии потребленной пищи. Эти результаты и вероятное предпочтение детритофагами частей листа с наибольшей концентрацией грибного мицелия свидетельствуют о том, что значение грибов в питании животных-потребителей листвы может быть значительно выше, чем это предполагалось после определения средней биомассы микроорганизмов на единицу веса разлагающейся листвы.
Trophic linkages in Glen and Grand Canyons of the lower Colorado River downstream from Glen Canyon Dam were examined using multiple stable isotope analysis. The δ 13C values of dissolved inorganic carbon (DIC), and the δ 13C, δ 15N, and δ 34S values of seston, aquatic and terrestrial plants, and aquatic animals were determined. The δ 13C value of DIC varied among sites. DIC from the epilimnion of the reservoir (Lake Powell) and from a tributary was more 13C-enriched than DIC in the Colorado River, probably as a result of variation in aquatic primary production and dissolution of carbonate among sites. Four potential bases of aquatic secondary production: upland vegetation, riparian vegetation, reservoir plankton, and benthic algae were isotopically (δ 13C and δ 15N) distinct from each other. Analysis of δ 13C, δ 15N, and δ 34S showed that seston from the dam tailwater (Glen Canyon) consisted of lotic algae and zooplankton from Lake Powell, except for the ultra fine fraction (
Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A. (editors), 1999, The controlled flood in Grand Canyon: Scientific experiment and management demonstration: Washington, D.C. American Geophysical Union, Geophysical Monograph 110, 367 p.
The Colorado River ecosystem in lower Glen Canyon and throughout Marble and Grand Canyons was greatly altered following closure of Glen Canyon Dam in 1963, as flood control and daily fluctuating releases from the dam caused large ecological changes. Ecosystem research was conducted from 1983 through 1990, and intensively from 1990 through 1995 when dam releases were modified both for scientific purposes and protection of the river ecosystem. High flows (e.g., beach/habitat building flows) were included in the Glen Canyon Dam Environmental Impact Statement (EIS), which identified a preferred strategy for dam operations and protection of the downstream ecosystem. Use of high flows partially fulfills recommendations of many river and riparian scientists for return of more natural flows, as part of initial efforts in river restoration. In 1996, a seven-day experimental controlled flood was conducted at Glen Canyon Dam to closely study the effects of a high flow event equivalent to those proposed for future dam management. It is an example of modification of operations of a large dam to balance economic gains with ecological pro- tection. Limited to 1274 m 3/s, the test flood was lower than pre-dam spring floods. The experiment was conducted to (1) test the hypothesis that controlled floods can improve sediment deposition patterns and alter important ecological attributes of the river ecosystem without negatively affecting other canyon resources and (2) learn more about river pro- cesses, both biotic and abiotic, during a flood event. Along with an explanation of the planning and background of this flood experiment, this paper summarizes expected and realized changes in canyon resources studied during the flood. Responses of specific re- sources to the flood are synthesized in the following compendium papers.
A beach/habitat-building flow (i.e., test flood) of 1274 m3/s , released from Glen Canyon Dam down the Colorado River through Grand Canyon, had little effect on distribution, abundance, or movement of native fishes, and only short-term effects on densities of some nonnative species. Shoreline and backwater catch rates of native fishes, including juvenile humpback chub (Gila cypha), flannelmouth suckers (Catostomus latipinnis), and bluehead suckers (C. discobolus), and all ages of speckled dace (Rhinichthys osculus), were not significantly different before and after the flood. Annual spring spawning migrations of flannelmouth suckers into the Paria River and endangered humpback chub into the Little Colorado River(LCR) took place during and after the flood, indicating no impediment to fish migrations. Pre-spawning adults staged in large slack water pools formed at the mouths of these tributaries during the flood. Net movement and habitat used by nine radio-tagged adult humpback chub during the flood were not significantly different from prior observations. Diet composition of adult humpback chub varied, but total biomass did not differ significantly before, during, and after the flood, indicating opportunistic feeding for a larger array of available food items displaced by the flood. Numbers of nonnative rainbow trout (Oncorhynchus mykiss)
Life histories and annual production were determined for six species of net-spinning caddisflies in a headwater stream of the Tallulah River in north Georgia, USA. Five species in the family Hydropsychidae were univoltine, whereas the sixth, a member of the Philopotamidae, had at least two generations per year. Combined annual production, as determined by the Hynes method, was 1.0 g/m2 (ash-free dry mass). Seventy-five percent of the production was concentrated in the two largest species, Arctopsyche irrorata and Parapsyche cardis. The remaining production from highest to lowest percent, was contributed by Dolophilodes distinctus, Hydropsyche sparna, Diplectrona modesta, and Hydropsyche macleodi. Analysis of gut contents alone indicated that detritus was the most important food source. How- ever, food preference and food-specific ecological efficiencies were utilized to calculate the amount of production attributable to each major food category. Surprisingly, almost 80% of all caddisfly production was attributed to animal food, 13% to detritus, and 8% to algae. Actual annual consumption required to account for this production was 2.28 g/m2 animals, 2.54 g/m2 detritus, and 0.51 g/m2 algae. We attempt to quantify the role that net-spinning caddisflies play in the "spiralling" of seston in mountain streams. Our results show that the omnivorous caddisflies are not the major consumer of detritus and algae, and that they produce more detritus in their feces than they consume, thus ap- pearing to lower the food quality of the seston. Net-spinning caddisfly production in this mountain stream appears to be limited by the amount of high quality food available in the seston.
