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Antagonistic and synergistic effects on a stream periphyton community under the influence of pulsed flow velocity increase and nutrient enrichment
... Most freshwater microphytobenthic community studies are based on field observations of diatom assemblages, while data from controlled mesocosm experimental settings are relatively scarce. The few experimental studies that have been conducted have provided valuable insights into the impact of single stressors, such as light availability, flow velocity, sediment, nutrients, temperature, and salinity, as well as their combinations, on the community structures and functional traits of benthic diatoms (Piggott et al., 2015;Bondar-Kunze et al., 2016;Nuy et al., 2018;Costello et al., 2018;Salis et al., 2019;Bondar-Kunze et al., 2021;Frost et al., 2023). However, the recovery process after the alleviation of these stressors is less studied, we could not find any prior mesocosm studies that have examined the recovery of microphytobenthos communities following the alleviation of stressors. ...
... C: control; T: increased temperature; S: increased salinity; S + Our amplicon, but not microscopy, results are consistent with previous studies, for example, Salis et al. (2019) found despite decreased flow, there was an increase in taxon richness and a decrease in algal biomass and cell density. Microalgae biofilms growing in slow flow velocity developed visible strands of diatoms and appeared to be thicker and more mature than biofilms in fast flow treatment (Battin et al., 2003a;Battin et al., 2003b;Bondar-Kunze et al., 2016). Rather than community structure, flow-related responses were associated with changes in biomass (Battin et al., 2003b). ...
... Furthermore, a weak but significant correlation was found between photosynthetic biomass (F 0 ) and reduced flow velocity. One quantitative effect of flow velocity was a drop in photosynthetic biomass (Bondar-Kunze et al., 2016). ...
OPEN ACCESS PAPER - Field observations form the basis of the majority of studies on microphytobenthic algal communities in freshwater ecosystems. Controlled mesocosm experiments data are comparatively uncommon. The few experimental mesocosm studies that have been conducted provide valuable insights into how multiple stressors affect the community structures and photosynthesis-related traits of benthic microalgae. The recovery process after the stressors have subsided, however, has received less attention in mesocosm studies. To close this gap, here we present the results of a riparian mesocosm experiment designed to investigate the effects of reduced flow velocity, increased salinity and increased temperature on microphytobenthic communities. We used a full factorial design with a semi-randomised distribution of treatments consisting of two levels of each stressor (2 × 2 × 2 treatments), with eight replicates making a total of 64 circular mesocosms, allowing a nuanced examination of their individual and combined influences. We aimed to elucidate the responses of microalgae communities seeded from stream water to the applied environmental stressors. Our results showed significant effects of reduced flow velocity and increased temperature on microphytobenthic communities. Recovery after stressor treatment led to a convergence in community composition, with priority effects (hypothesized to reflect competition for substrate between resident and newly arriving immigrant taxa) slowing down community shifts and biomass increase. Our study contributes to the growing body of literature on the ecological dynamics of microphytobenthos and emphasises the importance of rigorous experiments to validate hypotheses. These results encourage further investigation into the nuanced interactions between microphytobenthos and their environment and shed light on the complexity of ecological responses in benthic systems.
... In periphytons, the autotrophic microbial assemblages play a major role in primary productivity and autochthonous carbon production (Calapez et al., 2020). Moreover, heterotrophic communities can effectively biodegrade and remove organic pollutants, promoting the metabolism, mineralisation, and circulation of essential nutrients in aquatic ecosystems (Bondar-Kunze et al., 2016;Mai et al., 2020). Recently, to better utilise the periphytic biofilms' capacity of biological purification and degradation, various kinds of artificial substrates have been applied to immobilise periphytons, substituting natural substrates (Witt et al., 2011;An et al., 2018). ...
... Furthermore, the flow velocity in the flume was kept at 0.1-0.15 m/s, which is approximately the flow velocity in urban rivers (Bondar-Kunze et al., 2016). The substrates mentioned above were used as the biofilm supports for microbial colonisation. ...
Periphytic biofilms have been widely used in wastewater purification and water ecological restoration, and artificial substrates have been progressively used for periphyton immobilisation to substitute natural substrates. However, there is insufficient knowledge regarding the interaction network structure and microbial functions in biofilm communities on artificial substrates, which are essential attribute affecting their applications in biofilm immobilisation. This study compared the community structure, co-occurrence network and metabolic functions of bacterial and microeukaryotic periphytic biofilms during a 35-day indoor cultivation on artificial substrates, such as artificial carbon fibre (ACF) and polyvinyl chloride (PVC), and natural substrates, such as pebble and wood. Results demonstrated that different types of artificial substrates could affect the community composition and functional diversity of bacterial and microeukaryotic biofilms. The bacterial and microeukaryotic community on ACF and PVC showed significantly higher Simpson index compared to those on wood. Bacterial networks on artificial substrates were more complex than those on natural substrates, while the keystone species on natural substrates were more abundant, indicating that the bacterial communities on artificial substrates had stronger stability and resistance to external interference. Furthermore, the functional metabolic profiles predicted showed the abilities of bacterial communities to metabolise nitrogen and carbon sources colonised on artificial substrates were stronger than those on natural substrates. These findings demonstrated that artificial substrates could be special niches for microbial colonisation, possibly altering microbial compositions, interactions and functions. Therefore, this study provides a powerful theoretical basis for choosing suitable artificial substrates for microbial aggregation and immobilisation technology.
... Various negative effects on riverine life caused by artificial altering of river flow have been described (Schmutz et al., 2015;Shen & Diplas, 2010). Hydropeaking can negatively affect biomass development of the stream periphyton (Bondar-Kunze, Maier, Schönauer, Bahl, & Hein, 2016) as well as macrophyte (Bejarano et al., 2018) and insect diversity . Moreover, hydropeaking also causes artificial drift and a potentially critical decrease in the biomass of river benthic invertebrates (Bruno, Siviglia, Carolli, & Maiolini, 2013). ...
... The biofilm results indicated that the biotic conditions were important to consider and could influence egg detachment rates. Periphyton production increase with eutrophication (Mattila & Räisänen, 1998) and hydropeaking supports fast growing species (Bondar-Kunze et al., 2016). Dissolved organic matter in dammed river seams to also be increasing with global warming (Hejzlar, Dubrovský, Buchtele, & Rů žička, 2003). ...
Human‐induced changes in the hydrological regimes of lotic waters such as hydropeaking have significant negative impacts on riverine life. However, the impacts of dynamic changes in water flow on adhesive fish eggs are not very well known. We focused on the effects of hydropeaking on the spawning ground of a rheophilic cyprinid fish, the asp (Leuciscus aspius). We tested whether a sudden increase in water velocity caused by hydropeaking may have negative effect on the adhesive eggs by the combination of field observations and laboratory experiments. The main objectives of the study were to: i) investigate abiotic characteristics of an asp spawning ground, ii) monitor egg densities in relation to hydropeaking events and iii) test detachment rates of the asp eggs in laboratory conditions in relation to water velocity. The asp spawning ground was associated with shallow water depths (0.2–0.4 m) and flowing water (0.1–0.4 m.s‐1) during base flow. The water velocity that occurred on the spawning ground during the hydropeaking event was measured to be from 0.7–1.2 m.s‐1. Asp eggs nearly disappeared from the spawning ground before their hatching time probably due to several hydropeaking events. The laboratory experiments showed the significant dependency of egg detachment rates on the water velocity and substrate type with a critical value of 0.7 m.s‐1. Our data suggested that eggs may be negatively impacted by flow alterations. Avoiding hydropeaking or keeping water velocity below critical values is recommended for the management of rheophilous fish spawning grounds.
... For instance, primary production and algal biodiversity as well as biofilm internal mass transport were reported to decrease with increasing flow velocity where at the same time mass transport from the water column toward biofilms increase (Beyenal and Lewandowski, 2002;Larned et al., 2004;Soininen, 2004). The algal composition of biofilms can adapt to the present flow conditions (Graba et al., 2013;Bondar-Kunze et al., 2016). With increasing flow velocities, the diversity of the diatom community may decrease because only specialized species can withstand (Soininen, 2004). ...
... Indeed, binding mechanisms for herbicides have been described for DOC in soils, for long (Baskaran and Kennedy, 1999;Ling et al., 2006). Furthermore, the absorption of herbicides to humic acids, which is one fraction of the DOC pool in soils and aquatic systems, is well described in literature (Bollag and Myers, 1992;Senesi, 1992). Out of all binding mechanisms, e.g., hydrophobic adsorption or covalent binding, adsorption is considered to be the most important interaction (Senesi, 1992). ...
Multiple stressors pose potential risk to aquatic ecosystems and are the main reasons for failing ecological quality standards. However, mechanisms how multiple stressors act on aquatic community structure and functioning are poorly understood. This is especially true for two important stressors types, hydrodynamic alterations and toxicants. Here we perform a mesocosm experiment in hydraulic flumes connected as a bypass to a natural stream to test the interactive effects of both factors on natural (inoculated from streams water) biofilms. Biofilms, i.e., the community of autotrophic and heterotrophic microorganisms and their extracellular polymeric substances (EPS) in association with substratum, are key players in stream functioning. We hypothesized (i) that the tolerance of biofilms toward toxicants (the herbicide Prometryn) decreases with increasing hydraulic stress. As EPS is known as an absorber of chemicals, we hypothesize (ii) that the EPS to cell ratio correlates with both hydraulic stress and herbicide tolerance. Tolerance values were derived from concentration-response assays. Both, the herbicide tolerance and the biovolume of the EPS significantly correlated with the turbulent kinetic energy (TKE), while the diversity of diatoms (the dominant group within the stream biofilms) increased with flow velocity. This indicates that the positive effect of TKE on community tolerance was mediated by turbulence-induced changes in the EPS biovolume. This conclusion was supported by a second experiment, showing decreasing effects of the herbicide to a diatom biofilm (Nitzschia palea) with increasing content of artificial EPS. We conclude that increasing hydrodynamic forces in streams result in an increasing tolerance of microbial communities toward chemical pollution by changes in EPS-mediated bioavailability of toxicants.
... Hein, 2016;Greimel et al., 2018;Bruno, Siviglia, Carolli, & Maiolini, 2013;Hauer, Unfer, Holzapfel, Haimann, & Habersack, 2014;Schülting, Feld, & Graf, 2016;Young, Cech, & Thompson, 2011). Regarding fish, it has been well documented that hydropeaking entails stranding and downstream displacement of individual fish (Nagrodski, Raby, Hasler, Taylor, & Cooke, 2012;Young et al., 2011;Hunter, 1992;Saltveit et al., 2001;Halleraker et al., 2003;Auer, Zeiringer, Führer, Tonolla, & Schmutz, 2017;Führer et al., 2022), which, in turn, diminishes population vitality (Hayes et al., 2021;Schmutz et al., 2015).Linked to artificial flow alteration, hydropeaking possibly also causes unnatural short-term water temperature fluctuations, referred to as "thermopeaking" Ward & Stanford, 1979;Zolezzi, Siviglia, Toffolon, & Maiolini, 2011). ...
Intermittent water releases from hydropower plants, called hydropeaking, negatively affect river biota. The impacts mainly depend on hydrological alterations, but changes in physical habitat conditions are suspected to be co‐responsible. For example, hydropeaking accompanied by a sudden change of water temperature in the downstream river—called thermopeaking—is also presumed to impair aquatic ecosystems. Still, knowledge about these thermopeaking impacts on aquatic species and life‐stages is limited. We performed flume experiments under semi‐natural conditions to fill this knowledge gap, simulating single hydropeaking events with a change in water temperature. As response parameters, we quantified the drift and stranding of early life‐stages of European grayling (Thymallus thymallus L.), a key fish species of Alpine hydropeaking rivers. Hydropeaking events with a decrease in water temperature (“cold thermopeaking”) led to significantly higher downstream drift (mean = 51%) than events with increasing water temperature (“warm thermopeaking”, mean = 27%). Moreover, during cold thermopeaking, a comparably high fish drift was recorded up to 45 min after the start of peak flows. In contrast, drift rates quickly decreased after 15 min during warm thermopeaking. Remarkably, the spatial distribution of downstream drift along gravel bars during cold thermopeaking showed the opposite pattern compared to those triggered by warm thermopeaking events indicating different behavioral responses. Furthermore, the stranding rates of the cold thermopeaking trials were twice as high (mean = 31%) as those of the warm thermopeaking experiments (mean = 14%). The outcomes present vital information for improving mitigation measures and adapting environmental guidelines.
... Hydropeaking consists of sudden, artificial water releases by storage hydropower plants into rivers to address peaks of energy demand, thus affecting the subdaily flow regime through rapidly increasing and decreasing flow spates. A growing scientific and public awareness of the adverse effects of hydropeaking on stream ecology has developed in the last decades (e.g., Auer, Zeiringer, Fuhrer, Tonolla, & Schmutz, 2017;Bejarano, Jansson, & Nilsson, 2017;Bejarano, Sordo-Ward, Alonso, Jansson, & Nilsson, 2020;Bondar-Kunze, Maier, Schönauer, Bahl, & Hein, 2016;Boavida, Harby, Clarke, & Heggenes, 2017;Casas-Mulet, Alfredsen, & Killingtveit, 2014;Schülting, Feld, & Graf, 2016), leading to increasing attention to the study, design, and implementation of mitigation strategies, based on operational and structural measures (Barillier, Beche, Malavoi, & Gouraud, 2021;Bruder et al., 2016;Greimel et al., 2018;Hauer, Siviglia, & Zolezzi, 2017;Moreira et al., 2019). Operational measures rely on changes in the energy production schemes resulting, however, in a reduced flexibility of operations for the hydropower companies (Gostner et al., 2011), which may hamper their ability to profitably adapt to rapidly variable energy prices and peak requests from the grid. ...
