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Environmental stressors have profound implications for species, communities, and ecosystems by altering fundamental processes. With increasing human impacts on aquatic ecosystems, two main scenarios have been reported: (1) the spatiotemporal superposition of multiple stressors, leading to interactions among them and (2) intensifying environmental gradients, leading to threshold responses. However, studies designed to assess the effects of multiple stressors (e.g., pulse stressors) along environmental gradients (e.g., press stressors) are uncommon, and interactions between pulse and press stressors may cause abrupt changes in biological responses. We conducted a laboratory experiment to investigate the effects of osmotic stress along a nutrient enrichment gradient on a freshwater community composed of periphyton, microorganisms, and zebra mussels (Dreissena polymorpha). Our objectives were to (1) quantify the individual and combined effects of stressors, (2) delineate thresholds along the nutrient gradient (press) in the absence and presence of osmotic stress (pulse), and (3) test for interactions between the two stressors. We evaluated effects on metabolic rates in D. polymorphaand on microbial activity, as well as phototrophic periphyton biomass and physiological status. We observed interactions between the two stressors for metabolic rates in D. polymorphaand periphytic phaeopigments. In contrast, we found an individual effect of osmotic stress on microbial activity and chlorophyll acontent. Thresholds were only identified in the presence of osmotic stress for metabolic rates in D. polymorpha. Our work highlights the importance of combining multiple stressors with environmental gradients and the need to consider multiple biological compartments when evaluating the impacts of stressors on ecosystems.

The New Zealand mud snail Potamopyrgus antipodarum(NZMS) is a global invader that is readily spread through attachment to recreational fishing gear. Preventing the spread of NZMSs by decontaminating fishing gear such as waders is a key step toward limiting invasive NZMSs and their ecological impacts; however, the effectiveness of decontamination protocols depends on both the efficacy of the protocol and the willingness of anglers to implement it. We tested the efficacy of three decontaminants (Virkon Aquatic, Formula 409, and bleach) at killing NZMSs on waders using two application techniques (spray versus soak) and two exposure durations (10 versus 20 min). We coupled the results of these tests with responses to a self‐administered online survey that gauged the willingness of anglers to implement several decontamination strategies. Mortality of NZMSs differed widely among decontaminants, with the greatest mortality caused by Formula 409 (mean ± SE = 100 ± 0%), regardless of application type or duration. Bleach produced a mean mortality of 68.75 ± 11.97%, and Virkon Aquatic resulted in a mean mortality of 56.25 ± 11.97%. Neither exposure duration nor application method significantly influenced the degree of NZMS mortality, and their interaction was not significant. Anglers who responded to the survey (n= 339) revealed that Formula 409 was the decontaminant they would be most willing to use. Further, spraying was highly preferred over soaking for all decontaminants. Based on our experimental trials and the angler survey, we developed an angler decontamination metric (ADM) that helps to determine the decontamination strategy that optimizes NZMS mortality on fishing gear. Our ADM indicates that spraying gear with Formula 409 is the most effective NZMS decontamination strategy that anglers are willing to use. Our study is the first to combine the efficacy of NZMS decontaminants and angler willingness to adopt a decontamination strategy. By doing so, we hope to encourage the widespread use of NZMS decontamination of fishing gear to limit the spread and impacts of this increasingly relevant invasive species.

Climate change is altering the structure and functioning of river ecosystems worldwide. In mountain rivers, glacier retreat has been shown to result in systematic changes in aquatic invertebrate biodiversity, but the effects of ice loss on other biological taxa and on whole-ecosystem functions are less well understood. Using data from mountain rivers spanning six countries on four continents, we show that decreasing glacier cover leads to consistent fungal-driven increases in the decomposition rate of cellulose, the world’s most abundant organic polymer. Cellulose decomposition rates were associated with greater abundance of aquatic fungi and the fungal cellulose-degrading Cellobiohydrolase I (cbhI) gene, illustrating the potential for predicting ecosystem-level functions from gene-level data. Clear associations between fungal genes, populations and communities and ecosystem functioning in mountain rivers indicate that ongoing global decreases in glacier cover can be expected to change vital ecosystem functions, including carbon cycle processes.