Human alteration of Earth is substantial and growing. Between one-third and one-half of the land surface has been transformed
by human action; the carbon dioxide concentration in the atmosphere has increased by nearly 30 percent since the beginning
of the Industrial Revolution; more atmospheric nitrogen is fixed by humanity than by all natural terrestrial sources combined;
more than half of all accessible surface fresh water is put to use by humanity; and about one-quarter of the bird species
on Earth have been driven to extinction. By these and other standards, it is clear that we live on a human-dominated planet.
Freshwater ecosystems are at the forefront of the global biodiversity crisis, with more declining and extinct species than in terrestrial or marine environments. Hydrologic alterations and biological invasions represent two of the greatest threats to freshwater biota, yet the importance of linkages between these drivers of environmental change remains uncertain. Here, we quantitatively test the hypothesis that impoundments facilitate the introduction and establishment of aquatic invasive species in lake ecosystems. By combining data on boating activity water body physicochemistry, and geographical distribution of five nuisance invaders in the Laurentian Great Lakes region, we show that non-indigenous species are 2.4 to 300 times more likely to occur in impoundments than in natural lakes, and that impoundments frequently support multiple invaders. Furthermore, comparisons of the contemporary and historical landscapes revealed that impoundments enhance the invasion risk of natural lakes by increasing their proximity to invaded water bodies, highlighting the role of human-altered ecosystems as "stepping-stone" habitats for the continued spread of freshwater invaders.
We determined the trophic basis of production and quantified the food web of caddisfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Production was dominated by three net-spinning taxa (Cheumatopsyche spp., Hydropsyche rossi, and Chimarra moselyi), comprising 97-98% of the total among at least 14 trichopteran species. Annual production (as dry mass) was among the highest estimates reported for caddisflies, ranging from 43.5 to 63.9 g/m(2) of snag surface (Or from 14.2 to 25.5 g/m(2) of channel bottom) in two consecutive years. Although all taxa were present throughout the year, C. moselyi had much higher production in summer, H. rossi was highest from late summer to winter, and Cheumatopsyche spp. showed no pattern. Ingestion was determined from production estimates, bioenergetic efficiencies, and quantitative gut analyses. The two macrofiltering taxa (Cheumatopsyche spp. and H. rossi) were omnivorous, with 50.6 and 64.4% of their production, respectively, due to eating animals. Somewhat lower amounts of their production (40.7 and 23.5%) were due to ingestion of amorphous detritus. Production of the microfiltering species Chimarra moselyi was primarily due to eating amorphous detritus (91%). Removal of amorphous detritus from the system by the major taxa was highest in summer, and diatom removal was highest in fall, whereas animals were eaten consistently throughout the year. A quantitative food web showed that while the linkages among nine taxa were complex, the ingestion pathways were dominated by amorphous detritus (total consumption = 62.5 g.m(-2).yr(-1)) and animal prey (22.3 g.m(-2).yr(-1)), with >99% of food resources being consumed by the three dominant filter-feeding taxa. A connectivity food web was misleading because it implied equivalence of all food resources and consumers when great variation in strength of linkages actually existed. These comparisons provide a strong argument for considering energy flow as a measure of linkage strength when evaluating food webs, and they cast doubt on the usefulness of oversimplified connectivity webs as a basis for food web theory.
