Compound-specific δ15N analyses of amino acids for trophic level estimation from indigenous and invasive freshwater amphipods

Invasion of non-native species in freshwater ecosystems often alters the indigenous macroinvertebrate community and food web structure by changing the resource availability. One of these species is the invasive amphipod Dikerogammarus villosus, whose impact by predation, especially on coexisting amphipods, is still under debate. In this study, we aim to apply compound-specific stable isotope analysis of amino acid δ¹⁵N, which is the state-of-the-art approach for marine systems to estimate trophic positions, (1) to calculate β values (i.e., the differences in δ¹⁵N values of trophic and source amino acids in primary producer) for freshwater systems, based on field samples of freshwater primary consumers (i.e., mussels) from the River Rhine, and (2) use these β values in a case study to calculate the trophic position of the invasive D. villosus in comparison with coexisting indigenous and non-native amphipod species from the river Alb sampled in 2013 and river Speyerbach sampled in 2014, two tributaries of the River Rhine, Central Europe. Our results show that our freshwater β values calculated for six combinations of trophic and source amino acids were lower by between approximately 0.85‰ and 5.67‰ than those found for marine animals in previous studies. This highlights that more attention is needed on the variability of the natural differences in β values between ecosystems. By using the freshwater β values, we showed that the trophic position of D. villosus is comparable to those of coexisting amphipod species. These findings confirm that D. villosus has a flexible and omnivorous feeding strategy like other amphipod species, and suggest that predation is not the main responsible factor for the impact of D. villosus on other species.