We assessed the effect of a hydropeaking diversion mitigation measure that allows for additional hydropower production, which markedly reduced hydropeaking on a 10‐km stream reach in the north‐eastern Italian Alps. Hydropeaking, caused by a storage hydropower plant, affected the study reach from the 1920s to 2015, when a cascade of three small run‐of‐the‐river plants was installed to divert the hydropeaks from the plant outlet directly into the intake of the RoRs plants, and hydropeaking was released downstream the confluence with a major free‐flowing tributary. The flow regime in the mitigated reach shifted from a hydropeaking‐dominated to a baseflow‐dominated regime in winter, with flow variability represented only by snowmelt and rainfall in late spring and summer. The application of two recently proposed sets of hydropeaking indicators, the hydraulic analysis of the hydropeaking wave, together with the assessment of biotic changes, allowed quantifying the changes in ecohydraulic processes associated with hydropeaking mitigation. The flow regime in the mitigated reach changed to a residual flow type, with much less frequent residual hydropeaks; although an average two‐fold increase in downramping rates were recorded downstream the junction with the tributary, these changes did not represent an ecological concern. The functional composition of the macrobenthic communities shifted slightly in response to flow mitigation, but the taxonomic composition did not recover to conditions typical of more natural flow regimes. This was likely due to the reduced dilution of pollutants and resulting slight worsening in water quality. Conversely, the hyporheic communities showed an increase in diversity and abundance of interstitial taxa, especially in the sites most affected by hydropeaking. This effect was likely due to changes in the interstitial space availability, brought by a reduction of fine sediments clogging. Besides illustrating a feasible hydropeaking mitigation option for Alpine streams, our work suggests the importance of monitoring both benthic and hyporheic communities, together with the flow and sediment supply regimes, and physico‐chemical water quality parameters.
... Understanding the effects of temporary abrupt flow pulses, and their interaction with light and nutrient availability on the periphyton community is needed to allow predictions of effects on ecosystem functioning under future scenarios of riparian cover loss, accelerated eutrophication and increased discharge variability that can serve to inform stream management under climate change. To date, several studies have investigated the effects of light, nutrients and flow pulses as single factors or combining two of the parameters as stressors (Adámek et al., 2016;Baekkelie et al., 2017;Bondar-Kunze et al., 2016;Goldenberg-Vilar et al., 2021;Wu et al., 2019), but less is known about the simultaneous effects of the three factors. ...
Streams generally are affected by multiple stressors acting at different timescales. Periphyton, often the most important primary producer in these ecosystems, may respond to short‐term impacts as well as to different long‐term environmental conditions with potentially various changes in community structure.
Here, we experimentally investigated the effects of sudden flow pulses on periphyton communities as a way to mimic extreme precipitation events in lowland streams that are predicted to occur more often with climate change in some regions. Using outdoor flumes, we allowed periphyton to colonise nutrient‐diffusion substrates under two light conditions (50% shade and fully open) and nutrient availabilities (control, with access only to stream nutrients, and N–P‐enriched) along a gradient in baseflows (0.43 to 2.17 L/s). After one month, we exposed the communities to a flow pulse (two‐fold peak flow increase to simulate conditions of a potentially high disturbance) and analysed the responses of biomass and taxonomic composition.
Flow pulse promoted periphyton growth in the lowest range of the baseflow but led to biomass reduction in the highest range. Light was the second major driver of biomass accrual, whereas nutrient enrichment had a strong effect on community composition both before and after the pulse (i.e., diatom dominance vs. green algae dominance in scenarios without and with enrichment, respectively). In all treatments, the flow pulse promoted a higher taxonomic richness, suggesting a partial reset of the succession of the periphyton communities. However, independent of flow and resources, periphyton communities showed low ecological resistance against the pulse with changes in chlorophyll a , biovolume and taxonomic richness to the pulse.
We demonstrated that the effects of pulses on periphyton are similar in terms of biomass but varied strongly regarding composition depending on their initial structure, which is in turn mediated by the baseflow normally experienced by the systems, and on light and nutrient availability.
Our results highlight the importance of testing multiple stressors, such as an increase in extreme events, under a wide range of environmental conditions (i.e., flow regime, trophic state, light availability). This approach allows us to detect potential interaction effects and non‐linear responses, and highlights that the environmental settings ultimately determine the net effects of flow pulses on community structure and probably also on several important ecosystem processes.
... Example: Hydropeaking (peak flow events from hydropower plant operation) dampened/mitigated the effects of nutrient enrichment on benthic algae biomass in an oligotrophic alpine river experiment (Bondar-Kunze et al., 2016). ...
The article serves to introduce the environmental issue of ever-increasing multiple stressors acting on streams and rivers globally. It presents the most relevant stressors and outlines the state of knowledge in multi-stressor research. Stressor interactions are well documented, but full understanding of the cause-effect relationships remains scarce. This poses high uncertainties to predicting and effectively managing multi-stressor effects. Both a refined analytical approach and integrative, adaptive environmental management offer solutions to this intricate problem.
... By contrast, mitigation of abrupt changes in flow velocity due to e.g. daily hydro-peaking can worsen the effects of excessive nutrients, which are otherwise flushed downstream (Bondar-Kunze et al., 2016). A recent review suggests that non-additive responses predominate across aquatic ecosystems (Villar-Argaiz et al., 2018); however, these results mainly come from experimental studies in marine systems and from lakes. ...
Numerous anthropogenic stressors, including river regulation, excess loadings of nutrients and sediment, channelisation, as well as thermal and hydrological stressors driven by climate change impact riverine ecosystems worldwide. In a time when freshwater degradation and the rate of global warming are faster than ever, understanding the potential interactive effects of local and catchment‐scale stressors with large‐scale climatic conditions is essential to enhance our ability to plan effective conservation, restoration, and mitigation measures.
In this study we analysed a dataset spanning the whole of Sweden using a space‐for‐time approach to investigate interactive effects of land use, river regulation, and climate on brown trout ( Salmo trutta ) abundance in streams.
We found that in warmer regions trout populations were negatively affected in catchments with more intense river regulation by hydropower dams (i.e. ≥10 m ³ /km ² total reservoir storage volume). In such catchments, a 7°C warmer mean summer air temperature was associated with an average between 44% and 83% decline in trout abundance. In catchments with less intense river regulation, trout abundance instead increased moderately with increasing temperature.
We also found that brown trout abundance declined with increasing areal extent of urban areas when found in combination with ≥20% agricultural land use. When agricultural land use reached maximum values (84%), brown trout abundance decreased from an average of 13 individuals per 100 m ² in catchments with no urban areas to values ≤1 in catchments with ≥5% urban land use. Also, brown trout abundance declined with increasing agricultural land use in catchments with ≥3% urban land use.
Our study brings innovative empirical evidence of interactive effects between river regulation, land use and climate on brown trout populations. From a management perspective our findings suggest that: (1) restoring natural flows (e.g. through dam removal) and riparian vegetation could mitigate adverse effects of climate change; and (2) restoration measures that minimise the effects of agriculture and urban land use (e.g. reduction of nutrient levels and restored riparian buffer zones) could help rehabilitate brown trout in catchments with high anthropogenic land use change. However, given the large observed variation between streams, we advise for bespoke management actions stemming from sound knowledge of local habitat conditions and target populations, whenever possible, using an ecosystem management‐based approach.
... Hydropeaking effects on vegetation include periphyton biomass reduction [12] and reduced establishment and growth of riparian vegetation, with the exception of few floodtolerant species [13]. ...
As the global demand for renewable electricity grows, hydropower development of river basins increases across the world. Hydropeaking, i.e., streamflow alteration consisting of daily or subdaily rapid and marked discharge fluctuations, can affect river reaches below hydropower units. Environmental effects of hydropeaking include geomorphological alterations and possible modifications of the freshwater biota. Among affected instream communities, benthic macroinvertebrates are receiving increasing attention and the related scientific research has experienced significant progress in the last decade. In this context, this paper aims to summarize state-of-the-art methods for the assessment of hydropeaking impacts on benthic macroinvertebrate communities. The present review could support the proper design of monitoring plans aimed at assessing the ecological impacts of hydropeaking and the effects of possible mitigation strategies.
... If phytoplankton, its existence cannot describe the conditions in which it is located, because the organic matter / materials it needs and its utilization does not describe the conditions in which it is located [12], [13]. ...
Environmental damage due to natural resource extraction, especially in watershed areas, seems to be of increasing concern and so far, from the aspect of aquatic and water resources, plankton is commonly used as an indicator of environmental damage. This study explores the distribution of microphyta as a parameter of environmental damage. The research was conducted in Pager watershed, Central Kalimantan and sampled at 2 (two) stations, namely station A (for the right side of the river) and station B (for the left side of the river). The study was conducted 13 sampling times, which began in the period 25 May 2019 ending until 9 November 2019. Laboratory analysis to identify the type and number of microphyta was carried out at the Palangka Raya University Laboratory. The results showed that the number of microphyta taxa at station A (right side of the river) was 12-13 species, more than station B (left side of the river). The number of microphyta taxa at Station B is 8 - 9 species, it is suspected that there is an influence from the gray water settlements around the left side, especially when the water level drops. The distribution characteristics of microphyta in the Pager river are as follows: large number of taxa/species, low abundance and low diversity index. River/peat water environments are vulnerable to change, especially human interference. This research shows the potential use of microphyta as an indicator of environmental damage.
... Diatoms were most abundant at the beginning of our experiment, but P enrichment shifted the composition of algal communities towards a chlorophyte dominated community. Chlorophyte accrual is expected in P-rich environments with ample light and minimal disturbance (Biggs et al., 1998a) and algal communities amended with P in other mesocosm experiments have shown similar shifts in chlorophyte relative abundance (Baekkelie et al., 2017;Bondar-Kunze et al., 2016). Moreover, filamentous chlorophyte abundance is often associated with the increased trophic status of streams (Ch etelat et al., 1999;Stevenson et al., 2012). ...
Phosphorus (P) is an essential macronutrient for algal communities, but in excess can exacerbate stream eutrophication. However, P loadings to streams vary temporally from continuous to episodic as a result of inputs from point and non-point sources, respectively. P loading pattern can thus alter the temporal availability of P and may influence effects of P enrichment on algal communities. We assessed how P loading pattern influences algal biomass and composition by conducting a 29-day P enrichment experiment in nine artificial streams exposed to either: (1) continuous P enrichment; (2) episodic P enrichment, or; (3) no P enrichment. P enrichment increased algal biomass accrual, but peak biomass did not differ between continuously and episodically enriched treatments. Maximum absolute growth rates were also comparable between P enriched treatments. However, episodic P additions sustained elevated rates of biomass accrual, whereas absolute growth rates in the continuously enriched communities declined towards the end of the experiment. P enrichment resulted in comparable increases in relative abundance of chlorophytes and decreased proportions of bacillariophytes and charophytes in algal communities for continuously and episodically enriched treatments. However, composition of bacillariophyte (diatom) assemblages differed significantly among all P enrichment treatments in accordance with species autecological attributes for P. Our results demonstrate that episodic and continuous P enrichment may augment algal biomass similarly. Yet, P loading pattern regulated the composition of algal communities. Thus, remedial management strategies for the control of nuisance algae production may require focus on the predominant source of P to streams. Finally, species specific responses of diatom assemblages to P enrichment and associated loading patterns suggests this taxonomic group may have potential as diagnostic indicators for identifying the presence of key nutrient sources associated with eutrophication of stream ecosystems.
... Progress in Oceanography 183 (2020) 102309 4.2. Regional connection and separation of the MEC Ocean current plays an important role in shaping the community composition of microbial eukaryotes because dispersal of most plankton is a passive process, especially when the current velocity is high (Bondar-Kunze et al., 2016;Nemergut et al., 2013). As a result, local MEC exhibited temporal dynamics in response to the occurrence and disappearance of currents (Hwang and Wong, 2005;Liu et al., 2015b;Xu et al., 2017). ...
One interesting question in the study of marine microbial ecology is how ocean currents affect the geographic distributions of microorganisms. The Zhe-Min Coastal Current (ZMCC) is strong in winter and could influence the plankton communities in Hong Kong coastal waters by bringing species from far north. In this study, we investigated the spatial distribution of microbial eukaryotic communities (MEC) in the ZMCC and evaluated the relative contribution of environmental and spatial factors on the variations of MEC. The results showed that diverse taxa of diatoms (Bacillariophyceae) and Dinophyceae (Dinoflagellata) dominated the sequences in all samples. MEC compositions in the Hong Kong coastal waters were closer to the communities in the ZMCC than other adjacent areas. Abundant and rare taxa exhibited a similar geographic pattern of distribution. Environmental factors alone explained 14.1% and 27.6% of the variations in the entire and abundant communities, respectively, which were higher than pure spatial factors (8.3% and 19.5%, respectively). The lower importance of spatial factors (dispersal limitation) might be explained by the passive dispersal favored by current transportation and was further supported by the high immigration rate computed in the neutral community model (NCM). High fitness to the NCM (R² = 0.74) indicated the predominant role of neutral process in shaping the MEC. The large proportion of unexplained variations in rare taxa suggested the importance of other potential factors, especially biotic interactions. Our study demonstrates the influences of coastal current on local and regional structures of MEC.
... temperature) (Ferreira and Chauvet, 2011). As for algae, nutrient enrichment and flow variations combined effect had been previously found to be dependent on specific algae groups (Bondar-Kunze et al., 2016). Nevertheless, all assemblages responded significantly to sewage contamination and grazing activity and their interacting effect. ...
Freshwaters are constantly facing ecosystem functioning alterations and loss of biodiversity driven by multiple
anthropogenic and natural stressors, that by acting simultaneously create complex interactions, affecting the
quantity and quality of water resources.