We sampled macroinvertebrates in 51 rapids in regulated and unregulated north Swedish rivers. Our objective was to quantify the effects on the invertebrate fauna of altered flow patterns, habitat area and isolation. The remaining rapids in regulated north Swedish rivers have two types of flow regime: (a) Those with altered flow pattern, but unreduced flow, are characterized by low long-term variation and high day to day variation. Typically, the spring flood is strongly reduced, whereas winter flow is greatly increased in relation to the situation in unregulated streams. (b) Rapids from which a major part of the flow has been diverted usually show high seasonal and daily variation, although there are often long periods with constant flow within seasons. The effects of altered flow patterns were examined using the following method. Predictive models were built for biotic responses at unregulated sites. These models did not include variables affected by regulation, such as flow magnitude and variability. The models were then used to make predictions for affected sites. The differences between observed and predicted numbers were used as a measure of the effect of regulation. Sites with reduced flow had 0–30 species (mean=6) less than predicted, corresponding to a loss of 0–38% of predicted richness. The total abundance was 0–54% (mean=12%) less than predicted. Subdivision of the fauna into functional feeding groups showed that the abundances of collectors, grazers and predators, but not filter feeders and shredders,were negatively affected. For regulated sites,with no diversion of flow, significant losses were only recorded for the abundances of collectors and predators. A subsequent analysis of the relationship between the effects and the regulation-related variables, which had been left out in the first models, indicated that the occurrence of large and rapid changes of discharge was the most important factor. No effects on overall species richness of habitat size and isolation were found, suggesting that extinction and re-establishment of subpopulations are not prominent processes on the scale considered in this investigation. Our data suggest that avoiding large and rapid flow changes can increase both the abundance and the diversity of vertebrates. Increasing the flow will decrease flow variability but will also expand the habitable area and thus the production of invertebrates. The best effect is expected at sites where a considerable proportion of the flow has been diverted.
Coarse particulate organic mass varied seasonally and changed from autochthonous to allochthonous organic matter along a 386-km section of the Colorado River below Glen Canyon Dam, Arizona. Dam operations influenced drift components selectively throughout the hydrograph; however, ramping rate had no effect on drift mass. Eddies and pools collected ~50% of the main- stem drift with only detritus retained long enough to settle in pools. Coarse particulate organic drift mass was negatively correlated with stream gradient in the Colorado River Cladophora drift packets were pulverized by rapids, which subsequently increased fine particulate organic matter at downriver sites. Tributary type (spring stream or large desert watershed river) significantly altered the mass and composition of tributary drift; however, tributaries contributed <0.1% of the total organic matter to main-stem drift. Terrestrial insects represented <0.001% of the total invertebrate mass in main-stem drift. As a result of hydraulics and suspended sediment, organic drift mass throughout the river corridor is reach specific. Longitudinal change in drift composition corresponds with a shift in fish species.
The 1996 controlled flood provided evidence that elevated releases from Glen Canyon Dam can enhance short-term primary and secondary production of aquatic resources of the Colorado River in Grand Canyon National Park. The flood scoured substantial proportions of benthic algae and macroinvertebrates and removed fine sediments from the channel, which ultimately stimulated primary productivity and consumer biomass. Channel margin sand deposits buried riparian vegetation and leaf litter, entraining nutrients for later incorporation into the upper trophic levels. The flood restructured high-stage sand bars and associated eddy return channels (i.e., backwaters used as nurseries by native and non-native fish), but many were short-lived because reattachment bars were eroded shortly after the flood. The flood was of insufficient magnitude to permanently suppress non-native fish populations, even though there was significant population depletion at some collecting sites. Pre-spawning aggregations, spawning ascents of tributaries, and habitat use by native fishes were unaffected by the flood. Adult rainbow trout (Oncorhynchus mykiss) in the Lees Ferry tailwater fishery were also unaffected, but the proportion of juveniles <152 mm total length decreased by 10%; a strong year class following the flood indicated replacement through successful reproduction.