Examining patterns and processes along the aquatic continuum from headwaters to ocean can benefit from a landscape ecology approach, where hydrologic and ecological processes depend on landscape position. We conducted a study of freshwater wetland ponds subject to similar climatic conditions but distributed along a 20‐km trajectory from glacial headwaters to ocean in southcentral Alaska, U.S.A. Specifically, we investigated how proximity to glaciers and ocean influenced physical, chemical, and biological characteristics of ponds. Physicochemical patterns along a distance gradient from the ocean supported the hypothesized influence of elevation and potential atmospheric deposition of marine‐derived nitrogen, whereas those related to glacial flow path length may reflect inputs from glacial weathering. We expected that the effects of landscape and hydrology on physicochemical patterns would provide a template for shaping ecosystem processes, but ecosystem processes also appeared to contribute to physicochemical patterns across this landscape. Ponds more heavily influenced by glaciers tended to be more heterotrophic exhibiting greater rates of organic‐matter decomposition and ecosystem respiration, which were positively correlated with phosphorus and iron concentrations likely due to glacial weathering and remineralization processes. In contrast, ponds near the ocean tended to be more autotrophic exhibiting greater gross primary production and net ecosystem production, processes that may have contributed to greater total nitrogen, nitrogen to phosphorus ratios, and dissolved organic carbon concentrations. Consideration of the relative importance of hydrologic inputs across the landscape is needed because the acceleration of glacial melt and sea‐level rise by climate change may alter future broad‐scale patterns of ecosystem processes.

The invasion of round goby Neogobius melanostomusinto tributaries of the Laurentian Great Lakes poses concerns for the conservation of native fish communities and the functioning of stream ecosystems. In streams, where standard methods for detection are lacking, development of sensitive sampling methods to identify round goby-invaded sites is a prerequisite for the implementation of effective management strategies. We sampled 34 stream sites that spanned a range of goby densities with two active gears (electrofishing and seining) and one passive gear (minnow traps) to compare gear efficiency at detecting invasive round goby. Naive occupancy, defined as the number of sites where goby were detected with each gear divided by the total number of sites, was 0.79 (27/34 sites) for seining, 0.68 (23/34) for electrofishing, and 0.50 (17/34) for traps. The probability of detecting a single round goby in a single pass (determined with single-season, constant-probability models) was 0.75 ± 0.065 (mean ± SE) for minnow traps, 0.69 ± 0.056 for seining, and 0.47 ± 0.075 for electrofishing. Mean ± SE catch per unit effort (CPUE; round goby/min) was 0.716 + 0.158 for seining, 0.137 ± 0.043 for electrofishing, and 0.078 ± 0.022 for minnow traps, seining being significantly more efficient than minnow traps and electrofishing. The CPUE did not differ between electrofishing and traps. Mean goby size did not differ among gear types, but a size bias was detected when data from a related study were included in the analysis, seining capturing smaller round goby. Based on the sensitivity and efficiency of seining, we conclude that when stream conditions allow, this gear is a practical means of detecting round goby and determining their abundance in streams. Received August 11, 2011; accepted January 4, 2012

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. Bacteria and fungi contribute to the breakdown of leaf litter in rivers and floodplains. To break down leaf litter, these microbes need the nutrients nitrogen and phosphorus (P), and microbes can get nutrients either from the leaf litter itself or from the environment. Most leaf litter has low nutrient content and microbes must rely on the environment to supply nutrients. We studied microbial nutrient uptake from the environment during litter breakdown to determine whether it varies predictably across the globe and how it is influenced by changing climate and nutrient pollution. In 100 rivers and floodplains in 11 of Earth's major biomes we placed small strips of cotton as stand‐ins for leaf litter. Nutrient uptake was consistently greater on cotton strips that were submerged in the river compared to cotton on the floodplain. For microbes in the river, nutrient uptake was faster in instances where there was more P in the water. For microbes in the floodplain, nutrient uptake was faster where temperatures were warmer. Faster nutrient uptake by microbes was linked with faster cotton breakdown in rivers and floodplains. Our study shows that climate change and nutrient pollution can alter the activity of microbes in rivers and floodplains. Nitrogen (N) and phosphorus (P) immobilization was measured on organic matter (cotton) in 100 rivers and riparian zones representing 11 biomesElevated temperature in riparian zones and phosphate in rivers increased immobilization, and consequently accelerated decompositionN and P immobilization was strongly linked by microbial stoichiometry despite widely varied surface‐water nutrient ratios Nitrogen (N) and phosphorus (P) immobilization was measured on organic matter (cotton) in 100 rivers and riparian zones representing 11 biomes Elevated temperature in riparian zones and phosphate in rivers increased immobilization, and consequently accelerated decomposition N and P immobilization was strongly linked by microbial stoichiometry despite widely varied surface‐water nutrient ratios