Fish supply distinct nutrients and dissolved organic matter composition relative to ambient concentrations in northern lakes

Abstract Nutrient cycling in aquatic ecosystems is often facilitated by consumers through excretion. While fish play established roles in nitrogen (N) and phosphorus (P) cycling, their impact on carbon (C) cycling remains underexplored. This role could be significant in ecosystems where dissolved organic matter (DOM) regulates light and nutrient availability. However, the influence of DOM concentration (often measured as dissolved organic carbon [DOC]) and composition on biological processes is complex, with organisms showing diverse responses, sometimes leading to nonlinear patterns. We investigated fish excretion of N and P in 11 boreal lakes with varying DOC concentration and DOM composition and assessed fish excretion of DOC and DOM composition in a subset of three of those lakes representing low, medium, and high DOC levels. In line with our expectations, fish mass‐normalized N excretion rates had an inverse unimodal relationship with ambient DOC up to 7.3 mg C L −1 , yet with little evidence supporting a relationship between fish P excretion rates and ambient DOC. Fish exhibited lower DOC excretion rates in medium DOC environments compared to high DOC environments and excreted more terrestrially‐ and microbially‐derived DOM in low DOC environments. Overall, our findings underscore the dynamic role of fish as conduits of nutrients and carbon in aquatic ecosystems, shaped by ambient nutrients and light conditions, and physiological processes that warrant further investigation.