The controlled flood in the Colorado River below Glen Canyon Dam, Arizona, provided valuable information on short-term responses for both the riverine system and the biotic community, but the long-term effects of the flood on the aquatic food base were more difficult to assess. The 1274 m³/s discharge flushed the silt/clay fraction from the channel bottom throughout the river corridor. There were no significant differences in dissolved oxygen, specific conductance, and pH before and after the flood compared to during the flood. However, water clarity was dramatically reduced during the first 2 days of the flood event, but cleared after 7 days. Over 90% of the phytobenthos and ≥50% of the benthic invertebrates were scoured from the Lees Ferry reach, with biota associated with unstable fine sediment most vulnerable. Most of the dissolved organic carbon (DOC) and particulate organic carbon (POC) that passed through the river corridor was entrained in the initial hydrostatic wave; values for DOC and POC were significantly lower throughout the remainder of the flood. Stable isotope analyses indicated that riparian and upland vegetation made up most of the stream drift during the experimental flood, whereas phytobenthos was the dominant drift constituent during normal dam operations. Recovery rates to preflood levels were fast for phytobenthos (1 mon) and invertebrates (2 mon). We propose that the rapid recover rates and current high standing stock of aquatic benthos in the river corridor is more a function of higher water clarity, due to higher relatively constant dam releases, rather than solely related to the controlled flood. Our data indicate that consistent high discharges (≥400 m³/s) from Glen Canyon Dam mitigate the influence of suspended sediments delivered from tributaries on water clarity. Therefore, optimum conditions for management of the present exotic food base below Glen Canyon Dam may be achieved by steady discharges (~450 m³/s) with minimal fluctuation cycles (~50 m³/s).
Almost all reported benthos production levels were much too low to be able to support levels of fish production commonly reported in trout streams. This conclusions was taken as a reaffirmation of the "Allen paradox'. A number of other fish food resources (the hyporheos, snag habitat, floodplain fauna, terrestrial drift, meiofauna, other fishes) are suggested as additional sources of fish food which, when adequately measured, may solve the problem of the Allen paradox. -from Author
A gaging station has been operated by the U.S. Geological Survey at Lees Ferry, Arizona, since May 8, 1921. In March 1963, Glen Canyon Dam was closed 15.5 miles upstream, cutting off the upstream sediment supply and regulating the discharge of the Colorado River at Lees Ferry for the first time in history. To evaluate the pre-dam variability in the hydrology of the Colorado River, and to determine the effect of the operation of Glen Canyon Dam on the downstream hydrology of the river, a continuous record of the instantaneous discharge of the river at Lees Ferry was constructed and analyzed for the entire period of record between May 8, 1921, and September 30, 2000. This effort involved retrieval from the Federal Records Centers and then synthesis of all the raw historical data collected by the U.S. Geological Survey at Lees Ferry. As part of this process, the peak discharges of the two largest historical floods at Lees Ferry, the 1884 and 1921 floods, were reanalyzed and recomputed. This reanalysis indicates that the peak discharge of the 1884 flood was 210,000±30,000 cubic feet per second (ft3/s), and the peak discharge of the 1921 flood was 170,000±20,000 ft3/s. These values are indistinguishable from the peak discharges of these floods originally estimated or published by the U.S. Geological Survey, but are substantially less than the currently accepted peak discharges of these floods. The entire continuous record of instantaneous discharge of the Colorado River at Lees Ferry can now be requested from the U.S. Geological Survey Grand Canyon Monitoring and Research Center, Flagstaff, Arizona, and is also available electronically at http://www.gcmrc.gov. This 'record is perhaps the longest (almost 80 years) high-resolution (mostly 15- to 30-minute precision) times series of river discharge available. Analyses of these data, therefore, provide an unparalleled characterization of both the natural variability in the discharge of a river and the effects of dam operations on a river. Following the construction and quality-control checks of the continuous record of instantaneous discharge, analyses of flow duration, sub-daily flow variability, and flood frequency were conducted on the pre- and post-dam parts of the record. These analyses indicate that although the discharge of the Colorado River varied substantially prior to the closure of Glen Canyon Dam in 1963, operation of the dam has caused changes in discharge that are more extreme than the pre-dam natural variability. Operation of the dam has eliminated flood flows and base flows, and thereby has effectively "flattened" the annual hydrograph. Prior to closure of the dam, the discharge of the Colorado River at Lees Ferry was lower than 7,980 ft3/s half of the time. Discharges lower than about 9,000 ft3/s were important for the seasonal accumulation and storage of sand in the pre-dam river downstream from Lees Ferry. The current operating plan for Glen Canyon Dam no longer allows sustained discharges lower than 8,000 ft3/s to be released. Thus, closure of the dam has not only cut off the upstream supply of sediment, but operation of the dam has also largely eliminated discharges during which sand could be demonstrated to accumulate in the river. In addition to radically changing the hydrology of the river, operation of the dam for hydroelectric-power generation has introduced large daily fluctuations in discharge. During the pre-dam era, the median daily range in discharge was only 542 ft3/s, although daily ranges in discharge exceeding 20,000 ft3/s were observed during the summer thunderstorm season. Relative to the pre-dam period of record, dam operations have increased the daily range in discharge during all but 0.1 percent of all days. The post-dam median daily range in discharge, 8,580 ft3/s, exceeds the pre-dam median discharge of 7,980 ft3/s. Operation of the dam has also radically changed the frequency of floods on the Colorado River at Lees Ferry. The frequency of floods with peak discharges larger than about 29,000 ft3/s has greatly decreased, while the frequency of smaller floods, with peak discharges between 18,500 and 29,000 ft3/s, has increased substantially. Operation of the dam has greatly extended the duration of smaller floods; for example, each of the four longest periods of sustained flows in excess of 18,500 ft3/s occurred after closure of the dam.
