Colden V. Baxter’s research while affiliated with Idaho State University and other places

Stream ecosystems exhibit high degrees of spatial heterogeneity at nested scales from microhabitats to regions. This heterogeneity may facilitate the co-occurrence of biogeochemical processes that are favored under incompatible environmental conditions, like dinitrogen (N2 gas) fixation and denitrification. We hypothesized that environmental variation at the patch scale (1–10’s m) would facilitate the co-occurrence of N2 fixation and denitrification through the formation of hot spots. We measured rates of N2 fixation and denitrification and relative abundances of nifH and nirS (genes that encode for the enzymes nitrogenase and nitrite reductase, respectively) in patches determined by channel geomorphic units and substratum type in seven streams encompassing a gradient of N and P concentrations. We found hot spots, where rates of N2 fixation and denitrification were 1–4 times higher than reach-average rates, in all study streams. Most N2 fixation hot spots were in patches with rock substrata, while denitrification rates and relative abundances of nifH and nirS were higher in patches with fine sediment. Yet, in one of the streams, the same patches hosted rates in the top 25% of all patches for both denitrification and N2 fixation. Across all streams and patches, organic matter and dissolved oxygen concentrations were important predictors of rates of N2 fixation, denitrification, and nifH relative abundance, while P concentration was important to N2 fixation and denitrification. Our results demonstrate that understanding the spatial ecology of microbially driven nutrient cycling is required to characterize nutrient fluxes more completely in stream ecosystems.

Food webs vary in space and time. The structure and spatial arrangement of food webs are theorized to mediate temporal dynamics of energy flow, but empirical corroboration in intermediate‐scale landscapes is scarce. River‐floodplain landscapes encompass a mosaic of aquatic habitat patches and food webs, supporting a variety of aquatic consumers of conservation concern. How the structure and productivity of these patch‐scale food webs change through time, and how floodplain restoration influences their dynamics, are unevaluated. We measured productivity and food‐web dynamics across a mosaic of main‐channel and side‐channel habitats of the Methow River, WA, USA, during two study years (2009–2010; 2015–2016) and examined how food webs that sustained juvenile anadromous salmonids responded to habitat manipulation. By quantifying temporal variation in secondary production and organic matter flow across nontreated river‐floodplain habitats and comparing that variation to a side channel treated with engineered logjams, we jointly confronted spatial food‐web theory and assessed whether food‐web dynamics in the treated side channel exceeded natural variation exhibited in nontreated habitats. We observed that organic matter flow through the more complex, main‐channel food web was similar between study years, whereas organic matter flow through the simpler, side‐channel food webs changed up to ~4‐fold. In the side channel treated with engineered logjams, production of benthic invertebrates and juvenile salmonids increased between study years by 2× and 4×, respectively; however, these changes did not surpass the temporal variation observed in untreated habitats. For instance, juvenile salmonid production rose 17‐fold in one untreated side‐channel habitat, and natural aggregation of large wood in another coincided with a shift to community and food‐web dominance by juvenile salmonids. Our findings suggest that interannual dynamism in material flux across floodplain habitat mosaics is interrelated with patchiness in food‐web complexity and may overshadow the ecological responses to localized river restoration. Although this dynamism may inhibit detection of the ecological effects of river restoration, it may also act to stabilize aquatic ecosystems and buffer salmon and other species of conservation concern in the long term. As such, natural, landscape‐level patchiness and dynamism in food webs should be integrated into conceptual foundations of process‐based, river restoration.

Rivers and streams contribute to global carbon cycling by decomposing immense quantities of terrestrial plant matter. However, decomposition rates are highly variable, and large-scale patterns and drivers of this process remain poorly understood. Using a cellulose-based assay to reflect the primary constituent of plant detritus, we generated a predictive model (81% variance explained) for cellulose-decomposition rates across 514 globally distributed streams. A large number of variables were important for predicting decomposition, highlighting the complexity of this process at the global scale. Predicted cellulose-decomposition rates, when combined with genus-level litter-quality attributes, explain published leaf-litter-decomposition rates with impressive accuracy (70% variance explained). Our global map provides estimates of rates across vast understudied areas of Earth, and reveals rapid decomposition across continental-scale areas dominated by human activities.

