Nitrogen Assimilation in the Freshwater Amphipod <i>Gammarus fossarum</i> is Dominated by Animal-Derived Resources.

Abstract

Understanding how consumers assimilate nutrients from different resources is central to predicting nutrient cycling in ecosystems. Amphipods of the genus <i>Gammarus</i> are abundant detritivorous macroinvertebrates in European headwater streams, yet the relative contribution of plant- and animal-derived nitrogen (N) to their biomass remains poorly quantified. We conducted a 14-day laboratory experiment with the amphipod <i>Gammarus fossarum</i> using a stable isotope (δ¹⁵N) tracer approach. Individuals were supplied with either black alder leaves (<i>Alnus glutinosa</i>) or highly ¹⁵N-enriched hay to trace nitrogen assimilation and estimate tissue turnover rates. Despite large differences in isotopic values and nitrogen availability between plant resources, assimilation of plant-derived nitrogen was minimal. Instead, isotopic values indicated that most assimilated nitrogen originated from animal material, most likely through cannibalism. The estimated turnover rate constant was 0.10, corresponding to a nitrogen half-life of 6.7 days, with approximately 76% of metabolically active nitrogen replaced during the experiment. Our results show that nitrogen assimilation in <i>G. fossarum</i> can be dominated by animal-derived resources even when plant detritus is abundant. This highlights the importance of trophic flexibility in detritivorous macroinvertebrates and suggests that animal-derived nutrients may play a larger role in detritus-based stream food webs than commonly assumed.