Stream biofilms are complex communities, which are exposed to these alterations and, in addition, are
naturally stressed by invertebrate grazing. Therefore, they are expected to reflect these impacts through shifts in
community structure, composition and function. Here we used a mesocosm system to assess the single and
interacting effect of major anthropogenic stressors acting in Mediterranean streams (i.e. flow stagnation and
sewage contamination) in combination with a biological natural stressor (i.e. grazing) on the main assemblages
composing biofilm (i.e. bacteria, fungi and algae) by assessing communities’ relative abundance through
Denaturing Gradient Gel Electrophoresis (DGGE) operational taxonomic units (OTUs). Biofilm was submitted to
the three stressors, in a full-factorial design (2 flow conditions × 2 contamination conditions × 2 grazing
settings) in a 5-week experiment. Molecular data showed that the combined effect of anthropogenic stressors
(flow stagnation and sewage contamination) induced unequal OTUs responses on biofilm assemblages, with
antagonistic effects for bacteria, synergistic for fungi and additive for algae. Sewage and grazing interaction were
significant for all groups revealing a negative effect (antagonistic) on bacteria and algae diversity but positive on
fungi diversity (synergistic). The same overall response pattern was also found for the triple co-occurring
stressors, which increased fungi diversity while decreasing algae and bacteria.
In stream ecosystems in which low flow conditions and sewage contamination prevail in the presence of
natural herbivory, algae and bacterial diversity may be severely affected, while fungal diversity may be surprisingly
enhanced. Consequently, shifts in the relative proportions could led to the unbalancing of ecosystem
processes (e.g., photosynthesis, nutrient mineralization) defined by the microbial communities.
... For example, biofilms in Mediterraneanclimate rivers are thicker and metabolically more active downstream of flow-stabilizing dams (71). When flow variability increases below hydropower dams, fluctuations can reduce biofilm development and its quality as a food resource (86). Using experimental flumes subjected to hydropeaking, shifts have been found in periphyton fatty acid content, from important highly unsaturated compounds to nonessential saturated ones (87). ...
River restoration guided by research
Human activities have altered the flow regimes of many of Earth's rivers, with negative impacts on biodiversity, water quality, and ecological processes. In a Review, Palmer and Ruhi explain how restoration designs now attempt to mimic ecologically important aspects of natural flow regimes, guided by insights into how variations in flow affect biota and ecosystem processes. To be successful, such efforts must go beyond accounting for flood pulses to restore natural flow variability and achieve hydrological connectivity between a river and its surroundings.
Science , this issue p. eaaw2087
... Multiple-stressor effects of altered flow conditions have also been studied. Thus, Bondar-Kunze et al. (2016) found that streamflow dynamics (daily hydropeaking) and nutrient enrichment had interactive effects on algal biomass and community composition, whereas flow velocity reduction and fine sediment had interactive effects on stream biofilm communities (including algae) in another experiment (Nuy et al., 2018). In contrast, interactive effects of DCD with other stressors in streams have only been studied for fish and invertebrates , bacteria and leaf-associated fungi (Bruder et al., 2016). ...
Agricultural practices often result in multiple stressors affecting stream ecosystems, and interacting stressors complicate environmental assessment and management of impacted streams. The nitrification inhibitor dicyandiamide (DCD) is used for nitrogen management on farmland. Effects of leached DCD on stream ecosys- tems are still largely unstudied, even though it could be relevant as a stressor on its own or in combination with other agricultural stressors. We conducted two experiments in 128 outdoor stream-fed mesocosms to as- sess stressor effects on biomass, cell density, taxon richness, evenness and functional trait composition of benthic algal communities. First, we examined responses to a wide DCD gradient (eight concentrations, 0–31 mg L−1) and two additional stressors, deposited fine sediment (none, high) and nutrient enrichment (ambient, enriched). Second, we determined algal responses to four stressors: DCD, sediment, nutrients, and reduced flow velocity. Here DCD treatments included controls, constant application (1.4 mg L−1) and two pulsed treatments mimicking concentration patterns in real streams (peaks 3.5 mg L−1, 2.2 mg L−1). Sediment and nutrient enrichment were influential stressors in both experiments, with fine sediment having the most pervasive effects. In Experiment 2, reduced flow velocity had pervasive effects and stressor interactions were mainly restricted to two-way interac- tions. DCD had few, weak stressor main effects, especially at field-realistic concentrations (Experiment 2). At the highest concentrations in Experiment 1 (above levels observed in real streams), DCD effects were still rare but some significant stressor interactions occurred. Analyses of functional traits were helpful in identifying potential mechanisms driving changes in densities and community composition. These findings suggest that, while DCD on its own may be a minor stressor, it could have adverse effects on algal communities already exposed to other stressors, a scenario common in agricultural streams.
... Therefore, these anthropogenic induced rapid flow fluctuations may cause different ecological impacts, including periphyton biomass reduction (Bondar-Kunze et al., 2016), drift of macroinvertebrates ( Schülting et al., 2016), and physical as well as physiological constraints for riparian vegetation ( Bejarano et al., 2017a). Regarding fish biota, hydropeaking can reduce and alter spawning and rearing success (Becker et al., 1982;Casas-Mulet et al., 2014;McMichael et al., 2005), lead to downstream displacement and stranding (Auer et al., 2017;Boavida et al., 2017;Nagrodski et al., 2012), cause metabolic changes ( Costa et al., 2018;Flodmark et al., 2002;Taylor et al., 2012) and influ- ence fish growth ( Kelly et al., 2017;Korman and Campana, 2009;Puffer et al., 2017). ...
Hydroelectric power plants managed in response to sub-daily changes of the electricity market undergo rapid variations of turbine discharge, entailing quickly fluctuating water levels downstream. This operation regime, called hydropeaking, causes numerous adverse impacts on river ecosystems. The hydrological alterations which affect hydropeaking rivers can be described by five parameters that change over space and time (magnitude, rate of change, frequency, duration, and timing), where each parameter may be correlated with distinct environmental impacts and therefore may be used to define flow thresholds and set targets for operational mitigation strategies. Thus, this study aims to present an extensive review on the so far established hydropeaking targets and thresholds regarding the outputs from the scientific community as well as from national regulations. We found that only few European countries (Switzerland and Austria) have legal regulations regarding hydropeaking flow thresholds. Other countries, such as Canada and the USA, present environmental legislation that can force hydropeaking mitigation measures. Most mitigation thresholds and management recommendations in literature deal with the effect of downramping on the stranding of salmonids, as well as with minimum flows between peak-flows to avoid spawning ground desiccation. Regarding other fish species and parameters, information on mitigation targets or thresholds is scarcer or non-existent, as well as on hydropeaking mitigation case-studies, resulting in a lack of knowledge and guidelines for its implementation or regulation. Nevertheless, the available literature indicates that multiple aspects must be considered when assessing such values. Thus, to aid in that process, we propose that mitigation targets and thresholds must be based on key species, including particular features regarding season, life-stage and time of day, which must be combined with site-specific morphological characteristics. The presented approach may benefit impacted organism groups in hydropeaking reaches through the establishment of ecologically-based relevant mitigation thresholds and/or targets.
... Drift as a short-term effect of hydropeaking may reduce fitness and increase mortality of 26 benthic populations ( Gibbins et al., 2007b). On a long term other parameters like reduced food 27 availability and quality (Biggs et al., 1998;Bondar-Kunze et al., 2016;Cashman et al., 2017) or 28 reduced availability of sites for oviposition and emergence of merolimnic organisms may play 29 a major role for reported reductions of macroinvertebrate abundances ( Hoffmann and Resh, 30 2003;Kokavec et al., 2017) 31 ...
Intermittent storage hydropower production is the only renewable source offering both, a sufficient storage potential and a high temporal flexibility in production to ensure grid stability. However, hydropeaking (generation of peak discharges by hydro‐electric operation) causes fluctuations in the wetted width, water depth, flow velocity and bottom shear stress downstream of a hydro‐electric facility. River biota are known to be affected by these changes, which leads for example to increased drift. Correspondingly, abundance and biomass of macroinvertebrates are frequently found to be reduced in impacted river stretches. While there is sufficient evidence for increasing drift proportions due to hydropeaking, only few studies have highlighted the role of the rapidity of flow increase before and flow decrease after a peak event (ramping velocity). Here, we present the outcome of experimental hydropeaking in artificial flumes, mimicking two different ramping velocity treatments (T1: 0.5 cm/min, T2: 1 cm/min water table change) in early and late spring (June’14, March ‘15). Macroinvertebrate drift was significantly higher in treatments as compared to control flumes. Drift proportions peaked during the up‐ramping phase and were slightly lower during the peak discharge phase. Drift proportions of T2 treatments were significantly higher than those of T1 in June, but not in March. Our findings suggest that hydropeaking requires thoughtful ramping management, to allow macroinvertebrates to seek for refugia on the stream bottom during the peak event. However, the evidence of daytime‐dependent and seasonal drift patterns also plea for adaptive management, to account for the temporal variability of macroinvertebrate drift.
... If hydropeaking is reduced, the other effects become more important, meaning that also nutrient reduction measures will be needed. This interaction effect ranges from 18 % to 62 % and is based on two different MARS studies: two MARS river flume experiments in Norway (Baekkelie et al., 2017) and Austria (Bondar-Kunze et al., 2016). ...
Water resources globally are affected by a complex mixture of stressors resulting from a range of drivers, including urban and agricultural land use, hydropower generation and climate change. Understanding how stressors interfere and impact upon ecological status and ecosystem services is essential for developing effective River Basin Management Plans and shaping future environmental policy. The EU-funded project MARS (Managing Aquatic ecosystems and water Resources under multiple Stress) addressed the nature of these problems for Europe's water resources and the need to find solutions at a range of spatial scales.
MARS was operating at three scales: At the water body scale, the mechanistic understanding of stressor interactions and their impact upon water resources, ecological status and ecosystem services were examined through multi-factorial experiments and the analysis of long time-series. At the river basin scale, modelling and empirical approaches was adopted to characterise relationships between multiple stressors and ecological responses, functions, services and water resources. The effects of future land use and mitigation scenarios in 16 European river basins have been assessed. At the European scale, large-scale spatial analysis were carried out to identify the relationships amongst stress intensity, ecological status and service provision, with a special focus on large transboundary rivers, lakes and fish. The project offered support to managers and policy makers in the practical implementation of the Water Framework Directive (WFD), of related legislation and of the Blueprint to Safeguard Europe's Water Resources by advising the 3rd River Basin Management Planning cycle, the revision of the WFD and by developing new tools for diagnosing and predicting multiple stressors.
The final report at hand overviews the project’s objectives and provides a concise summary of the main scientific results obtained in MARS. It furthermore outlines the potential impact and the main dissemination activities. Given that the project has published about 230 scientific publications, the results presented in this report are necessarily selective and aim at a comprehensive overview of the MARS outcome illustrated by a few examples, which are described in more detail.
... For instance, benthic algae are highly impacted by flow velocities above 10-15 cm/s, because taxonomic composition and nutrient cycling may change (Biggs et al. 1998;Hondzo and Wang 2002). Bondar-Kunze et al. (2016) found in an experimental study that, in an oligotrophic stream ecosystem, daily hydropeaking significantly retarded the development of periphyton biomass with no interference in the relative abundance of the three main algal groups (diatoms, chlorophyta, cyanobacteria) or the photosynthetic activity. The lower biomass could be related to cell abrasion due to a fivefold increase in flow velocity compared to base flow conditions (Biggs and Thomsen 1995). ...
This open access book surveys the frontier of scientific river research and provides examples to guide management towards a sustainable future of riverine ecosystems. Principal structures and functions of the biogeosphere of rivers are explained; key threats are identified, and effective solutions for restoration and mitigation are provided.
Rivers are among the most threatened ecosystems of the world. They increasingly suffer from pollution, water abstraction, river channelisation and damming. Fundamental knowledge of ecosystem structure and function is necessary to understand how human acitivities interfere with natural processes and which interventions are feasible to rectify this.
Modern water legislation strives for sustainable water resource management and protection of important habitats and species. However, decision makers would benefit from more profound understanding of ecosystem degradation processes and of innovative methodologies and tools for efficient mitigation and restoration. The book provides best-practice examples of sustainable river management from on-site studies, European-wide analyses and case studies from other parts of the world. This book will be of interest to researchers in the field of aquatic ecology, river system functioning, conservation and restoration, to postgraduate students, to institutions involved in water management, and to water related industries.
... For instance, benthic algae are highly impacted by flow velocities above 10-15 cm/s, because taxonomic composition and nutrient cycling may change (Biggs et al. 1998;Hondzo and Wang 2002). Bondar-Kunze et al. (2016) found in an experimental study that, in an oligotrophic stream ecosystem, daily hydropeaking significantly retarded the development of periphyton biomass with no interference in the relative abundance of the three main algal groups (diatoms, chlorophyta, cyanobacteria) or the photosynthetic activity. The lower biomass could be related to cell abrasion due to a fivefold increase in flow velocity compared to base flow conditions (Biggs and Thomsen 1995). ...
Flow is a major driver of processes shaping physical habitat in streams and a major determinant of biotic composition. Flow fluctuations play an important role in the survival and reproductive potential of aquatic organisms as they have evolved life history strategies primarily in direct response to natural flow regimes (Poff et al. 1997; Bunn and Arthington 2002). However, although the organisms are generally adapted to natural dynamics in discharge, naturally caused flow fluctuations may entail negative consequences (e.g., stranding, drift, low productivity), especially if the intensity is exceptionally high or the event timing is unusual (Unfer et al. 2011; Nagrodski et al. 2012). Aside from natural dynamics in discharge, artificial flow fluctuations with harmful impacts on aquatic ecology can be induced by human activities. Hydropeaking—the discontinuous release of turbined water due to peaks of energy demand—causes artificial flow fluctuations downstream of reservoirs. High-head storage power plants usually induce flow fluctuations with very high frequencies and intensities compared to other sources of artificial flow fluctuations (Fig. 5.1). However, run-of-the-river power plants and other human activities may also create artificial hydrographs due to turbine regulation, gate manipulations, and pumping stations.