Despite growing interest in conservation and re‐establishment of ecological connectivity, few studies have explored its context‐specific social–ecological outcomes. We aimed to explore social and ecological outcomes to changing stream connectivity for both stakeholders and native fish species impacted by habitat fragmentation and nonnative species. We (1) investigated stakeholder perceptions of the drivers and outcomes of stream connectivity, and (2) evaluated the effects of stakeholder‐identified connectivity and nonnative species scenarios on Yellowstone cutthroat trout (YCT) populations. Our study was conducted in the Teton River, Idaho, USA. We integrated two modeling approaches, mental modeling and individual‐based ecological modeling, to explore social–ecological outcomes for stakeholders and YCT populations. Aggregation of mental models revealed an emergent pattern of increasing complexity as more types of stakeholders were considered, as well as gaps and linkages among different stakeholder knowledge areas. These results highlight the importance of knowledge sharing among stakeholders when making decisions about connectivity. Additionally, the results from the individual‐based models suggested that the potential for a large, migratory life history form of YCT, in addition to self‐preference mating where they overlap with rainbow trout, had the strongest effects on outcomes for YCT. Exploring social and ecological drivers and outcomes to changing connectivity is useful for anticipating and adapting to unintended outcomes, as well as making decisions for desirable outcomes. The results from this study can contribute to the management dialogue surrounding stream connectivity in the Teton River, as well as to our understanding of connectivity conservation and its outcomes more broadly.

Emergence of adult aquatic insects from rivers is strongly influenced by water temperature, and emergence timing helps to determine the availability of this ephemeral food resource for birds and other terrestrial insectivores. It is poorly understood how spatial heterogeneity in riverine habitat mediates the timing of emergence. Such spatiotemporal variation may have consequences for terrestrial insectivores that rely on aquatic-derived prey resources. We investigated emergence phenology of the giant salmonfly, Pteronarcys californica, at three spatial scales in two Idaho river networks. We examined the influence of tributary confluences on salmonfly emergence timing and associated insectivorous bird responses. Salmonfly emergence timing was highly variable at the basin-scale during the period we sampled (May–June). Within sub-drainage pathways not punctuated by major tributaries, emergence followed a downstream-to-upstream pattern. At the scale of reaches, abrupt changes in thermal regimes created by 10 major tributary confluences created asynchrony in emergence of 1–6 days among the 20 reaches bracketing the confluences. We observed 10 bird species capturing emerged salmonflies, including 5 species typically associated with upland habitats (e.g., American robin, red-tailed hawk, American kestrel) but that likely aggregated along rivers to take advantage of emerging salmonflies. Some birds (e.g., Lewis’s woodpecker, western tanager, American dipper) captured large numbers of salmonflies, and some of these fed salmonflies to nestlings. Emergence asynchrony created by tributaries was associated with shifts in bird abundance and richness which both nearly doubled, on average, during salmonfly emergence. Thermal heterogeneity in river networks created asynchrony in aquatic insect phenology which prolonged the availability of this pulsed prey resource for insectivorous birds during key breeding times. Such interactions between spatial and temporal heterogeneity and organism phenology may be critical to understanding the consequences of fluxes of resources that link water and land. Shifts in phenology or curtailment of life history diversity in organisms like salmonflies may have implications for these organisms, but could also contribute to mismatches or constrain availability of pulsed resources to dependent consumers. These could be unforeseen consequences, for both aquatic and terrestrial organisms, of human-driven alteration and homogenization of riverscapes.

Food web analyses offer useful insights into understanding how species interactions, trophic relationships, and energy flow underpin important demographic parameters of fish populations such as survival, growth, and reproduction. However, the vast amount of food web literature and the diversity of approaches can be a deterrent to fisheries practitioners engaged in on‐the‐ground research, monitoring, or restoration. Incorporation of food web perspectives into contemporary fisheries management and conservation is especially rare in riverine systems, where approaches often focus more on the influence of physical habitat and water temperature on fish populations. In this review, we first discuss the importance of food webs in the context of several common fisheries management issues, including assessing carrying capacity, evaluating the effects of habitat change, examining species introductions or extinctions, considering bioaccumulation of toxins, and predicting the effects of climate change and other anthropogenic stressors on riverine fishes. We then examine several relevant perspectives: basic food web description, metabolic models, trophic basis of production, mass‐abundance network approaches, ecological stoichiometry, and mathematical modeling. Finally, we highlight several existing and emerging methodologies including diet and prey surveys, eDNA, stable isotopes, fatty acids, and community and network analysis. Although our emphasis and most examples are focused on salmonids in riverine environments, the concepts are easily generalizable to other freshwater fish taxa and ecosystems. This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Water and Life > Conservation, Management, and Awareness Water and Life > Methods

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low‐nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low‐nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature‐dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature. Collectively, we demonstrated that exogenous nutrient supply and immobilization are critical control points for decomposition of organic matter.