Ecological scale mediates nonlinear relationships of animal nutrient excretion with dissolved organic matter

Abstract Ecosystems can exhibit nonlinear dynamics resulting from interactions and feedbacks across various ecological scales. For example, large‐scale nutrient cycling incorporates small‐scale biogeochemical processes that may vary locally. However, it remains uncertain whether biogeochemical processes such as animal‐mediated nutrient recycling exhibit nonlinearities across space and levels of biological organization. Animal‐mediated nutrient recycling may vary with animal taxonomic rank, trophic guild and abiotic factors such as light and nutrient supply. In aquatic ecosystems, dissolved organic matter (DOM) modulates light and nutrient supply, which may indirectly affect animal‐mediated nutrient recycling. In this study, we examined nitrogen (N) and phosphorus (P) excretion of fish and mayflies and the stoichiometry of mayfly, periphyton and seston in 11 streams that varied in dissolved organic matter (DOM) composition and concentration, the latter measured as dissolved organic carbon (DOC). We analysed reach‐scale animal nutrient excretion at two levels (individual and community) and by taxonomic rank (vertebrate and invertebrate). We found nonlinear relationships between animal nutrient excretion and DOC and DOM that followed monotonic, unimodal and bimodal patterns and varied with levels of biological organization and taxonomic rank. We also identified two critical DOC thresholds (4.5–5.5 and 6.5–7.5 mg C L −1 ) beyond which animal nutrient excretion and biomass shifted positively or negatively and that were within the range of previously identified thresholds with DOC. However, we could not establish a causal relationship between animal nutrient excretion and mayfly or periphyton stoichiometry. Animal‐mediated nutrient recycling is responsive to changes at fine ecological scales, while the magnitude and direction of this change is taxon‐specific. Overall, our study provides a new set of evidence that nonlinearity is inherent to dynamic ecological systems and that its patterns ought to be studied across various ecological scales and in the context of global change.