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Abstract: Freshwater ecosystems rely on allochthonous resources. Integration of these subsidies depends on diversity of both terrestrial resources and aquatic shredder and decomposer communities, but the diversity effects on leaf litter breakdown and decomposition are less clear in aquatic than terrestrial ecosystems. We need a better understanding of this relationship because aquatic communities are rapidly changing with species invasions and anthropogenic impacts. Here, we experimentally disentangled the effects of leaf and shredder richness on leaf litter breakdown by macroinvertebrates in mesocosm experiments using three species of amphipods, a dominant guild of crustaceans in European freshwater ecosystems. Increased leaf richness led to lower‐than‐predicted leaf consumption by native shredders, with mixed evidence of resource‐switching or prioritization of preferred food items within a leaf mix. Higher shredder species richness never promoted leaf consumption rates compared to predictions from relevant single‐species experiments, and instead sometimes substantially decreased leaf consumption. We then conducted a meta‐analysis of leaf litter consumption rates by seven widely distributed amphipod species (the three used in the experiments and four additional species). As expected based on our own experiments, nonnative amphipod species generally had lower biomass‐adjusted leaf litter consumption rates, although their larger body size led to higher per‐individual leaf consumption rates. Contamination of the water by metals, pesticides, and other chemicals additionally significantly decreased leaf litter consumption by multiple native and nonnative species compared to unpolluted systems. While the meta‐analysis suggested that litter consumption, and thus breakdown, would decline if native shredders are replaced by nonnative heterospecifics, complete species replacement is not the only outcome following immigration in a meta‐community context. Our experiments suggest that breakdown rates could remain reasonably high where native species coexist with nonnative arrivals. Experiments that neglect the ecological realism of species coexistence will necessarily mischaracterize effects on ecosystem functioning. [ABSTRACT FROM AUTHOR]

Shredders play a central role in the breakdown of leaf material in aquatic systems. These organisms and the ecological function they provide may, however, be affected by chemical stressors either as a consequence of direct waterborne exposure or through alterations in food-quality (indirect pathway). To unravel the biological relevance of these effect pathways, we applied a 2 × 2-factorial test design. Leaf material was microbially colonized for 10 days in absence or presence of the fungicide epoxiconazole (15 μg/L) and subsequently fed to the shredder Asellus aquaticus under exposure to epoxiconazole (15 μg/L) or in fungicide-free medium over a 28-day period ( n = 40). Both effect pathways caused alterations in asselids' food processing, physiological fitness, and growth, although not always statistically significantly: assimilation either increased or remained at a similar level relative to the control suggesting compensatory behavior of A. aquaticus to cope with the enhanced energy demand for detoxification processes and decreased nutritional quality of the food. The latter was driven by lowered microbial biomasses and the altered composition of fatty acids associated with the leaf material. Even with increased assimilation, direct and indirect effects caused decreases in the growth and lipid (fatty acid) content of A. aquaticus with relative effect sizes between 10 and 40%. Moreover, the concentrations of two essential polyunsaturated fatty acids (i.e., arachidonic acid and eicosapentaenoic acid) were non-significantly reduced (up to ~ 15%) in asselids. This effect was, however, independent of the exposure pathway. Although waterborne effects were generally stronger than the diet-related effects, results suggest impaired functioning of A. aquaticus via both effect pathways. [ABSTRACT FROM AUTHOR]

The River Continuum Concept (RCC) predicts that riverine food webs shift from a reliance on allochthonous energy in headwaters to autochthonous production in lower reaches. However, estimates of resource reliance often fail to account for resource segregation within the food web. Stable-isotope biomarkers can be used to estimate the relative importance of allochthonous and autochthonous material to specific groups and within the food web. δ13C and δ15N isotope ratios were calculated for allochthonous (conditioned leaf litter) and autochthonous (biofilm and bryophytes) basal energy sources, macroinvertebrate functional feeding groups (shredder, grazer, collector-filterer, collector-gatherer, predator), and resident fish species at 17 locations spanning the headwaters to lower reaches of 2 river systems in eastern Canada. Stable-isotope mixing models and correlations were used to identify longitudinal trends in the food webs of both rivers. In headwater streams, allochthonous material was the predominant resource for shredders, whereas all other primary, secondary, and tertiary consumers predominantly used autochthonous basal resources. Stable-isotope ratios of allochthonous material varied minimally between sites. Both autochthonous basal sources were significantly 13C-enriched and 15N-depleted in lower-reach relative to headwater sites. Shredders displayed minimal variation across sites, whereas longitudinal variation in all other primary consumers, macroinvertebrate predators, and fishes was closely correlated with autochthonous basal sources. These results highlight the role of resource segregation within river food webs and indicate that the RCC may not predict energy pathways in all temperate river systems. [ABSTRACT FROM AUTHOR]

