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Abstract 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 supportgreen food websbased on primary production andbrown food websbased 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.
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Aggregated filter‐feeders govern the flux and stoichiometry of locally available energy and nutrients in rivers
Abstract 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 (, 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 freePlain Language Summarycan be found within the Supporting Information of this article.
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- Award ID(s):
- 1831512
- PAR ID:
- 10453227
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Functional Ecology
- Volume:
- 35
- Issue:
- 5
- ISSN:
- 0269-8463
- Page Range / eLocation ID:
- p. 1183-1195
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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