Nitrogen Assimilation in the Freshwater Amphipod Gammarus fossarum is Dominated by Animal-Derived Resources

Understanding how consumers assimilate nutrients from different resources is central to predicting nutrient cycling in ecosystems. Amphipods of the genus Gammarus 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 Gammarus fossarum using a stable isotope (δ¹⁵N) tracer approach. Individuals were supplied with either black alder leaves (Alnus glutinosa) 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 G. fossarum 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.