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In freshwater ecosystems, consumers can play large roles in nutrient cycling by modifying nutrient availability for autotrophic and heterotrophic microbes. Nutrients released by consumers directly support green food webs based on primary production and brown food webs based on decomposition. While much research has focused on impacts of consumer driven nutrient dynamics on green food webs, less attention has been given to studying the effects of these dynamics on brown food webs.Freshwater mussels (Bivalvia: Unionidae) can dominate benthic biomass in aquatic systems as they often occur in dense aggregations that create biogeochemical hotspots that can control ecosystem structure and function through nutrient release. However, despite functional similarities as filter‐feeders, mussels exhibit variation in nutrient excretion and tissue stoichiometry due in part to their phylogenetic origin. Here, we conducted a mesocosm experiment to evaluate how communities of three phylogenetically distinct species of mussels individually and collectively influence components of green and brown food webs.We predicted that the presence of mussels would elicit a positive response in both brown and green food webs by providing nutrients and energy via excretion and biodeposition to autotrophic and heterotrophic microbes. We also predicted that bottom‐up provisioning of nutrients would vary among treatments as a result of stoichiometric differences of species combinations, and that increasing species richness would lead to greater ecosystem functioning through complementarity resulting from greater trait diversity.Our results show that mussels affect the functioning of green and brown food webs through altering nutrient availability for both autotrophic and heterotrophic microbes. These effects are likely to be driven by phylogenetic constraints on tissue nutrient stoichiometry and consequential excretion stoichiometry, which can have functional effects on ecosystem processes. Our study highlights the importance of measuring multiple functional responses across a gradient of diversity in ecologically similar consumers to gain a more holistic view of aquatic food webs. [ABSTRACT FROM AUTHOR]

Positive biodiversity–ecosystem functioning (BEF) relationships observed in experiments can be challenging to identify in natural communities. Freshwater animal communities are disproportionately harmed by global change that results in accelerated species loss. Understanding how animal‐mediated ecosystems functions may change as a result of global change can help determine whether biodiversity or species‐specific conservation will be effective at maintaining function. Unionid mussels represent half of imperiled species in freshwater ecosystems globally and perform important ecological functions such as water filtration and nutrient recycling. We explored BEF relationships for 22 naturally assembled mussel aggregations spanning three river basins. We used the Price equation to partition the contributions of species richness, composition, and context dependent interactions to two functions of interests: spatially‐explicit standing‐stock biomass (indirect proxy for function) and species‐specific nitrogen (N) excretion rates (direct measure of N recycling). Random and non‐random species loss each reduced biomass and N recycling. Many rare species with low contributions to biomass contributed to standing‐stock biomass in all basins. Widespread species had variable function across sites, such that context dependent effects (CDEs) outweighed richness effects on standing‐stock biomass in two basins, and were similar to richness effects in the third. Richness effects outweighed CDEs for N recycling. Thus, many species contributed a low proportion to overall N‐recycling; a product we attribute to the high evenness and functional effect trait diversity associated with these communities. The loss of low‐functioning species reduced the function of persisting species. This novel result using observational data adds evidence that positive species interactions, such as interspecific facilitation, may be a mechanism by which biodiversity enhances ecosystem functions. Our work stresses the importance of evaluating species‐specific contributions to functions in diverse systems, such as nutrient cycling when maintaining specific animal‐mediated functions is a management goal because indirect proxies may not completely characterize BEF relationships. [ABSTRACT FROM AUTHOR]

In streams, unionoid mussels and fish form aggregations that exert bottom‐up and top‐down effects on food webs, but the magnitude and spatial extent of their effects are controlled by species traits. Sedentary mussels live burrowed in the sediment in patchily distributed dense aggregations (mussel beds) where they filter seston and provide a local, relatively constant nutrient subsidy. In contrast, fish move on and off mussel beds, and thus comprise a transient nutrient subsidy.We asked how overlap between fish and mussels influences nutrient recycling and resource distribution in streams. We conducted an 8‐week study in experimental streams where we created mussel beds (comprised of two species, Actinonaias ligamentina and Amblema plicata), manipulated the occurrence of a grazing minnow (Campostoma anomalum), and tracked nutrient (nitrogen and phosphorus) and resource (algae, detritus, and chironomids) abundance up and downstream of the mussel beds.In general, neither consumer had strong effects on the concentration or spatial distribution of nutrients. Water turnover time in our experimental streams may have diluted fish and mussel nutrient excretion effects, making it difficult to detect spatial patterns during a given sampling period.Fish controlled the abundance and productivity of algae. In treatments without fish, large mats of filamentous algae formed early in the experiment. These algae senesced, decomposed, and were not replaced. When fish were present, algae consisted of attached biofilms with consistent biomass and spatial distribution over time.Although previous work has shown that mussels can have strong, seasonal bottom‐up effects on both primary and secondary production, our results suggested that adding grazing mobile fishes, led to a more consistent and homogenous supply of algal resources. Because mussels rarely occur in the absence of fish, considering their combined influence on ecosystem dynamics is likely to be important. [ABSTRACT FROM AUTHOR]

