Consumer‐Driven Nutrient Cycling Interacts With Light to Impact Stream Sediment Nutrient Dynamics

ABSTRACT Consumers can strongly impact nutrient cycling in freshwater ecosystems. However, their effects are mediated by interactions between species traits and environmental conditions. While the direct effects of consumer‐driven nutrient dynamics (CND) on ecosystem function have been extensively investigated, their indirect effects have received less attention. In streams, dense and diverse freshwater mussel assemblages can create nutrient cycling hotspots. Here, we performed a series of in situ dark–light incubation experiments using benthic metabolic chambers, along with nutrient excretion and organic matter (OM) biodeposition assays, to examine how four mussel species ( Amblema plicata , Fusconaia cerina , Lampsilis ornata , and Pustulosa kieneriana ) with contrasting traits—specifically, ammonium (NH 4 + –N) and soluble reactive phosphorus (SRP) excretion and OM biodeposition rates—affect sediment NH 4 + –N, SRP, and microbial N 2 cycling. We hypothesized that: (H1) mussels directly enhance NH 4 + –N and SRP fluxes through excretion, with the magnitude of their effect depending on species‐specific excretion rates; (H2) mussels indirectly stimulate microbially driven N 2 fluxes via NH 4 + –N excretion and OM biodeposition, with the extent of these influences varying by species‐specific rates of both processes; and (H3) light would modulate all nutrient fluxes by enhancing benthic photoautotrophic activity. We observed interspecific differences in mass‐specific NH 4 + –N excretion and OM biodeposition rates, but not in mass‐specific SRP excretion rates. Our results support our first hypothesis (H1), as mussels directly influenced NH 4 + –N and SRP fluxes through excretion, and species with greater biomass contributed more to these fluxes. Conversely, our second hypothesis (H2) was not supported, as mussels did not indirectly affect ambient or potential microbial N 2 fluxes. We also found support for our third hypothesis (H3), as light significantly modulated all nutrient fluxes, likely by enhancing benthic photoautotrophic activity. Our findings demonstrate that mussel CND can influence stream nutrient cycling at the patch‐scale and that their direct and indirect effects are modulated by light availability. This study highlights the importance of integrating local environmental conditions into trait‐based research to fully understand the impact of CND on ecosystem function.