The life cycles of mass species of Tubificidae: Isochaetides newaensis, Tubifex tubifex and Limnodrilus hoffmeisteri have been studied in rivers, reservoirs, lakes and under experimental conditions. The length of the life cycles depend on abiotic and biotic factors of the environment. Thus, changes in the temperature regime, in the level of productivity of the mud, and in the population density result in changes in the time, duration, and intensity of reproduction of the worms causing transformations in the structure and productivity of the populations.
Stream Ecology: Structure and Function of Running Waters is designed to serve as a textbook for advanced undergraduate and graduate students, and as a reference source for specialists in stream ecology and related fields. The Second Edition is thoroughly updated and expanded to incorporate significant advances in our understanding of environmental factors, biological interactions, and ecosystem processes, and how these vary with hydrological, geomorphological, and landscape setting. The broad diversity of running waters - from torrential mountain brooks, to large, lowland rivers, to great river systems whose basins occupy sub-continents - makes river ecosystems appear overwhelming complex. A central theme of this book is that although the settings are often unique, the processes at work in running waters are general and increasingly well understood. Even as our scientific understanding of stream ecosystems rapidly advances, the pressures arising from diverse human activities continue to threaten the health of rivers worldwide. This book presents vital new findings concerning human impacts, and the advances in pollution control, flow management, restoration, and conservation planning that point to practical solutions. Reviews of the first edition: ".. an unusually lucid and judicious reassessment of the state of stream ecology" Science Magazine "..provides an excellent introduction to the area for advanced undergraduates and graduate students-" Limnology & Oceanography "- a valuable reference for all those interested in the ecology of running waters." Transactions of the American Fisheries Society.
Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.
The method of Hynes and Coleman is modified to make it more adaptable and more consistent with their underlying assumptions. If most organisms comprising a community are univoltine and have approximately the same maximum size, the average standing crop can be used to obtain a realistic estimate of annual production. The size frequency distribution can be regarded as a first estimate of an “average cohort” when the number of “average cohorts” equals the number of size classes through which the organisms grow. If growth, in terms of the size units used, is assumed to be linear, then numerical differences in adjacent size classes can be attributed to mortality. When all size classes are considered together, the effect of nonlinear growth on the estimate of annual production is not large. In contrast, a serious error is introduced if the organisms are not univoltine. When the growth pattern and generation time are known, it is relatively simple to modify the average size frequency distribution to improve the estimate of the “average cohort” and hence improve the estimate of annual production. A reply to the criticisms of Fager is included.
We used five in situ experiments to test the influence of fluctuations in river discharge on the structure and function of the tailwaters benthos associated with cobble substrata in the Colorado River downstream from Glen Canyon Dam, Arizona, USA. Periods of daily desiccation and freezing during river fluctuation significantly limited community biomass and energy. The permanently submerged channel supported 4-fold higher macroinvertebrate mass than the varial zone. Daily harvests of benthos showed a 50% reduction in mass of Cladophora glomerata after 2 d of repeated 12-h summer exposure. Five days of repeated exposure resulted in >70% reduction in C. glomerata and >50% reduction in epiphyton mass. We observed a greater than or equal to 85% reduction in benthic macroinvertebrate mass after only one 12-h summer exposure. One night time exposure to subzero winter air temperatures resulted in greater than or equal to 50% loss of chlorophyll a and mass of C. glomerata and greater than or equal to 90% loss in macroinvertebrate mass. Gastropod densities on resubmerged cobbles that were subjected to long-term desiccation (similar to 6 mo) equalled submerged control cobbles within 1 wk, whereas recolonization by C. glomerata, Gammarus lacustris, and chironomid larvae was significantly slower; i.e., less than or equal to 30% of controls after 4 mo. Hence, our data showed that two 12-h exposure periods may require >4 mo for recovery to achieve the mass of permanently submerged benthos. Incremental increases in benthic energy (joules/m(2)) over increased base discharge from 142 to 793 m(3)/s may result in an increase in trout mass of approximately 42.5 kg/ha in the tailwaters at Lees Ferry.