Biogeochemical cycling has often been characterized by physical and microbial processes, yet animals can be essential mediators of energy and nutrients in ecosystems. Excretion by aggregated animals can be an important local source of inorganic nutrients in green food webs; however, whether animals are a source of dissolved energy that can support brown food webs is understudied.We tested whether animal aggregations are a substantial flux of bioavailable dissolved organic matter (DOM) by studying spatially stable, biogeochemical hotspots formed by filter‐feeding freshwater mussels. We used parallel‐factor analysis to quantify DOM fluorescent components composition of mussel excretion and expected digestive breakdown of particulate food sources would lead to excretion of labile DOM. Next, we combined measured excretion rates of DOM, ammonium (NH4+, N) and phosphorous (SRP; P) for 22 species with biomass estimates for 14 aggregations to quantify contributions of DOM, N and P to local availability. Because mussels occupy distinct stoichiometric niches, we anticipated that differences in species biomass and assemblage structure would elicit different flux and stoichiometries of aggregate excretion.Aggregate dissolved organic carbon (DOC) excretion was minor (1%–11%) compared to N (12%–2,860%) and P (1%–97%), yet generalities across assemblages emerged regarding organic matter transformation by mussels towards labile protein‐like compounds compared to abundant aromatic, humic compounds in ambient water.Aggregate excretion of labile DOM was a substantial pool of bioavailable energy, contributing 2%–114% of local labile DOM. Spatial differences in assemblage structure led to strong differences in aggregate flux and stoichiometry driven by biomass and stoichiometric trait expression of species with contrasting dominance patterns.Under the nutrient conditions of our study (high C:nutrient), biogeochemical hotspots associated with low‐trophic position animal biomass may indirectly control energy flow to the brown food web by shifting C:nutrient stoichiometry available to microbes or directly by increasing the flux of microbially available DOM. Collectively, our results highlight a potentially substantial flux of labile energy and nutrients to microbial communities through the transformation of ingested organic matter by aggregations of animals and emphasize that shared functional trait classification may not translate into shared ecological function. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Animals contribute significantly to nutrient cycling through excretion, but most studies consider their effects under relatively benign abiotic conditions. Disturbances such as drought may alter animals' nutrient contributions through shifts in species composition and biomass. Headwater streams are particularly vulnerable to extreme climate events and thus might show rapid changes in stream biota and their ecosystem effects.We tested how biomass and subsequent ecosystem effects (nutrient cycling) of an intermittent prairie stream community changed during a drought. We quantified the biomass and contributions to nutrient cycling for assemblages comprising fishes, crayfish, and tadpoles in 12 isolated pools over 3 months encompassing the harshest drought on record for Kings Creek, KS, U.S.A. We predicted that macroconsumer biomass would decline with pool surface area and that differences in macroconsumer biomass and taxonomic composition would lead to different contributions of pool assemblages to nutrient cycling.The biomass of pool assemblages declined with decreasing pool size, a pattern apparently driven by mortality, emigration, or metamorphosis. We also observed a change in assemblage structure of drying pools during drought relative to pool size, shifting dominance toward species with more drought‐resistant traits. Accordingly, assemblage nitrogen (N) excretion rates declined as pool biomass was reduced, leading to a 58% reduction in N available to epilithic biofilms. Phosphorus (P) excretion rates declined from June to July, but increased in August, as species with high P excretion rates maintained similar proportional biomass and biomass of a non‐native fish increased. Molar N:P of pool assemblage excretion declined significantly throughout the drought and coincided with loss of southern redbelly dace (Chrosomus erythrogaster: Cyprinidae).Animal‐mediated nutrient cycling was altered by the loss of biomass and stoichiometric traits of taxa that differed in their occurrences and ability to tolerate abiotic conditions during drought. Elevated availability of dissolved N in isolated pools may increase N uptake rates by biofilms during drought conditions, indicating the importance of N excreted by aggregated macroconsumers, especially those with unique stoichiometric traits. While the significance of shifts in the composition of freshwater communities to ecosystems is not entirely known, additional losses in ecosystem function and changes in community structure may follow episodes of severe drought. [ABSTRACT FROM AUTHOR]

Aggregations of freshwater mussels create patches that can benefit other organisms through direct habitat alterations or indirect stimulation of trophic resources via nutrient excretion and biodeposition. Spent shells and the shells of living mussels add complexity to benthic environments by providing shelter from predators and increasing habitat heterogeneity. Combined, these factors can increase primary productivity and macroinvertebrate abundance in patches where mussel biomass is high, providing valuable subsidies for some fishes and influencing their distributions. We performed a 12-wk field experiment to test whether fish distributions within mussel beds were most influenced by the presence of subsidies associated with live mussels or the biogenic habitat of shells. We used remote underwater video recordings to quantify fish occurrences at fifty 0.25-m2 experimental enclosures stocked with either live mussels (2-species assemblages), sham mussels (shells filled with sand), or sediment only. The biomass of algae and benthic macroinvertebrates increased over time but were uninfluenced by treatment. We detected more fish in live mussel and sham treatments than in the sediment-only treatment but found no difference between live mussel and sham treatments. Thus, habitat provided by mussel shells may be the primary benefit to fishes that co-occur with mussels. Increased spatiotemporal overlap between fish and mussels might strengthen ecosystem effects, such as nutrient cycling, and the role of both fish and mussels in freshwater ecosystems. [ABSTRACT FROM AUTHOR]