... Increased dissolved inorganic nutrients often stimulate microbial decomposition (Kominoski et al., 2015), fungal richness (Noel, Bärlocher, Culp, & Seena, 2016) and algal biomass accrual (Sabater et al., 2011). However, the effects of enrichment on communities and ecosystem functioning are context dependent and may not always be positive, depending on other environmental conditions and stressors (Bondar-Kunze, Maier, Schönauer, Bahl, & Hein, 2016). ...
... result in increased decomposition (Baldy et al. 2007;Garcia et al. 2017) and in highly polluted streams enrichment may even inhibit decomposition (Pascoal and Cassio 2004;Dunck et al. 2015). Similarly, although enrichment generally increases algal biomass (Slavik et al. 2004), this effect is often overridden by other environmental factors (Schneider et al. 2013;Bondar-Kunze et al. 2016), further implying that ecosystem-level responses to nutrient enrichment are highly variable and context dependent. ...
We examined how short-term (19 days) nutrient enrichment influences stream fungal and diatom communities, and rates of leaf decomposition and algal biomass accrual. We conducted a field experiment using slow-releasing nutrient pellets to increase nitrate (NO3-N) and phosphate (PO4-P) concentrations in a riffle section of six naturally acidic (naturally low pH due to catchment geology) and six circumneutral streams. Nutrient enrichment increased microbial decomposition rate on average by 14%, but the effect was significant only in naturally acidic streams. Nutrient enrichment also decreased richness and increased compositional variability of fungal communities in naturally acidic streams. Algal biomass increased in both stream types, but algal growth was overall very low. Diatom richness increased in response to nutrient addition by, but only in circumneutral streams. Our results suggest that primary producers and decomposers are differentially affected by nutrient enrichment and that their responses to excess nutrients are context dependent, with a potentially stronger response of detrital processes and fungal communities in naturally acidic streams than in less selective environments.
... In the experiment, P-limitation (below 0.6 mg/L P) influenced biomass, phosphatase activity, and rates of photosynthesis and P assimilation, but did not influence algal community structure more than the colonization stage of the biofilms, which shifted from Chlorophyta to Cyanophyta and from non-filamentous to filamentous species.Multiple stressors.The subsidy effect of nutrients on benthic biofilm biomass and metabolism partly compensated for the toxic effects of emerging contaminants in experimental streams, although toxic effects become apparent after a longer period (3-4 weeks) of exposure to contaminants at environmentally relevant concentrations(Aristi et al., 2016). When a multi-stressor experiment on stream biofilms combined nutrient enrichment and hydropeaking, the negative effect of high flow on biomass cancelled out potential subsidy effects of nutrients on algal growth, although the interaction of these treatments was positive for diatoms and negative for Chlorophyta(Bondar- Kunze et al., 2016). The same multiple stressor combination may be detrimental or beneficial to different algal groups that are linked within a periphyton community, so it is important to consider successional stage and community composition of biofilms and the frequency of stressors in multiple stressor studies. ...
This survey of literature on substratumassociated microbiota from 2016 includes highlights of research findings associated with algae, cyanobacteria, and bacteria from a variety of aquatic environments, but primarily freshwaters. It covers topics of relevance to the Water Environment Federation along with those of emerging or recent interest such as nuisance, bloom forming and harmful algae, fossil fuel related contamination, and other environmental pollutants like nanoparticles. Additional interesting findings reported on include general ecology, method development, multistressor interactions, nutrient cycling, taxonomy and systematics, trophic interactions, and biomonitoring, bioassessment, and bioremediation.
Hydropower is commonly considered a renewable energy source. Nevertheless, this does not imply an absence of impacts on the riverine ecosystem, the extent of which is expected to increase in the coming years due to the energy transition from fossil fuels to renewable sources and for the climate change. A common consequence of hydroelectric power generation is hydropeaking, which causes rapid and frequent fluctuations in the water flow downstream of hydropower plants. The review incorporates 155 relevant studies published up until November 2023 and follows a systematic review method, Preferred Reporting Items for Systematic Reviews and Meta- Analyses (PRISMA), which is a multi-stage systematic procedure for the identification and selection of research documents. The selected studies highlighted several prominent impacts of hydropeaking on aquatic environments. The primary effects include alterations in flow patterns, modification of water temperature, changes in sediment dynamics and fluctuations in dissolved gas levels. These alterations have been found to affect various aspects of aquatic ecosystems, including fish growth, behavior, reproductive success, habitat, and migration patterns, and benthic macroinvertebrate communities. Furthermore, hydropeaking can also lead to habitat fragmentation, erosion, and loss of riparian vegetation, thereby impacting terrestrial ecosystems that depend on the aquatic environment. Despite the body of literature reviewed, several knowledge gaps were identified, underscoring the need for further research. There is limited understanding of the long-term ecological consequences of hydropeaking and its cumulative effects on aquatic ecosystems. Additionally, there is lack of consensus regarding the quantification of ecosystem services, economic impact, soil moisture content, and weighted usable area due to flow fluctuation and global evolution of energy production from renewable energy sources. Addressing the identified research gaps is crucial for achieving a balance between energy production and the conservation of freshwater ecosystems in the context of a rapidly changing global climate.
Biofilms are the consortium of microorganisms encased within the slimy sheath of Extracellular Polymeric Substance. These are rampant in nature, comprising a pivotal strategy used by microbes to endure severe environmental conditions. The microorganisms inhabiting the biofilms display a stratified structure, in which the role of every species is fixed and predetermined as per the environmental suitability and synchronization. The role of quorum sensing in facilitating the communication processes in biofilms in magnificent. Biofilms can have positive impacts as well as negative repercussions, particularly with respect to industrial setting or on medical devices. In recent years, the role of biofilms in bioremediation has been realized and its potential for bioremediation is enormous and magnificent with respect to water, soil and air. The only prerequisite for optimising such things is that the complete microbial profile, architecture and kinetics of biofilms should be known. The role of biofilms in bioremediation has attracted the role of microbiologists towards this field because of its widespread application in environment, industry and health.
An increase in the demand for renewable energy is driving hydropower development and its integration with variable renewable energy sources. When hydropower is produced flexibly from hydropower plants, it causes rapid and frequent artificial flow fluctuations in rivers, a phenomenon known as hydropeaking. Hydropeaking and associated hydrological alterations cause multiple impacts on riverine habitats with cascading effects on ecosystem functioning and structure. Given the significance of its ecological and socio‐economic implications, mitigation of hydropeaking requires an inter‐ and transdisciplinary approach. An interdisciplinary network called HyPeak has been conceived to enrich international research initiatives and support hydropower planning and policy. HyPeak has been founded based on exchange and networking activities linking scientists from several countries where hydropeaking has been widespread for decades and numerous studies dedicated to the topic have been carried out. HyPeak aims to integrate members from other countries and continents in which hydropower production plays a relevant role, and grow to be a reference group that provides expert advice on the topic to policy‐makers, as well as researchers, stakeholders, and practitioners in the field of hydropeaking.
Free-flowing rivers support diverse, complex and dynamic ecosystems, as well as provide societal and economic services. Globally, however, the water flow of many rivers has been regulated by hydropower or other sources. Flow modification affects crucial ecosystem functions and processes, and organism’s capacity to fulfil its life cycle requirements. In light of these widespread effects, it is urgent to mitigate ecological impacts caused by existing water infrastructures. To achieve environmental objectives, as well as to manage water uses in a sustainable way, a thorough understanding of ecological responses to hydrological alterations on different temporal levels (e.g., environmental flow, hydropeaking) is essential. This work aims to establish holistic approaches for restoring flows in modified rivers, and to develop environmental flows able to sufficiently mitigate the ecological effects of short-term and annual flow modifications in fluvial ecosystems.
This thesis disentangles the effects of multiple stressors and shows that flow regulation is a primary predictor of fish populations. Moreover, by assessing flow-ecology relationships on annual, seasonal, and sub-daily levels, this work identifies fundamental principles to implement flow restoration measures in rivers affected by water abstraction and hydropeaking. While more dynamic flows are generally recommended as environmental flows, flow restrictions are needed for hydropeaking mitigation. Regarding the latter, a seasonal framework for hydrological mitigation based on fish life-history stages is established, and thresholds are synthesized.
Overall, this thesis advances the establishment of guidelines for successful flow restoration in river systems affected by competing water uses by establishing holistic flow restoration schemes and by subsuming quantitative and qualitative hydropeaking thresholds. Moreover, this thesis sets the topic of flow restoration into the broader context of hydromorphological river rehabilitation. Hence, this work contributes to a more balanced discussion on trade-offs between societal and environmental water uses.
Downstream of hydroelectric plants, hydropeaking can cause frequent flow variations, resulting in habitat modifications (e.g. hydraulics, reach morphology, temperature, water quality), which can impact organisms (stranding, dewatering, forced drift, growth disturbances) and ultimately may have negative and lasting impacts on biological communities, reducing resilience. Nevertheless, the severity of habitat disturbances vary depending on other existing pressures and local site conditions, which need to be taken into account to achieve effective hydropeaking mitigation. Preserving hydropower flexibility is also a priority to ensure the stability of electric systems without recourse to more polluting alternatives. Given these apparently opposing objectives, we propose a consensual technico-economic framework to guarantee the feasibility and effectiveness of site-specific hydropeaking mitigation, based on our experience as a hydropower operator and a literature review. While existing tools (such as habitat models) can be used to predict expected local effects of proposed mitigation and compare scenarios, predicting biological community responses is not currently possible (lack of in-situ evaluations of mitigation efficacy). These uncertainties and complex socio-ecosystems necessitate a forward-looking global approach that accounts for climate change, multi-purpose water use and electric system requirements, combined with site-specific analyses of the relative importance of hydropeaking impacts with respect to other pressures.
Sudden instream releases of water from hydropower plants (hydropeaking [HP]) can cause abrupt temperature variations (thermopeaking [TP]), typically on a daily/sub-daily basis. In alpine rivers, hydropeaking and thermopeaking waves usually overlap, which leads to a multiple stressor of flow velocity pulses and temperature alteration. Periphytic communities could give important insights into the effects of combined thermo- and hydropeaking (THP) in stream ecosystems. Thus, the study's first aim was to assess the combined effects of thermo-hydropeaking on structural (composition, biomass) and functional (photosynthesis, enzyme activity) properties of periphyton. The second aim was to assess the interaction between periphytic algae and the heterotrophic communities (bacteria) and determine how biotic and abiotic factors explain the variability of bacterial enzymatic activities in the periphyton. We assessed the effects of repeated cold and warm thermo-hydropeaking for 24 days on periphyton, by manipulating discharge and temperature in six experimental flumes directly fed by an Alpine stream. Our study revealed that THP had structural and functional effects on periphyton in oligotrophic streams, where the effects depending on the direction of the temperature change (cold/warm) and on the morphological setting (pool/riffle). The results showed that even a short-term increase in flow velocity and temperature decrease could induce better growth conditions for diatoms. Additionally, an increase in the interaction between periphytic algae and bacteria during thermo-hydropeaking was also shown, this coupling being more pronounced in pool than in riffle sections. Our results clearly showed that riffle sections develop less periphytic algal biomass and activity and therefore, THP can reduce biomass availability for primary consumers in large areas of impacted streams. These findings highlight the importance of mitigation measures, focusing on establishing heterogeneous stream bed areas, with frequent pool and riffle sequences.
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.
Anthropogenic increases in nitrogen (N) and phosphorus (P) concentrations can strongly influence the structure and function of ecosystems. Even though lotic ecosystems receive cumulative inputs of nutrients applied to and deposited on land, no comprehensive assessment has quantified nutrient‐enrichment effects within streams and rivers. We conducted a meta‐analysis of published studies that experimentally increased concentrations of N and/or P in streams and rivers to examine how enrichment alters ecosystem structure (state: primary producer and consumer biomass and abundance) and function (rate: primary production, leaf breakdown rates, metabolism) at multiple trophic levels (primary producer, microbial heterotroph, primary and secondary consumers, and integrated ecosystem). Our synthesis included 184 studies, 885 experiments, and 3497 biotic responses to nutrient enrichment. We documented widespread increases in organismal biomass and abundance (mean response = +48%) and rates of ecosystem processes (+54%) to enrichment across multiple trophic levels, with no large differences in responses among trophic levels or between autotrophic or heterotrophic food‐web pathways. Responses to nutrient enrichment varied with the nutrient added (N, P, or both) depending on rate versus state variable and experiment type, and were greater in flume and whole‐stream experiments than in experiments using nutrient‐diffusing substrata. Generally, nutrient‐enrichment effects also increased with water temperature and light, and decreased under elevated ambient concentrations of inorganic N and/or P. Overall, increased concentrations of N and/or P altered multiple food‐web pathways and trophic levels in lotic ecosystems. Our results indicate that preservation or restoration of biodiversity and ecosystem functions of streams and rivers requires management of nutrient inputs and consideration of multiple trophic pathways.
Hydropeaking corresponds to rapid artificial discharge variations, designed to address sub-daily peaks in electricity demand. It generates rapid changes in physical habitat (e.g., flow velocity and water depth) with potential impacts on stream assemblages. For assessing the generality of hydropeaking effects on fish assemblages, we present an original combination of spatial (among 45 reaches, including six groups of nearby reaches) and temporal (over 3-17 years) analyses of these effects. Our analyses involved descriptions of natural and artificial hydraulic variations in reaches, obtained after translating hourly discharge data into hydraulics. We found that the influence of hydropeaking was secondary compared to well-known spatial variations in fish assemblage structure along longitudinal gradients, and negative influences of floods on annual densities. However, the spatial and temporal analyses consistently suggested that hydropeaking may disfavour fish species typical of medium-sized streams relative to species of headwater streams (Salmo trutta, Phoxinus phoxinus, Cottus gobio). The magnitude of hydropeaking effects observed here, as well as an apparent weaker effect of ramping rates than the frequency of hydropeaks, may be due to lower ramping rates in our data set than in other studies.