Leaf litter decomposition is an important process providing energy to freshwater biota. Flow regulation and dams can strongly alter freshwater ecosystems, but little is known about the effect of small impoundments on leaf litter decomposition rates in headwater streams. In this study, we examined the effect of small water storage impoundments (80 to 720 m3) on leaf litter decomposition by comparing sites located within 10-m upstream and downstream of nine impoundments (Rhineland Palatinate, Germany) and sites located further upstream and downstream. The impoundments did not have a statistically significant effect on most physico-chemical variables. However, the abundance of shredders and leaf litter decomposition rates decreased in study sites located within 10-m upstream of the area flooded by impoundments. Small impoundments can locally reduce leaf litter decomposition rates in headwater streams. The effect of small impoundments on ecosystem functioning is minor and may require less attention by freshwater managers than other stressors, though this may differ for other ecological aspects such as connectivity. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

1. We compared leaf litter conditioning in two artificial stream channels that were connected to a second-order stream (both in- and outflow). The water in one channel had the same temperature as the stream and in the other channel was warmed (3 °C above ambient stream water temperature). In addition to measuring leaf decomposition by microbes, we ran feeding trials in the artificial streams and the laboratory that addressed the feeding preferences, resource acquisition and performance of a dominant detritivore. 2. Oak leaves were conditioned for three weeks in both channels and then offered during the following five weeks to Sericostoma vittatum larvae kept in cages in the same channels. Additionally, leaves conditioned in both conditions were simultaneously offered to the larvae in a feeding preference laboratory trial. Leaves conditioned in the channel with elevated temperature had reduced toughness, fewer phenols and lower carbon : nitrogen ratios. However, individuals did not prefer these leaves over those conditioned at ambient stream temperature. 3. We observed lower carbon : nitrogen imbalances and increased consumption rates in the stream channel at warmer temperature but these were not translated into higher growth rates. Higher metabolic rates of the S. vittatum individuals and other temperature-mediated factors such as changes in leaf quality could explain the increased consumption. We observed higher lipid storage and earlier pupation of S. vittatum at warmer temperatures, suggesting that individuals allocated resources to metamorphosis and reproduction, rather than to somatic growth. 4. We argue that microbial and invertebrate-mediated organic matter cycling will be faster in streams under future warming scenarios. However, these systems will likely become less efficient in carbon retention via effects on larval performance (growth and developmental time) and timing of prey availability to the stream and coupled riparian food webs. [ABSTRACT FROM AUTHOR]