Disturbance is an important component of many ecosystems, and variations in disturbance regime can affect ecosystem and community structure and functioning. The “intermediate disturbance hypothesis” suggests that species diversity should be highest at moderate levels of disturbance. However, disturbance is also known to increase the invasibility of communities. Disturbance therefore poses an important problem for conservation management, Here, we review the effects of disturbances such as fire grazing, soil disturbance and nutrient addition on plant species diversity and invasion with particular emphasis on grassland vegetation. Individual components of the disturbance regime can have marked effects on species diversity, but it is often modifications of the existing regime that have the largest influence. Similarly, disturbance can enhance invasion of natural communities, but frequently it is the interaction between different disturbances that has the largest effect. The natural disturbance regime is now unlikely to persist within conservation areas since fragmentation and human intervention have usually modified physical and biotic conditions. Active management decisions must now be made on what disturbance regime is required and this requires decisions on what species are to be encouraged or discouraged.
Invertebrate secondary production, or the formation of invertebrate biomass through time, has been estimated in many freshwater benthic habitats. It has been a major research theme for the North American Benthological Society (NABS), and many of its members have made significant contributions to the subject, both before and during the existence of J-NABS. Although some benthic production work occurred before 1960, the major methods were developed primarily during the 1960s and 1970s. Most of these methods also were applied in terrestrial and marine environments. The main focus of our paper is how secondary production has been used as an essential variable in facilitating answers to a wide variety of ecological questions. Benthic freshwater production studies before the inception of J-NABS were primarily related to life history, interpopulation comparisons, niche overlap/competition, predator–prey relationships, differences in production/biomass (P/B), energy flow, the trophic basis of production, habitat-specific microdistributions, effects of pollution and dams, and quantification of aquatic–terrestrial linkages. Since that time, new applications have been related to habitat-specific macrodistributions, quantitative food webs, experimental and tracer-based studies of trophic resources, chemical flows/stoichiometric relationships, diversity/function relationships, influence of nonnative species and landuse changes, implications of metabolic theory, and the importance of meiofauna vs macrofauna. J-NABS has been a major outlet for many of these applications, has probably included a higher fraction of papers incorporating secondary production analysis than any other journal, and probably will continue to be a leader in this area.
The serial discontinuity concept (SDC; Ward and Stanford, in Ecology of River Systems, 1983) predicts that recovery of large regulated rivers over distance downstream from a dam is limited by relative tributary size; however, channel geomorphology may also influence the recovery process. We examined the spatial variation in water quality, benthic composition and ash-free dry standing biomass (AFDM) among the bedrock-defined geomorphological reaches in three turbidity segments of the Colorado River between Glen Canyon Dam and Diamond Creek, Arizona, including most of the Grand Canyon. This 387-km long study area supported virtually no Ephemeroptera, Plecoptera or Trichoptera, probably because cold, stenothermic, hypolimnetic releases limited maximum aestival warming to 17·1°C. The benthos displayed abrupt, physically related decreases in AFDM over distance from the dam and in the varial zone. The 26-km long clear water segment between the dam and the Paria River supported a depauperate Cladophora glomerata/epiphyte/chironomid/Gammarus lacustris/lumbricine/Physella sp. assemblage, and ooze-dwelling oligochaetes. This segment contained 6·9% of the aquatic habitat below the 140 m³/s (normal minimum) discharge stage of the Colorado River study area, but supported 63·5% of the benthic primary producer AFDM and 87% of the benthic consumer AFDM in the entire study area. Turbidity increased and light penetration decreased immediately downstream from the confluence of the small, turbid Paria River, and further downstream from the Little Colorado River confluence. The benthos downstream from the Paria River was abruptly replaced by an Oscillatoria/Simuliium assemblage with a mean AFDM of <0·12 g C/m².