The EU FP 7 project MARS – “Managing Aquatic ecosystems and water Resources un- der multiple Stress” (duration 01.02.2014 – 31.01.2018) investigated how multiple human stressors affect rivers, lakes, groundwater, transitional and coastal waters. Outcomes of this research project are more than 200 scientific publications, more than 4000 pages of deliverables, various tools and background information about multiple stressors; all available in the Freshwater Information System.
This document provides recommendations and highlights relevant outcomes of the MARS project aiming to inform River Basin Managers and stakeholders in other sectors (energy, water industry, agriculture) on how to best assess and mitigate impacts of multiple stressors acting on Europe’s aquatic ecosystems. The document aims at supporting those who implement the EU Water FrameworkDirective (WFD) and who have to make recommendations or take decisions based on existing monitoring data. MARS mainly addressed pressures regarding hydromorphology, nutrients and climate change, while the focus of this document is on the most com mon and typical stressor combinations of European waters. For convenience and better readability, this document uses hyperlinks to relevant MARS results, models and tools to enable readers to directly access the respective web- sites.
Freshwater ecosystems are threatened by multiple anthropogenic stressors, which might be differentiated into two types: those that reduce biological activity at all concentrations (toxic contaminants), and those that subsidize biological activity at low concentrations and reduce it at high concentrations (assimilable contaminants). When occurring in mixtures, these contaminants can have either antagonistic, neutral or synergistic effects; but little is known on their joint effects. We assessed the interaction effects of a mixture of assimilable and toxic contaminants on stream biofilms in a manipulative experiment using artificial streams, and following a factorial design with three nutrient levels (low, medium or high) and either presence or absence of a mixture of emerging contaminants (ciprofloxacin, erythromycin, diclofenac, methylparaben, and sulfamethoxazole). We measured biofilm biomass, basal fluorescence, gross primary production and community respiration. Our initial hypotheses were that biofilm biomass and activity would: increase with medium nutrient concentrations (subsidy effect), but decrease with high nutrient concentrations (stress effect) (i); decrease with emerging contaminants, with the minimum decrease at medium nutrient concentrations (antagonistic interaction between nutrients subsidy and stress by emerging contaminants) and the maximum decrease at high nutrient concentrations (synergistic interaction between nutrients and emerging contaminants stress) (ii). All the measured variables responded linearly to the available nutrients, with no toxic effect at high nutrient concentrations. Emerging contaminants only caused weak toxic effects in some of the measured variables, and only after 3–4 weeks of exposure. Therefore, only antagonistic interactions were observed between nutrients and emerging contaminants, as medium and high nutrient concentrations partly compensated the harmful effects of emerging contaminants during the first weeks of the experiment. Our results show that contaminants with a subsidy effect can alleviate the effects of toxic contaminants, and that long-term experiments are required to detect stress effects of emerging contaminants at environmentally relevant concentrations.
The potential for complex synergistic or antagonistic interactions between multiple stressors presents one of the largest uncertainties when predicting ecological change but, despite common use of the terms in the scientific literature, a consensus on their operational definition is still lacking. The identification of synergism or antagonism is generally straightforward when stressors operate in the same direction, but if individual stressor effects oppose each other, the definition of synergism is paradoxical because what is synergistic to one stressor's effect direction is antagonistic to the others. In their highly cited meta-analysis, Crain et al. (Ecology Letters, 11, 2008: 1304) assumed in situations with opposing individual effects that synergy only occurs when the cumulative effect is more negative than the additive sum of the opposing individual effects. We argue against this and propose a new systematic classification based on an additive effects model that combines the magnitude and response direction of the cumulative effect and the interaction effect. A new class of “mitigating synergism” is identified, where cumulative effects are reversed and enhanced. We applied our directional classification to the dataset compiled by Crain et al. (Ecology Letters, 11, 2008: 1304) to determine the prevalence of synergistic, antagonistic, and additive interactions. Compared to their original analysis, we report differences in the representation of interaction classes by interaction type and we document examples of mitigating synergism, highlighting the importance of incorporating individual stressor effect directions in the determination of synergisms and antagonisms. This is particularly pertinent given a general bias in ecology toward investigating and reporting adverse multiple stressor effects (double negative). We emphasize the need for reconsideration by the ecological community of the interpretation of synergism and antagonism in situations where individual stressor effects oppose each other or where cumulative effects are reversed and enhanced.
Periphyton succession was studied over 89 d in longitudinally adjacent reaches (a riffle and run) in Sycamore Creek, a spatially intermittent desert stream. Effects of nitrogen limitation were assessed by comparing algal development on clay saucers containing either nitrate-enriched or unenriched agar. We evaluated effects of grazing on periphyton accrual by amending agar in half the substrata from each enrichment condition with an insecticide (Malathion). Early successional (25-d) communities on unenriched substrata were dominated by Epithemia sorex, a diatom capable of N2 fixation; non-fixing diatoms dominated enriched communities. NO3-N enrichment increased algal diversity (H′) and delayed late-successional dominance by Calothrix, a heterocystous cyanobacterium. Replacement of diatoms by cyanobacteria was likely facilitated by autogenic changes in nutrient and light conditions within the periphyton and temporal increases in water temperature. Three measures of algal biomass exhibited nearly linear increases in all treatments over the 3-mo study. Enrichment enhanced standing crops of chlorophyll a and ash-free dry mass (AFDM) in both reaches, an effect most pronounced within the first 3-4 wk. Algal populations unable to fix N2 were stimulated by enrichment but remained in low abundance on unenriched substrata whereas biovolume of N2-fixing populations was lower on enriched substrata relative to controls. Malathion reduced the density of only one common grazer, the moth larva Petrophila jaliscalis, which was abundant only in the riffle. Addition of Malathion to enriched substrata in this habitat had no significant effect on net primary productivity or total algal biovolume but resulted in increased chlorophyll a and AFDM, suggesting that Petrophila did influence algal communities, but by a mechanism that remains unclear. Spatial heterogeneity of nutrient supply in this system contributes to maintenance of algal diversity; but over long successional seres diversity declines regardless of enrichment owing to autogenic changes that occur during succession. Our results confirm earlier observations that low nitrogen availability limits accrual of algal biomass following spates and constrains algal community structure in Sycamore Creek. However, highly significant early successional differences in algal standing crop and community structure among enrichment treatments were eliminated or much reduced in mid to late stages of succession. We propose that this temporal change in enrichment response was caused by development of a thick periphyton mat that reduced availability of both water-column or substratum-derived nutrients to algae, and increased reliance on internal nutrient recycling.
Laboratory experiments were conducted to investigate the effect of fluid motion on the growth and metabolism of the freshwater periphyton Spirogyra and Stigeocloniom. The only growth-limiting factor was the effect of fluid motion; all the other environmental conditions, such as light, temperature, and nutrients were not limiting growth. The growth rates of periphyton were measured in terms of the chlorophyll-a concentration, and the metabolism was characterized by dissolved oxygen concentration measurements. The periphyton growth and corresponding photosynthetic rates were the lowest in a stagnant fluid. In a moving fluid the periphyton growth was mediated by the fluid flow conditions at the channel bottom. The results indicate critical conditions, corresponding to the detachment of periphyton in the channel bed. A functional dependence between the periphyton detachment and fluid flow conditions is proposed.
The effects of hydropower dams, and in particular the impacts of reduced river flows on the periphyton community, were assessed in the Soča River, Slovenia. Sampling sites were selected upstream and downstream of the Podsela and Ajba dams. Sampling was carried out in 1998 during a period of low flows. Reaches downstream from the dams experienced prolonged periods of reduced flows, and a corresponding decrease in flow velocity and water depth. The chain of hydropower dams has stopped sediment inflow from the upstream reach. Below the dams the oscillations of water temperature, dissolved oxygen, and oxygen
saturation are much larger in comparison with unregulated sites upstream. The impact of prolonged periods of reduced flows, a lack of sediment supply from upstream and changes in physico-chemical variables has caused high periphyton biomass, proliferation of green algae and increases in the number of periphytic algae species below the dams. This has significant implications for the design of environmental flow strategies that provide a sediment supply to maintain a healthy periphyton community.
Flow variation associated with hydropower production causes periodic exposure of zones along the banks of regulated rivers. These zones have reduced algal biomass and lower productivity. We investigated whether algal assemblages in regulated rivers differed in tolerance to aerial exposure because understanding such tolerance could be used to alter water releases to improve downstream productivity. In field experiments, we tested algal tolerance to aerial exposure in 3 assemblages: green filamentous and cyanobacterial assemblages in Thailand and a mixed (filamentous green + cyanobacteria) assemblage in New Zealand. Algae-bearing stones were exposed at night, during the day (in sun or shade, with or without simulated rain), or continuously, or were continuously submerged (with a handling control). Exposure reduced cyanobacterial chlorophyll a, and reduction was greater with day than with night exposure. Mixed algal assemblages showed a similar pattern in chlorophyll a, but ash-free dry mass (AFDM) did not change (bleached algae remained). In contrast, green filamentous algae survived better when exposed than when submerged, and when exposed during the day than at night. Snails consumed green filamentous algae, and submersion resulted in high grazing loss of filamentous algae but not cyanobacteria. Grazing pressure was presumably lower at night during water release. For cyanobacteria, shade and rain slightly increased chlorophyll a retention. For filamentous green algae, shade had no effect, and rain was beneficial in the shade but not the sun. Our experiments demonstrated an interaction among variable hydraulic conditions, activity of grazing snails, and availability of grazing-susceptible algae. A longer wet–dry cycle, daytime water release, and a shaded riparian zone reduced aerial exposure effects on algal assemblages, indicating that water releases can be regulated to improve productivity in the downstream exposed zone.
Nutrient availability in ecosystems is patchy both in space and in time. Whereas temporal trends have often been studied, less information exists on spatial patterns of nutrient availability, particularly in aquatic ecosystems. The goals of this study were (1) to describe and quantify patterns of nutrient concentration in surface waters of an arid land stream and (2) to compare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concentrations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel every-25 m over a 10-km section of a Sonoran Desert stream during three periods: early succession (2 wk post-flood), middle succession (2 mo post-flood), and late succession (9 mo post-flood). Nutrient concentrations were extremely variable in space (coefficients of variations as high as 145%). Coefficients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-variogram analysis showed that nutrient concentrations were spatially dependent on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as measured by the semi-variogram range, tended to decrease with successional time. The strength of spatial dependence, as measured by the slope of the ascending limb of the semivariogram, increased with successional time. The limiting nutrient, nitrogen, was consistently more spatially heterogeneous than phosphorus or conductivity, both in terms of patch size (range) and strength of spatial dependence. In streams, downstream transport combined with nutrient transformation produces patches of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils variation in nutrient concentration is likely to affect the spatial distribution of organisms and rates of ecosystem processes.
The effects of nutrient ratios on algal community structure and algal growth have been examined extensively in lakes and marine environments, but rarely in streams. We manipulated stream water N:P ratio (65:1, 17:1, 4:1) and total nutrient concentration (low and high) in a factorial experiment using once-through streamside flumes and measured responses in abundance, community structure, and elemental composition of periphyton communities. Early in the experiment, periphyton chlorophyll a and total algal biovolume were higher for treatments where N was added (high total nutrient concentration) but were not affected by N:P ratio. This response is contrary to our prediction that P would limit periphyton growth based on the high N:P ratio in the source water and unamended periphyton mate. The relative abundance of nine of eleven common algal taxa was affected by N:P ratio, total nutrient concentration, or both. Overall, algal community structure was more sensitive than bulk measures of periphyton abundance to changes in N:P ratio and total nutrient concentration. Periphyton %N and %P increased with the N and P concentration of stream water, and periphyton N:P tracked stream water N:P ratio. Responses in periphyton chemical composition to nutrients could affect the food quality of periphyton for consumers.
Few studies have examined the effects of low flows on stream biota—a paradoxical situation considering the mounting pressure on streams as a result of increased human abstraction. We present a
conceptual model describing processes operating in rivers during low flows and argue that such processes reflect both direct effects of reduced flow on benthic invertebrates and indirect effects of enhanced plant growth occurring during such periods. Our model suggests that the longer the duration of low flow, the more the plant community will change and, in turn, the more habitat quality will change, with consequences to benthic invertebrate communities and higher trophic levels. A fundamental part of the model is the recognition of different stream types, based on a habitat-template matrix of resource supply, interflood velocity, and substrate stability during flood events. This habitat template results in 12 stream
types, each of which supports specific plant communities that set the antecedent plant conditions prior to a low-flow period. During low flows, hydraulic factors interact with the antecedent plant communities, which undergo specific responses. For example, in low-nutrient streams with gravel substrates and dominated by diatoms, low-flow conditions will cause little or no change to the antecedent plant community because this plant community is structured by top-down grazing pressure. Consequently, benthic invertebrate composition will not change. In contrast, cover and biomass of filamentous green algae in higher-nutrient gravel-bed streams will increase during low-flow events, and this increase will affect habitat suitability for invertebrates over and above those caused solely by hydraulic changes. The greatest changes to invertebrate communities are expected in macrophyte-dominated streams because these plants change instream habitat conditions the most during extended periods of low flow. Therefore, current flow-management techniques must consider the type of plant communities that exist prior to low
flows and be cognizant of that fact that these plant communities can have dramatic influences on benthic communities during low-flow periods.
