Tracing Consumer-Derived Nitrogen in Stream Communities

The flux of consumer-derived nutrients has been increasingly recognized as an important ecosystem process, yet few have quantified the impact of these fluxes on food webs. The objective of our study was to determine the importance of unionid mussel-derived nitrogen (MDN) relative to ecosystem demand and the food web. We fed mussels (Lampsilis siliquiodea, n=249) an enriched δ¹⁵N algae and placed them into an N-limited stream for 63 days. Mussel hemolymph was non-lethally sampled over the course of the experiment to measure tissue turnover of δ¹⁵N and excretion experiments were done to model the amount of N mussels provided in comparison to stream N uptake demand. Multiple food web components were sampled twice prior and five times following the mussel addition to trace the δ¹⁵N entering the food web. Our mussel excretion rates in comparison to areal uptake demand suggested that mussel excretion can account for 40% N demand in this stream. Our enrichment showed that MDN was entering the food web and supplied up to 19% of the N in the food web near the mussel bed. Our results show that N supplied by mussels can be an important nutrient subsidy that provides food web support.