The intluence of thermal regime upon community-level growth rates and voltinism was estimated for larval Chironomidae inhabiting litter accumulations in four streams located in an Appalachian Mountain basin. Groups of larvae were confined in growth chambers and incubated in situ at time intervals representing the observed range of annual thermal variation. Estimates of daily growth rates (g) were derived from change in average length over the incubation period. Using multiple regression, I found temperature and larval size to have significant positive and negative effects on g, respectively. Equations derived for each stream described a substantial proportion of the variance among observed g values (X2 = 0.7 l-0.82) but did not differ significantly. Therefore, the data from all streams were combined to derive a single general equation which, along with larval size dis- tribution, biomass, and temperature data, was used to model the variation in annual biomass turnover(G) and hypothetical size-dependent voltinism among the study streams. Size distribution of larvae did not differ significantly among streams and variation in G (range = 12.8-18.6) was attributed primarily to variation in thermal regime. Differences in voltinism were predicted to be minor but were closely dependent on both terminal size of larvae and thermal regime. The model provides evidence that spatial variation of G on the order of 31% can be expected for midge communities within a < 30 km2 area of this Appalachian Mountain basin.
Twelve categories/traits were used to classify and rank aquatic invertebrates based on their propensity to drift and importance as a food resource for salmonids. Invertebrate availability was based on their (i) propensity to intentionally drift, (ii) likelihood of being accidentally dislodged by the current, (iii) drift distance, (iv) adult drift, (v) benthic exposure, (vi) body size, and (vii) abundance. This study represents the first attempt to characterize the intentional drift propensity of stream invertebrates. A ranking procedure separated invertebrates into Baetis and three groups decreasing in availability. Predicted ranks were significantly correlated with the actual rank of invertebrates in trout guts taken in three separate studies conducted in the central Rocky Mountains, suggesting that this procedure can effectively rank invertebrates based on their availability as a food resource for salmonids. A cluster analysis separated the 95 taxa into four drift guilds and six availability groups. This study provides criteria for determining when alterations in invertebrate community composition will affect food resources for higher trophic levels by causing a decline in the most available taxa. This research also supports previous findings that floods are important in maintaining invertebrates that represent an important food resource for salmonids.
The accepted procedure for de termining production of multivoltine invertebrates by use of the Hynes method is to multiply the Hynes value by the number of generations per year. For aquatic insects, if pupal, adult, or egg stages comprise a significant portion of total generation time, this procedure will underestimate production. For crustaceans, if reproduction occurs before attaining the final size class, the procedure, using generation time, will overestimate production. It is necessary to multiply the Hynes value by 365/ CPI , where CPI is the cohort production interval (in days) from hatching to the attainment of the largest aquatic size class.
Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic fresh-water snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.
A century of hydrologic data (1895–1999) and 50 yr of aquatic macroinvertebrate assemblage data (1947–1999) were examined for two tailwater reaches of the Green River downstream from Flaming Gorge Dam in northeastern Utah, USA (40°54′ N, 109°25′ W). One reach was located upstream of an intermittent tributary, and the other downstream. The purpose of the study was to chronicle long-term dynamics and the effect of partial thermal restoration on invertebrate assemblages. The immediate hydrologic effect of the dam was a large decrease in annual maximum daily discharges, water temperatures, and sediment transport. Upstream of the intermittent tributary, macroinvertebrate genera declined from >70 to <30, and the mean macroinvertebrate density increased from 1000 to 10 000/m2 after dam closure. In 1978, a multilevel water intake structure was installed on the dam. Mean summer water temperatures increased from 6° to 12°C, and the number of annual degree days warmed from 2340 to 3200, which was similar to pre-dam conditions; but the rate and timing of warming remained different than before the dam. In contrast to an expected increase in taxon richness, the number of taxa routinely collected after partial thermal restoration was similar to or lower than that observed before thermal restoration. Downstream of the intermittent tributary, taxon richness was about twice that found upstream after dam closure, and the mean annual per-sample taxon richness increased from 7.6 to 11.0 following partial thermal restoration. In both reaches, invertebrate densities were near 10 000/m2 before and after thermal restoration. The lack of an appreciable increase in insect taxon richness upstream from an intermittent tributary following partial thermal restoration is likely due to the combined effects of three factors: (1) the competitive dominance of insect taxa by amphipods, (2) low rates of immigration and colonization, and (3) low reproductive success of insects due to a few degrees difference in the water temperatures between the regulated river and natural streams in the area. These results suggest that we should not only evaluate traditional habitat attributes, but biological interactions as well, when determining or monitoring the effects of river regulation on aquatic biota.