Long-term discharge records (17-81 yr) of 78 streams from across the continental USA were analyzed. Based on overall flow variability, flood regime patterns and extent of intermittency, 11 summary statistics were derived from the entire record for each stream. Using a nonhierarchical clustering technique, 9 stream types were identified: harsh intermittent, intermittent flashy, intermittent runoff, perennial flashy, perennial runoff, snowmelt, snow + rain, winter rain, and mesic groundwater. Stream groups separated primarily on combined measures of intermittency, flood frequency, flood predictability, and overall flow predictability, and they showed reasonable geographic affiliation. The positions of the streams in a continuous three-dimensional flow space illustrate the wide range of ecologically important hydrologic variability that can constrain ecological and evolutionary processes in streams. -from Authors
Grazer–periphyton interactions are shaped, in part, by indirect effects of nutrient regeneration. They are an important model system with which to test predictions of ecological stoichiometry and the Growth Rate Hypothesis. We conducted a laboratory experiment to test how nutrient enrichment and grazer identity interact to regulate the nutrient content and stoichiometry of both periphyton and consumers. We considered a situation in which P concentration in the water column was high, in contrast to previous experiments in which P was the limiting nutrient. We added N and P, alone and in combination, to the water in experimental aquaria that contained periphyton communities on clay tiles and grazers (1 of 3 snail species) or no grazers (ungrazed control). Benthic algae incorporated nutrients in close proportion to their availability in each nutrient treatment. Algal biomass increased significantly with þNþP enrichment, but not with þN or þP enrichment alone. Grazers had no effect on periphyton C:N ratios and positive effects on periphyton C:P and N:P ratios. P content of grazers (% dry mass) increased and C:P and N:P molar ratios of grazers decreased in response to N enrichment of the water. Grazer P content increased in response to N enrichment, probably because of increased grazer growth rates. We hypothesize that the addition of N under N-limiting conditions led to increased P uptake or retention by grazers because of high growth rates and RNA production, consistent with the Growth Rate Hypothesis.
Statistical models for predicting the effects on algal biomass of eutrophication are much better developed for lentic systems than for lotic systems, partly because of the dynamic physical nature of streams as controlled by flood regimes. I analyzed data from 30 sites in 25 runoff-fed streams and rivers to develop statistical models for mean monthly and maximum chlorophyll a as a function of soluble nutrient concentrations and days of accrual (reflecting the frequency of flood disturbance events). Variation in stream-water nutrients explained 12-22.6% of the variation in mean monthly chlorophyll a and 29.5-32.5% of the variation in maximum chlorophyll a among sites. Days of accrual explained 39.7% and 61.8% of the variation in mean monthly and maximum chlorophyll a, respectively. Multiple regression models combining dissolved nutrient data and days of accrual explained 43.7-48.8% of the variation in mean monthly chlorophyll a and 72.1-74.1% of the variation in maximum chlorophyll a among sites. In streams with infrequent floods and long accrual periods (e.g., >100 d), a relatively small increase in dissolved nutrients greatly increased the frequency of high biomass events. However, as could be anticipated, this result did not occur in more flood-prone streams. A nomograph to predict oligo-, meso-, and eutrophic conditions as a function of nutrient concentrations and days of accrual is presented based on the regression models for maximum chlorophyll a. The models need further testing, but might be useful for predicting the effects of changes in nutrients on benthic algal biomass in other temperate streams and rivers. I suggest that variable nutrient criteria for the prevention of benthic algal proliferations could be set in streams in relation to regimes of local flood frequency and expected time available for biomass accrual. The present analysis suggests that managing nutrient supply could not only reduce the magnitude of maximum biomass, but also reduce the frequency and duration of benthic algal proliferations in streams.
We examined,effects of disturbance,timing,on resistance and,resilience of epilithic algal communities,growing,in fast- (29 cm/s) and slow- (1 2 cm/s) current outdoor experimental stream channels in Kentucky, USA that were either left undisturbed (control) over 33 d following a simulated spate, or were subjected to an additional spate after either 9, 18, 27, or 33 d. On day 33, all channels were subjected to a final spate to assess effects of short-term disturbance,history on resistance. independently,of seasonal,influences. Suc- cession proceeded,from,a sparsely populated,community,dominated,by a small. monora- phid diatom (.4chnanthes rninutissirna) immediately after the initial spate, to dominance
Three long-term phosphate enrichment experiments were conducted at the Experimental Troughs Apparatus (EXTRA), South Thompson River British Columbia to determine the relationship between external orthophosphate (PO43−) concentration and peak areal biomass (PB) of periphytic diatom communities. Levels of PO43− which saturated PB were two orders of magnitude greater than those required to saturate specific growth rates in thin film periphyton communities of similar taxonomic composition. With PO43− additions between 0.1 to 1.0 μg P∙L−1, PB responded in a hyperbolic fashion, initially increasing rapidly, then showing signs of saturation PB continued to increase in a slow, linear manner above 1.0 μg P∙L−1. Maximum PB (PBmax) was calculated to occur at ca. 28 μg P∙L−1. At higher PO43− concentrations (> 30–50 μg P∙L−1) PB was no longer P limited. Below the saturation point, PB was approximated by a log–linear function of PO43−.
Under high flows, the biomass of riverine phytoplankton can be constrained by short transport times and advective losses. However, under slower flows and longer transport times, secondary factors and sometimes their interaction with flow may constrain phytoplankton biomass. To contribute to a wider understanding of the riverine conditions that constrain phytoplankton biomass, we tested the hypothesis that phytoplankton of the Daly River (tropical Australia) was constrained by transport time during dry-season base flow. The river is virtually undisturbed, with oligotrophic nutrient concentrations during the dry season. The most frequently occurring taxa were planktonic, rather than benthic, and dominated by the dinoflagellate Peridinium inconspicuum which has r-strategist traits that favour rapid growth in a nutrient-deficient environment. Our hypothesis was not supported because increased downstream loads of Chlorophyll a and the domination of P. inconspicuum inferred phytoplankton net growth. Instead, phytoplankton biomass was more likely to be nutrient-limited, although transport time may limit phytoplankton growth over some reaches and for specific taxa. The present study demonstrated that even in the fast-flowing middle reaches of a river (~0.4 m s–1), a population of phytoplankton can be sustained.
The within-day, between-day and month-to-month variability of light attenuation and microalgal chlorophyll a (chl a) and photosynthetic response was measured in San Antonio Bay, Texas, USA. Waters were shallow (<2 m) and turbid (attenuation coefficients of 0.7 to 15.3 m(-1), yet daily rates of primary production (0.1 to 2.5 g C m(-2) d(-1)) were comparable to those in much deeper and clearer estuaries. Chl a concentrations in the sediment (459 to 7837 mg m(-3) in the surficial millimeter) were much higher than those in the water column (4 to 48 mg m(-3)). The benthic assemblages were photosynthetically competent, but daily benthic primary productivity was low (0.00 to 0.09 g C m(-2) d(-1); an average of 2 % of productivity in the water column) because of the very low irradiances at the sediment-water interface. The high rates of production by the suspended microalgae were largely due to high chlorophyll-specific light-saturated rates of photosynthesis, P-m(chl) [3.0 to 24.4 g C (g chl a)(-1) h(-1)], which were correlated positively with temperature and inversely with the mean irradiance in the water column. The between-day and temperature-independent variation in P-m(chl) was also correlated with F(v)Chl(-1), an index of the proportion of functional photosystem II reaction centers. In turn, within-day variability in F(v)Chl(-1) was inversely correlated with the mean irradiance in the water column in 8 of 10 observations, a pattern consistent with photoinhibition. A depression of P-m(chl) caused by photoinhibition throughout the water column may therefore be responsible for the inverse trend of P-m(chl) with mean irradiance in the water column. Short-term (h) variability in suspended chl a and turbidity was high (coefficient of variation = 13 to 75 %), but estimates of daily productivity could be predicted with reasonable fidelity (mean error 27 %) from a single midday determination of chl a, the photosynthesis versus irradiance response and the attenuation coefficient, along with daily incident radiation. The predictive power of a single observation was due to coherence in the variation of chl a, P-m(chl) and the attenuation coefficient in the water column: the decrease in mean irradiance caused by resuspension was compensated for by concomitant increases in P-m(chl) and suspended chl a. Between-day variability in productivity of 15 to 52% approached month-to-month differences, so the optimal use of resources in monitoring productivity would be to take single samples daily.
The photosynthetic performance of sea ice microalgae, benthic microalgae, coralline red algae (Phymatolithon foecundum) and brown macroalgae (Laminaria saccharina, Fucus evanescens and Desmarestia aculeata) in an ice-covered high-Arctic fjord (Young Sound, NE Greenland) was studied with in situ instruments for oxygen microprofiling and active chlorophyll fluorescence measurements. In situ measurements of the effective quantum yield of Photosystem II (PSII) under ambient low irradiance ranged between 0.3 and 0.7. Net oxygen production of benthic microalgae and sea ice microalgae was measured in situ at low irradiance of similar to2 to 30 mumol photons m(-2) s(-1). The compensation irradiance determined in the laboratory for coralline algae arid brown macroalgae was 1.6 and 1.9 mumol photons m(-2) s(-1) respectively., The same experiments showed irradiances at onset of saturation, E-k, for coralline red algae and brown macroalgae of 17 and 12.8 mumol photons m(-2) s(-1) respectively. In situ measurements of rapid light curves, measured over 1 to 2 min by the saturation pulse method, enabled snap shots of the in situ adaptation of the photosynthetic apparatus in the different phototrophs under ambient irradiance conditions. These measurements showed Ek values for PSII related electron transport (relative ETR) at 7.9 (sea ice microalgae), 4.6 (benthic microalgae), 11 (coralline red algae) and 4.4 to 7.3 (brown macroalgae) mumol photons m(-2) s(-1). At higher irradiance levels, relative ETR decreased, indicating inhibition and/or downregulation of photosynthesis. When light curves were measured with longer application of the different actinic light levels, a less pronounced or zero decrease of relative ETR was found at higher irradiance, and Ek values were shifted to higher irradiance, i.e. 6.9 mumol photons m(-2) s(-1) (benthic microalgae) and 11.5 to 16.1 mumol photons m(-2) s(-1) (brown macroalgae). This apparent short-term acclimation to increasing irradiance reversed to the initial characteristics of the light curve within 15 to 20 min after irradiance returned to ambient intensity. Our measurements add to the small database on in situ photosynthetic performance of surface-associated algae below ice cover. All phototrophs were well adapted to low irradiance under the ice, and active fluorescence measurements indicated that the phototrophs are able to adapt reversibly within minutes to moderate changes in irradiance levels.
Short-term (3 day) diatom colonization of clean ceramic tiles and tiles conditioned with a thin, nonalgal biofilm was examined in fast- and slow-current outdoor experimental stream channels to assess effects of organic conditioning and current regime on diatom colonization. Colonization rates onto unconditioned tiles were 10 times lower in fast current that in slow. Substratum conditioning reduced the effects of current because organic conditioning of tiles significantly enhanced colonization in fast current, but not in slow current. Two diatom taxa (Nitzschia acicularis W. Sm. and Synedra radians Kütz) colonized unconditioned tiles more rapidly than conditioned tiles in slow current, suggesting the existence of negative interactions between these diatoms and bacteria and/or organics in conditioning films.
1. Periphyton chlorophyll a and ash free dry weight (AFDW) were monitored in nine rivers to examine the relative importance of flows and nutrients for regulating periphyton biomass in gravel bed rivers.
2. Mean annual flows in the rivers ranged from 0.94 to 169 m ³ s ⁻¹ , mean dissolved reactive phophorus (DRP) from 1.3 to 68 μ g 1 ⁻¹ , periphytic chlorophyll a from 4.6 to 73 mg m ⁻² . and AFDW from 2.8 to 16 g m ⁻² .
3. For eight of the nine rivers NH 4 ‐N. DRP, total Kjeldahl nitrogen, total phosphorus and total suspended solids were correlated ( P <0.01) with flow, and for seven rivers conductivity was inversely correlated ( P <0.05) with flow.
4. There was a hyperbolic relationship between flows and biomass, with chlorophyll a >100 mg m ⁻² and AFDW >20 g m ⁻² occurring most frequently in flows of <20 m ³ s ⁻¹ .
5. Floods prevented the development of medium term (i.e. up to 2 months) maxima in biomass in five of the rivers, but maxima occurred over summer‐autumn and winter‐spring in the three rivers where floods were absent.
6. Chlorophyll a biomass was more resistant to flooding than AFDW. Only 5993 of the forty‐six recorded floods caused chlorophyll a scouring, whereas 74% of the floods caused AFDW scouring. The efficiency of scour was more influenced by the pre‐flood biomass than the magnitude of the event.
7. Biomass maxima were significantly correlated ( P <0.01) with mean DRP concentration during the accrual period. Overall, up to 53% of the mean annual biomass difference between rivers was explained by the mean annual DRP concentrations. However, the high correlations between nutrient concentrations and flow indicated that the nutrient data were also carrying hydrological information and that simple causal relationships between nutrients and biomass are difficult to establish in rivers.
8. It is concluded that hydrological factors contribute at least equally with nutrients to the differences in periphyton biomass between the gravel‐bed study rivers. They combined to explain up to 63.3% of the variance in biomass, compared with 57.6% for nutrients. It is recommended that periphyton data from gravel‐bed rivers should always be viewed within the context of the flow history of the site, and not just as a function of nutrient concentrations.
