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We tested these hypotheses by combining experimental manipulations of epiphytes and macronutrient fertilization with elemental analyses and community metabarcoding (fungi and prokaryotes). Specifically, we compared wood stick and cellulose decomposition among three treatments: an unaltered trunk section, an epiphyte mat, and a ‘removal treatment’ where an epiphyte mat was removed to test the effect of soil resources. We also performed a factorial fertilization experiment to test the effects of nitrogen (N) and phosphorus (P) on the decomposition of suspended cellulose.

Decomposition rates were fastest on the epiphyte mats, intermediate in the removal treatment and slowest in the controls. Phosphorus addition increased decomposition rates in the fertilization experiment, and greater P concentrations, along with some micronutrients, were associated with increased rates of decomposition on the epiphyte mats and in the removal treatments. Locally dispersed fungi dominated the wood stick communities, indicating that fungal dispersal is limited in the canopy, and fungal saprotrophs were associated with increased rates of decomposition on the epiphytes.

These experiments show that slowed decomposition above the forest floor is caused, in part, by separation from soil resources. Moreover, our findings provide support for both the nutrient limitation and dispersal limitation hypotheses and indicate that mechanisms regulating canopy‐level decomposition differ from those documented on the forest floor. This demonstrates the need for a holistic approach to decomposition that considers the vertical position of necromass as it decomposes. Further experimentation is necessary to quantify interactions between community assembly processes, nutrient availability, substrate traits, and microclimate.

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.