We examined the effects of dam operation on the relative abundance and relative condition of rainbow trout Oncorhynchus mykiss captured by electrofishing between 1991 and 1997 in the Lee's Ferry tailwater below Glen Canyon Dam, Arizona. Higher minimum, higher mean, and more stable flow releases from the dam after 1991 provided conditions that supported greater relative abundance of rainbow trout. Though the relative abundance of most length-classes increased following the onset of a stabilized flow regime, relative condition declined, particularly in large rainbow trout (≥305 mm). Correlation analyses suggested that the small rainbow trout (
Coarse particulate organic mass varied seasonally and changed from autochthonous to allochthonous organic matter along a 386-km section of the Colorado River below Glen Canyon Dam, Arizona. Dam operations influenced drift components selectively throughout the hydrograph; however, ramping rate had no effect on drift mass. Eddies and pools collected similar to 50% of the main-stem drift with only detritus retained long enough to settle in pools. Coarse particulate organic drift mass was negatively correlated with stream gradient in the Colorado River. Cladophora drift packets were pulverized by rapids, which subsequently increased fine particulate organic matter at downriver sites. Tributary type (spring stream or large desert watershed river) significantly altered the mass and composition of tributary drift; however, tributaries contributed <0.1% of the total organic matter to main-stem drift. Terrestrial insects represented <0.001% of the total invertebrate mass in main-stem drift. As a result of hydraulics and suspended sediment, organic drift mass throughout the river corridor is reach specific. Longitudinal change in drift composition corresponds with a shift in fish species.
Closure of Glen Canyon Dam in 1963 transformed the Colorado River by reducing the magnitude and duration of spring floods, increasing the magnitude of base flows, and trapping fine sediment delivered from the upper watershed. These changes caused the channel downstream in Glen Canyon to incise, armor, and narrow. This study synthesizes over 45 yr of channel-change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations. Although bed lowering in lower Glen Canyon began when the first cofferdam was installed in 1959, most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2.6 m of sediment from the center of the channel. The average grain size of bed material has increased from 0.25 mm in 1956 to over 20 mm in 1999. The magnitude of incision at riffles decreases with distance downstream from the dam, while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream. Analysis of bed-material mobility indicates that the increase in bed-material grain size and reduction in reach-average gradient are consistent with the transformation of an adjustable-bed alluvial river to a channel with a stable bed that is rarely mobilized. Decreased magnitude of peak discharges in the post-dam regime coupled with channel incision and the associated downward shifts of stage-discharge relations have caused sandbar and terrace erosion and the transformation of previously active sandbars and gravel bars to abandoned deposits that are no longer inundated. Erosion has been concentrated in a few pre-dam terraces that eroded rapidly for brief periods and have since stabilized. The abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage-discharge relations. In the downstream part of the study area where riffles controlling channel elevation have not incised, channel narrowing has resulted from decreased magnitude of peak discharges and minor post-dam deposition. These physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non-native species. Models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit, the pre-dam surface and subsurface grain size, and the magnitude of post-dam average flows. These results suggest that it is also necessary to consider (1) the possibility of variable responses among different channel elements and (2) the potential importance of exceptional flows resulting from management decisions.
Acknowledgments,. ................ ................ ........... 129 References ................ ................ ................ .. 131 I. SUMMARY Previous studies were collated with new,data to produce,an exceptionally detailed connectance,web for Broadstone,Stream (UK) that contained,131 species, including the permanent meiofauna (i.e., species that are always passing through a mesh of 500 � m), and 842 links. Despite its apparent ADVANCES IN ECOLOGICAL RESEARCH VOL. 36,2005 Elsevier Inc.
THE idea that the connections between species in ecological assemblages
are characterized by a trait called 'interaction strength' has become a
cornerstone of modern ecology. Since the classic paper of
Watt1, ecologists have acknowledged the importance of
distinguishing pattern (static community features) from process
(dynamically based mechanisms), which can determine the immediate
observed details. Food webs display pathways of implied dynamics, and
thus potentially unite pattern and process in a single
framework2,3. Most analyses4-8 have focused
entirely on web topology and the derived descriptive properties. By
contrast, attempts to generalize how natural communities are
organized9,10 or summary statements about whole
communities6,11-13 have emphasized critical processes.
Elton's2 insights and May's3generalizations and
analyses have stimulated current developments. In May's approach, the
idea of interaction strength is precise, reflecting coefficients in a
jacobian matrix associated with a community dynamics model. He found
striking dependence of community stability both on web complexity and on
the number and strength of interactions. By contrast, empiricists have
usually determined relative interaction strength from single-species
removals analysed by multivariate statistics14,15.I report
here the first experimental study designed to estimate interaction
strengths in a species-rich herbivore guild, documenting on a per capita
basis mainly weak or positive interactions, and a few strong
interactions, a pattern which has profound implications for community
dynamics.