We demonstrated that differences in habitatrequirements by C. glomerata and Oscillatoria havea profound bottom-up influence on the foodweb in thetailwaters below Glen Canyon Dam in the Colorado Riverthrough Grand Canyon National Park, USA. We examinedthe effects of suspended sediment and desiccation onthe colonization sequence of Cladophora glomerata andOscillatoria spp. and the consequent effects onmacroinvertebrates in each algal community in a seriesof reciprocal transplants in the regulated ColoradoRiver, AZ. Our experiments showed that C. glomeratagrows best in continuously submerged, clear-water,stable habitats, whereas Oscillatoria forms densemat-like matrices of trichomes and sand in varialzones and habitats with high suspended sedimentstypical of many southwestern USA streams. Varial zoneconditions have a stronger influence on communitystructure than habitats with high suspended sediments. Recruitment by chironomid larvae was less dependent onC. glomerata and less affected by suspended sedimentand periodic desiccation than Gammarus lacustris. Weestimated the energy from macroinvertebrate biomassassociated with tufts of C. glomerata to be an orderof magnitude higher than that in Oscillatoriamatrices. Therefore, loss of C. glomerata andreplacement of habitat more suitable for Oscillatoriaas a result of regulated flows indirectly reducespotential energy flow in the Colorado Riverfoodweb.
Protecting the worlds freshwater resources requires diagnosing threats over a broad range of scales, from global to local. Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly 80% of the worlds population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global water security for both humans and freshwater biodiversity.
Of the 23 algae recorded, Stigeoclonium, Oedogonium, Oscillatoria, Lyngbya and Pleurocapsa were dominant at some point in the 15 month sampling period. Nutrient concentrations were consistently high (N 1.1-21.4 mg.L-1; P 0.1-10.4 mg.L-1); therefore, changes in temporal distribution of algae were probably dependent on seasonal changes in light and temperature. Small unicellular algae were the first autotrophs to attach, followed by larger filamentous forms. -from Authors
Periphyton can play an important role for biogeochemical processes in large regulated rivers. The development of benthic algae during low flow periods are especially suitable in braided sections, near bank zones and shallow areas. Light availability, fine sediment accumulation and flow
velocity are assumed to be the main factors controlling periphyton growth, while the effect of water level fluctuations in large rivers is often neglected. However, the latter could also have severe impacts on the development of benthic algae. The present study aims to analyse how short term
(hourly and daily) water level fluctuations influence the periphyton development and species richness and how desiccation stress affects the specific primary production. An in-situ experiment was conducted for one month on a Danube River bank in the free-flowing stretch of the river east of
Vienna. Plates with artificial substrates were exposed at several defined depths along a fluctuating water level gradient. A short term laboratory experiment was also conducted to assess the effect of drying and rewetting on benthic algal production. Our results showed the benthic chlorophyll-a
content was 40 to 60 times lower and diatom species were 5 times fewer at a desiccation-affected zone compared with the permanently-immersed zone. However, after a short term flow pulse lasting 3 days, the differences between the two zones were alleviated. In the laboratory experiment, the
benthic algal community from two different colonisation stages with different chlorophyll-a content and species composition (days 12 and 21 of development) were capable of efficient photosynthesis for 2 and 5 hours of desiccation, respectively, followed by no measurable photosynthetic
activity for an additional hour. Complete recovery of photosynthesis took 3 and 5 hours after rehydration. The results of this study showed that the periphyton on a riparian zone of a large regulated river is a very dynamic community with a high capacity for resilience with regard to the highly
fluctuating environmental conditions. Furthermore, short-term desiccation stress should be considered as an important form of disturbance that affects algal communities in altered river systems.
In-situ, nutrient amendment experiments (nutrient-diffusing substrata, NDS) were conducted in 12 New Zealand gravel-bed streams to investigate seasonality of biomass accrual and nutrient limitation of benthic algal communities. Benthic algal biomass accrual rates exhibited significant (p = 0.019, repeated measures ANOVA) seasonal differences; rates were greatest in summer and least in winter. The degree of nutrient limitation also differed (p = 0.003) seasonally; periphyton community biomass was most responsive to nutrient amendments in summer and least responsive in winter. Temperature may be the underlying cause of these patterns. The ratios of dissolved inorganic nitrogen to soluble reactive phosphorus (DIN:SRP) in streamwater and of streambed periphyton communities were of limited use for predicting which nutrient limited NDS bioassays; cellular nutrient content was weakly predictive. This study demonstrates the need to consider temporal changes (i.e., seasonality) when assessing the influence of nutrients on stream ecosystems, and indicates that the use of nutrient ratios to ascertain which nutrient may limit benthic algal biomass should be validated with field experiments.
Ecological theory predicts that multiple nutrients, added independently, could each stimulate algal community biomass. In contrast to this theoretical prediction, individual nutrient amendment experiments frequently detect significant increases in algal community biomass in response to addition of a single nutrient, whereas the independent addition of other nutrients often does not result in a statistically significant increase in biomass. However, because of the low replication and high variability typical of ecological field experiments, only large responses can be detected as statistically significant in any single experiment. It is therefore possible that smaller, yet still real and biologically important responses occur, but remain undetected because of the low statistical power of individual experiments. Meta-analysis of lotic nutrient (N and P) amendment experiments indicated that simultaneous stimulation of benthic algal community biomass by >1 nutrient was the rule, not the exception. Addition of a limiting nutrient typically doubled algal biomass, whereas addition of another nutrient generally increased algal biomass ∼1.25-fold. N was approximately equally likely as P to be limiting. The power of the typical experiment for detecting biomass stimulation resulting from limiting and “nonlimiting” nutrients was >85% and <50%, respectively. This study presents yet another line of evidence that multispecies communities are unlikely to be limited by a single nutrient, and illustrates the utility of meta-analysis for conducting statistically powerful syntheses of ecological experiments.
Diatom immigration rates were affected by changes in current patterns similar to those found around substrates in streams and were increased by changes in microhabitat characteristics similar to those autogenically generated during diatom community development. Diatom abundances on tiles exposed for 24 h, i.e., diatom immigration rates, ranged from 30 t 1500 cells .cm^-^2.d^-^1. Immigration rates in areas sheltered from the 27 cm/s currents of two Michigan streams were greater than in areas exposed to the current. Diatom immigration rates increased by a factor of 6 when microhabitat conditions were altered by interrupting currents near the substrate surface, and by a factor of 2 when substrates wee coated with agar. These two microhabitat conditions, respectively, simulated changes in current patterns near the substrate, and in adsorptive characteristics of the substrate that could occur autogenically as diatoms accumulate on substrates. Species-specific effects of current velocity and microhabitat conditions were related to size and growth habits of cells.
Proliferations of periphyton associated with the ecological degradation of streams have been linked to changes in flow regime, most frequently as a product of water abstraction and impoundment. We used descriptive and experimental studies to examine the relationship between water velocity and the distribution of two distinct, often high biomass, periphyton patch types and associated macroinvertebrate communities in the Waipara River, a meso‐eutrophic stream in Canterbury, New Zealand.
One patch type (‘ Phormidium’ ) consisted mainly of two filamentous cyanobacterial taxa: a Phormidium/Lyngbya complex and Oscillatoria . The other type (‘filamentous greens’) consisted mainly of several filamentous chlorophyte taxa ( Mougeotia sp .) along with various diatom epiphytes. Ash‐free dry mass, chlorophyll a and particulate N and P concentrations were significantly greater in the Phormidium than in the filamentous green patches.
There was a significant relationship between patch cover and water velocity, with filamentous greens negatively associated with velocity and dominant in areas with a velocity <∼0.20 m s ⁻¹ , while Phormidium was dominant at velocities >∼0.40 m s ⁻¹ . A 7‐day experimental reduction in velocity in Phormidium ‐dominated habitats resulted in a significant reduction in% cover (84–21%) with a corresponding increase in filamentous greens (16–79%). There was a small but significant reduction in the % cover of filamentous greens (90–82%) with experimentally increased velocity, but no significant change in Phormidium cover (7–2%), perhaps due in part to the slow growth rate of taxa in this patch type.
Characterisation of macroinvertebrate assemblages collected from Phormidium and filamentous green patches and the substratum immediately below showed patch‐specific assemblages, although patch type was a less important determinant than vertical location, with 85% of the macroinvertebrates found in the substratum below algal mats.
The differential responses of Phormidium and filamentous green patches to velocity are considered in the context of a subsidy‐stress model that examines how trade‐offs between flow‐mediated biomass accrual and loss processes vary for different algal growth forms.
Collectively, these results indicated that key ecological characteristics of rivers can be quite sensitive to spatial and temporal variations in hydraulic conditions that are much smaller than those typical of floods and droughts.
The factors underlying population and community dynamics are almost invariably multivariate, and ecosystems worldwide are affected by many anthropogenic stressors. Inorganic dissolved nutrients and deposited fine sediment are common stressors in agricultural streams and can be expected to influence benthic algae and cyanobacteria.
We determined population‐ and community‐level responses of phototrophic periphyton along twin‐stressor gradients of dissolved nutrients and deposited fine sediment after 20 days of exposure in stream mesocosms. Multiple linear regression and an information‐theoretic approach were used to select the best predictive models for each response variable, accounting for potential subsidy‐stress responses for each stressor (where at low stressor levels, an ecological variable responds positively up to an inflection point, beyond which the effect is negative) and for multiple‐stressor interactions. This approach enabled us to test the subsidy‐stress hypothesis for each stressor and to investigate whether the two stressors operated singly or in concert (additively or with an interaction) on periphyton attributes.
Focussing further on three ecological guilds of periphyton (low profile, high profile, motile), we also tested the specific hypotheses, that sediment augmentation would produce a proportional increase in motile and a decrease in high‐profile forms, and that nutrient augmentation would produce proportional increases in both motile and high‐profile forms.
Unimodal subsidy‐stress patterns along the sediment gradient were found only for densities of two common N itzschia species. By contrast, unimodal patterns along the nutrient gradient occurred frequently at both population and community levels, supporting the subsidy‐stress hypothesis for nutrient enrichment. Some single‐stressor responses were observed, but the twin stressors sediment and nutrients mainly acted in a simple, additive way, probably because their modes of action are different. As a notable exception from this rule, potentially harmful cyanobacteria ( P hormidium and O scillatoria species) responded synergistically to the stressors. Our hypothesis about the representation of ecological guilds across the sediment gradient was fully supported, whilst the hypothesis regarding nutrients received partial support.
Categorisation of periphyton taxa into physiognomic guilds revealed predictable response patterns to the stressors. The prevalence of motile taxa in stream communities may be suitable for detecting sedimentation impacts and early signs of nutrient enrichment. Finally, based on periphyton response shapes along the nutrient gradient, thresholds could be identified at which community variables changed abruptly.
We carried out a benthic survey and two experiments in runs at eight sites down the Kakanui River (South Island, New Zealand) during summer low flows, to investigate the interaction between nutrients, periphyton, and macro‐grazers. Benthic periphytic biomass was generally low (< 20 mg m chlorophyll a) at most sites, but high densities of macro‐grazers (mainly snails) were observed at six of the eight sites. Chlorophyll a and cellular P concentrations were generally higher on artificial substrates in the first‐ to third‐order tributaries, compared with downstream. Macro‐grazer densities (mainly snails) were also highest in the second‐ and third‐order tributaries. Enrichment of patches with N and P did not translate into significant increases in chlorophyll a concentrations. Instead there was a general increase in macro‐grazers, and an increase in the relative abundance of Cocconeis placentula. In a second experiment, the chlorophyll a level was five‐fold higher on the substrates where macro‐grazers were excluded and there was no significant response of chlorophyll a to nutrient addition on these substrates. On the grazed substrates, densities of snails and caddis‐larvae were two‐fold higher with N+P enrichment. These experiments provided evidence for a tight coupling between first and second trophic levels, and strong grazer control of periphyton, in this river.
A correlation was found between the seasonal epiphyte cycle in Lake Memphremagog (Québec‐Vermont), with a maximum in mid‐June, and the abundance of grazers (mainly oligochaetes and chironomids). Evidence for a cause and effect relationship was provided by exclosure‐enclosure experiments. When grazers were excluded epiphyte biomass did not decline as it did in the lake. Addition of grazers to a dense epiphyte cover resulted in a decline qualitatively and quantitatively similar to that in the lake. Grazing pressure, calculated from general empirical equations for aquatic deposit feeders, was of the required order of magnitude to explain the decline in epiphyte biomass.
Because similar seasonal epiphyte cycles have been described elsewhere these findings should have a wider relevance.
The comparison of two treatments generally falls into one of the following two categories: (a) we may have a number of replications for each of the two treatments, which are unpaired, or (b) we may have a number of paired comparisons leading to a series of differences, some of which may be positive and some negative. The appropriate methods for testing the significance of the differences of the means in these two cases are described in most of the textbooks on statistical methods.
Most methods for assessing the ecological status of streams focus on structural characteristics (water quality, community composition, riparian vegetation) but neglect functional properties of the ecosystem because routine methods to assess stream function are scarce. Metabolism, one of the most integrative ecosystem functions, can be a good indicator of stream function because it is relevant across all sizes and types of streams, is sensitive to stressors, such as eutrophication or changes in riparian cover, and can be measured continuously. Environmental controls on whole-ecosystem metabolism were measured at 19 contrasting stream reaches in the Basque Country (northern Spain). Discharge, temperature, and O2 were monitored continuously for 15 mo, reaeration rate was calculated with the nighttime regression method, and whole-stream metabolism was calculated by the single-station open-channel method. The effect of discharge on reaeration coefficients was highly site-specific. Average gross primary production (GPP) ranged from 2.7 to 11.0 g O2 M-2 d-1, was highest at eutrophic sites, and showed no relationship with periphyton biomass. Ecosystem respiration (ER) ranged from 6.3 to 42.6 g O2 m-2 d-1 and was highest at polluted sites. Differences among sites increased in summer. All sites were heterotrophic on an annual basis, but 3 were autotrophic during summer. Turbidity was the main controller of primary production during summer and explained 20% and 39% of the spatial variation in GPP and net ecosystem production, respectively. Biological O2 demand of water explained 40% of ER variance. Catchment activities also controlled GPP, which decreased as population density increased. To our knowledge, our study is the first report of continuous monitoring of whole-stream metabolism at many reaches simultaneously, and it shows the potential of this technique for routine monitoring of stream function.