Streams receiving agricultural runoff are typically enriched with nutrients, which variously impact stream communities. We examined the effects of phosphate and nitrate enrichment on leaf litter breakdown, microbial biomass and the nutrition of an invertebrate shredder to determine how nutrients are transferred through the stream detrital food web. Using artificial streams, individuals of Anisocentropus kirramus (Trichoptera: Calamoceratidae) were fed leaves of Apodytes brachystylus (Icacinaceae) under different nutrient regimes. We measured the amount of leaf material consumed or decomposed and the microbial biomass colonising the leaves. The dry mass, and protein, lipid and carbohydrate composition of A. kirramus larvae were determined after 28-day feeding on the leaves. Supplements of phosphorus, but not nitrogen, enhanced leaf breakdown, microbial growth and growth of larvae. Microbial biomass and dry mass of larvae increased with nutrient enrichment and they were significantly correlated. Thus, the phosphorus supplement was transmitted through the detrital food web via the microbial pathway, resulting in higher nutritional quality of leaves and enhanced physiological condition of the shredder. Understanding such subtle relationships is important in determining the impacts of anthropogenic contaminants on freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: The annual variation in population density of the stream macroinvertebrate Gammarus pulex is frequently linked to annual courses of both, resource availability and temperature, which is calling for a mechanistic analysis of the system. To this effect, we coupled a size structured population model for the consumer and a quality structured resource model for its main resource, leaf litter standing stock. We introduced temperature dependence of processes and a lumped internal energy storage for reproduction and energy reallocation. One-year''s field data on body length frequencies of G. pulex, in-stream leaf litter standing stock, leaf fall and temperature were used to calibrate the model for parameters not yet determined in independent experiments. The model reproduced the seasonal pattern of both dynamics acceptably. Independently measured feeding rates of G. pulex were used to confirm the simulated relationship between resource and consumer. Such, we revealed the contribution of temperature and food response to changes in population density. Temperature effects of mortality and reproduction roughly concur with each other, while the energy response of reproduction is pivotally distinct. In the face of model application for projection of climate change effects, further attention should particularly be paid to the specific temperature response of G. pulex and to the contribution of non-shredding decomposition (physical abrasion and microbial colonization) to loss of leaf litter standing stock. [Copyright &y& Elsevier]

Current aquatic environmental risk assessment of plant protection products or biocides does not consider effects on organisms involved in leaf litter breakdown, a fundamental ecosystem process in streams. Therefore, direct ecotoxicological implications of tebuconazole, a frequently used triazole fungicide, on the leaf-shredding amphipod Gammarus fossarum, were assessed. While acute toxicity was low (96h-LC50=1347μg/L), feeding rate, a sublethal endpoint, was significantly reduced after seven days of exposure to 600μg/L. At the same concentration, but during a three week exposure under semi-static conditions, gammarids showed significant reductions in feeding, but also in assimilation and growth. At 200μg/L, however, only assimilation was significantly affected. As these endpoints can be used to evaluate the ecotoxicity of a broad range of chemicals and to deduce possible implications in the functioning of ecosystems, the inclusion of similar experimental set-ups might further improve aquatic environmental risk assessment. [Copyright &y& Elsevier]

Microplastics are an emerging pollutant of high concern, with their prevalence in the environment linked to adverse impacts on aquatic organisms. However, our knowledge of these impacts on freshwater species is rudimentary, and there is almost no research directly testing how these effects can change under ongoing and future climate warming. Given the potential for multiple stressors to interact in nature, research on the combined impacts of microplastics and environmental temperature requires urgent attention. Thus, we experimentally manipulated environmentally realistic concentrations of microplastics and temperature to partition their independent and combined impacts on metabolic and feeding rates of a model freshwater detritivore. There was a significant increase in metabolic and feeding rates with increasing body mass and temperature, in line with metabolic and foraging theory. Experimental warming altered the effect of microplastics on metabolic rate, which increased with microplastic concentration at the lowest temperature, but decreased at the higher temperatures. The microplastics had no effect on the amount of litter consumed by the detritivores, therefore, did not result in altered feeding rates. These results show that the metabolism of important freshwater detritivores could be altered by short-term exposure to microplastics, with greater inhibition of metabolic rates at higher temperatures. The consequences of these metabolic changes may take longer to manifest than the duration of our experiments, requiring further investigation. Our results suggest little short-term impact of microplastics on litter breakdown by gammarid amphipods and highlight the importance of environmental context for a better understanding of microplastic pollution in freshwater ecosystems. Image 1 • Metabolic and feeding rates of detritivores increase with body mass and temperature. • Microplastics pollution reduces metabolic rates but not feeding rates. • Experimental warming alters the effect of MPs on metabolic rates. • Increased MP concentrations only inhibited metabolism at the highest temperatures. • Effects of MPs on organismal physiology thus depend on environmental context. Results summary: Warming alters the effect of microplastics on metabolic rates. Increased microplastic concentrations only inhibited metabolism at the highest temperatures. [ABSTRACT FROM AUTHOR]