1. Colonization of nutrient-diffusing substrata by periphyton and invertebrates was investigated at forested and open sites in a small, mountain stream and a spring in the South Island of New Zealand.
2. Substrata had colonization surfaces made from 100μm mesh plankton netting that enabled algal assemblages to be removed intact for scanning electron microscopy. They also allowed small volumes of solvent to be used for the extraction of photosynthetic pigments.
3. At all sites, periphyton assemblages were dominated by species of Achnanthes, Cocconeis and Gomphonema, and except in the forest in winter, periphyton biomass was always greater on enriched (N + P added) than control substrata.
4. Invertebrates colonizing diffusion substrata were principally larval Chironomidae (Orthocladiinac). No larvae were present in winter, but in three spring and summer trials mean larval densities were higher on nutrient-enriched than control substrata at all sites.
5. The inclusion of an insecticide Malathion in diffusion substrata, reduced insect colonization at open and forested sites. After 28 days, no concurrent increases in algal pigment concentration were observed on nutrienl-enriched or control substrata at the forested site. However, pigment concentrations were higher on substrata incorporating Malathion at the open site suggesting that algal standing crop was depressed by the activities of grazers.
Attached algal populations were sampled at weekly or biweekly to characterize successional changes in the secondary clarifiers of a wastewater treatment plant. Three communities were compared from areas of slow, medium and rapid current velocities. In general, the algae resembled those reported for other hypereutrophic flowing water. Of the twenty-three algae recorded, Stigeoclonium, Oedogonium, Oscillatoria, Lyngbya, and Pleurocapsa were dominant at some point in the 15 month sampling period. Nutrient concentrations were consistently high (N = 1.1–21.4 mg·L−1; P = 0.1–10.4 mg·L−1); therefore, changes in temporal distribution of algae were probably dependent on seasonal changes in light and temperature. Colonization of artificial substrates was also observed. Small unicellular algae were the first autotrophs to attach and these were followed by larger filamentous forms.
1. The articles in this Special Issue on pressure‐response relationships in stream ecology cover a range of pressures including acidification, excess loading of metals, nutrients and organic matter to stream ecosystems and changes in riparian and catchment land use. Impacts on the biota (macroinvertebrates), ranging from single species to community responses, are addressed over a multitude of spatial scales using large data sets.
2. The majority of articles are based on monitoring data compiled as part of the EU‐funded project REBECCA, but the Special Issue also includes articles from outside Europe (US and New Zealand) as well of studies using other data sources.
3. Results from the six articles gave new insights of relevance to water managers and documented that large‐scale monitoring can be an asset to freshwater science. The large data sets enable a range of analytical approaches that single out important patterns in highly variable data.: importance of humic substances in reducing the negative impact of low pH; impact of very low levels of BOD (<2 mg L ⁻¹ ); higher sensitivity of trait‐based metrics compared with identity‐based metrics and importance of catchment and riparian vegetation for the ecological quality of streams.
3. We advocate an increased use of monitoring and survey data in addressing questions relevant to scientist and end‐users, but issues relating to especially data quality should be considered. In future, the use of these data could increase knowledge exchange between the scientific community and managers with the ultimate aim of improving our freshwater resources.
1. Periphyton species composition, chlorophyll a concentration, organic matter biomass, and metabolic activity were analysed at a site in a regulated river with low nutrient concentrations to investigate population and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hydraulic range [0.1–0.5m depth, 0.1–1.5ms ⁻¹ velocity, and 0.01–1.5 Froude number ( Fr )] with Cymbella kappii, Synedra ulna , and Gomphoneis herculeana having the highest relative biovolumes.
2. Unexpectedly, Cymbella kappii and Synedra ulna were abundant or co‐dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within the mid‐velocity range (velocity = 0.8–1.2ms ⁻¹ ).
3. The physiognomy of the communities changed with a change in hydraulic conditions. There was progressively more diatom mucilage as velocity and Fr increased which resulted in a macroscopic change from relatively thin films (1–2mm) at low velocities and Fr (in pool habitats), to thick (approximately 10mm) mats at higher velocities and Fr (in riffle habitats). Associated with this, ash‐free dry mass (APDM) increased strongly, but chlorophyll a concentration did not, resulting in a decrease in % chlorophyll a over the gradient.
4. The results of experiments conducted in an in situ benthic chamber showed no significant differences in gross primary production between two communities of different biomass, but indicated a slightly decreasing trend with increasing velocity (0.14 to 0.38 ms ⁻¹ ). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combined AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with variable amounts of mucilage being secreted below, depending on hydraulic conditions.
5. The diatom community had considerable physiognomic plasticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help ameliorate environmental stress. If so, present theory on velocity control of periphytic algal development in streams, which is essentially based on passive responses, needs expanding. It is concluded that the response of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydraulic habitat preference curves for periphyton communities in nutrient‐poor streams.
The resistance of stream periphyton to structural disturbance was investigated in a laboratory flow tank. Overall, the nonfilamentous diatom communities were the most resistant, and the filamentous communities were the least resistant. A community dominated by Melosira varians/Gomphonema parvulum lost 50% of its biomass with only a 3-fold increase in shear stress. In contrast, a community dominated by the nonfilamentous diatoms Fragilaria vaucheria/Cymbella minuta lost <50% of its biomass after a 70-fold increase in shear stress. These results show that spates without bedload movement can potentialy hve widely differing disturbance effects on periphyton loss among streams depending on the initial taxonomic composition. -from Authors
1. Laboratory and field experiments were performed to develop and then apply a nutrient-diffusing substratum (NDS) design suitable for use in large, fast-flowing rivers.
2. Initial laboratory experiments quantified diffusion of PO4 and NO3 from new and previously used clay pots, which were soaked in deionized distilled water. Mean release rates initially exceeded 2.4 and 725 μmol l–1 day–1 P and 0.22 and 18 μmol l–1 day–1 N from new and used pots, respectively, but declined rapidly with increasing time spent in deionized distilled water and were below detectable levels after about 18 and 29 days, respectively.
3. A phosphorus (P) dose–response experiment in a P-limited reach of the Athabasca River, Alberta, Canada showed that epilithic biomass and macroinvertebrate density on NDS increased with increasing concentrations of KH2PO4 up to about 0.5 m. Beyond this threshold, biomasses and densities were unaffected by initial KH2PO4 concentration. Coefficients of variation of epilithic biomass estimates declined with increasing KH2PO4 whereas invertebrate density appeared to be unaffected by KH2PO4 levels.
4. Release rates of both P and N from NDS filled with 0.5 m KH2PO4 or 0.5 m NaNO3 declined at a log-negative rate from about 5000 μmol N-NO3 l–1 day–1 and 3500 μmol
P-PO4 l–1 day–1 on day 2, to 200 μmol l–1 day–1 for both N and P on day 32.
5. After development, we used the diffusing substrata to identify spatial patterns in nutrient limitation at seven sites along a 120 km reach in the Athabasca River, that receives two known point-source nutrient inputs. NDS consisting of N, P, N + P and unenriched controls were attached to the river bottom for 22–23 days and then retrieved and sampled for epilithic chlorophyll a. Physicochemical parameters and epilithic biomasses on upper stone surfaces were also quantified when NDS were deployed and retrieved from each site.
6. Sites located immediately downstream of the two point source inputs had higher water column concentrations of PO4 and epilithic biomasses than the site immediately upstream; epilithic biomass was positively related to PO4 in the late autumn (r2= 0.58) but not in early autumn. Sites located immediately below nutrient inputs were not nutrient-limited, whereas upstream reference sites were P-limited.
Previous studies have shown major differences in the way biomass of stream periphyton is controlled by spatial variations in velocity. We hypothesize that these differences may be the result of different growth forms within the community. Some dense and coherent growth forms (e.g. mucilaginous diatom/cyanobacterial mats) may be resistant to diffusion and also resistant to dislodgment by shear stress. Higher velocities applied to such communities could therefore be expected to enhance biomass accrual by increasing rates of mass transfer, but without greatly increasing losses through sloughing. Conversely, other growth forms (e.g. long filamentous green algae) have an open matrix, and high rates of diffusion into the mats can potentially occur even at low velocity. However, as velocities increase, high skin friction and form drag should lead to higher rates of sloughing. The overall result of these processes should be that maximum biomass occurs at low velocities. This “subsidy-stress” hypothesis was tested twice with each of three different periphytal growth forms: a coherent, mucilaginous, diatom community; a moderately coherent, stalked/ short, filamentous diatom community; and an open-weave, long, filamentous green algal community. A monotonic increase in chl a biomass occurred as a function of near-bed velocities for the first of the two mucilaginous diatom communities investigated. No biomass-velocity relationship was found, however, with the second mucilaginous community, probably because the waters were highly enriched and mass transfer driven by molecular diffusion was probably high throughout the velocity gradient. Biomass was moderate at low velocities, peaked at near-bed velocities from 0.18 to 0.2 m·s−1 (∼0.40–0.45 m·s−1 mean column velocity), and then decreased at higher velocities in both of the stalked/ short filament communities of diatoms analyzed. With the long filamentous green algal communities, a monotonic reduction in biomass occurred as a function of increases in velocity. Proliferations greater than 100 mg·m−2 chl a occurred at low near-bed velocities (i.e. <0.2 m·s−1), after which biomass declined nearly exponentially as a function of increasing velocity to less than 10 mg·m−2 chl a at velocities greater than 0.4 m·s−1. These biomass-velocity trends support our hypothesis that community growth form determines periphytal responses to spatial variations in velocity within stream reaches.
Respiration rates in light- and dark-incubated biofilms were determined using simple flux calculations on measured O2 concentration profiles and photosynthetic rates. A significantly higher areal O2 consumption was found in illuminated biofilms than in dark-incubated biofilms. Although photorespiration accounted for part of the increase, the enhanced areal O2 consumption of illuminated biofilms could also be ascribed to a deeper oxygen penetration in light as well as an enhanced volumetric O2 respiration in and below the photic zone. Gross photosynthesis was largely unaffected by increasing flow velocities, whereas the O2 flux out of the photic zone, that is, net photosynthesis, increased with flow velocity. Consequently, the amount of produced O2 consumed within the biofilm decreased with increasing flow velocity. Our data indicated a close coupling of photosynthesis and respiration in biofilms, where the dissolved inorganic carbon requirement of the photo-synthetic population may largely be covered by the respiration of closely associated populations of heterotrophic bacteria consuming a significant part of the photosynthetically produced oxygen and organic carbon.
1. An artificial glass substratum was incubated in the River Danube for a period of 28 days in order to detect the sequential colonization of microorganisms.
2. Light and fluorescent microscopy showed that microalgae and the picoalgal fraction on the slides increased rapidly over the first 2 weeks of colonization. Diatoms were numerically the most abundant component of the periphyton and their species richness and diversity increased rapidly in the early phase of colonization whereas diversity subsequently increased moderately.
3. Evenness of the diatom community was initially high, lower in the intermediate phase and again higher later on. Succession involving early, intermediate and late colonizer species was observed. Community composition during the first 5 days of colonization was very different from later stages whereas there were only minor changes subsequently.
4. Molecular community analysis by means of terminal restriction fragment length polymorphism analysis of PCR amplified 16S rRNA and 18S rRNA genes pointed to even larger differences between the composition of samples obtained early and late in the period.
5. The number of 18S rRNA and 16S rRNA terminal restriction fragments (T-RF-s) was variable over the colonization period and the fragment patterns of both the bacterial and eukaryotic portion of the microbial community were variable, with most T-RF-s unique to a single sample, suggesting a wide diversity and dynamic properties of periphytic organisms.
Managers must understand the effects of stressors on ecosystems in order to identify thresholds of harm but, to be meaningful, thresholds will usually need to be defined for situations where multiple stressors are operating.
We investigated the individual and combined effects of the principal stressors (nutrient concentration and streambed fine sediment cover) operating in native grassland streams converted to pasture in New Zealand, using two different approaches: a survey of 32 small streams and an experiment involving nine streams where the stressors were manipulated in a factorial design. We investigated the consequences for populations of benthic invertebrates and for the structure of communities, including taxon richness and the representation of species traits.
Up to half the taxa and most community metrics responded to at least one stressor. Our results suggest that in these streams, an increase in fine sediment loading from anthropogenic causes had more widespread effects than augmented nutrient concentrations. Of most significance is our finding, both from the survey and, in particular, the experiment, of a variety of complex interactions among the stressors.
Synthesis and applications . The development of indices of stream health that distinguish the effects of sediment from those of nutrients should help prioritize catchment management actions. Of more general importance is our finding that the consequences of stressors are often unpredictable on the basis of knowledge of single effects; if managers only consider the effects of individual stressors, their assessment of risk may be higher or lower than reality.
A method for the determination of lipophilic photosynthetic pigments in algal cultures and lake water samples is described. The extraction of these substances is carried out in a homogenisator with acetone/water (9:1, v/v) and leads to small extraction volumes without further concentration steps during sample preparation. A binary gradient system based on acetonitrile-containing solvents and a non-endcapped C18-RP-column is used for the chromatography of the obtained extracts. Pigments are detected by monitoring the absorbance at 440 nm and identified by their on-line recorded absorption spectra. An evaluation of a variety of C18-RP-columns using one of the most similar pairs of xanthophylls, lutein and zeaxanthin, demonstrates that non-endcapped instead of fully-endcapped C18-RP-phases are able to resolve both substances. Calibration curves for relevant pigments present in lake waters using algal cultures as pigments sources are linear in the tested range from 10 ng to 300 ng for carotenoids and 50 ng to 1200 ng for chlorophylls, respectively. The described method allows the determination of lipophilic algal pigments in lake water samples down to 0.4 g/l carotenoid or 2 g/l chlorophyll. The reproducibility of the whole procedure is quite satisfactory and does not exceed 15% for any identified pigment.