Isotopic evidence for zooplankton as an important food source for the mysid Paramysis lacustris in the Curonian Lagoon, the South-Eastern Baltic Sea

To evaluate the contribution of mesozooplankton to the diet of a coastal mysid Paramysis lacustris in the Curonian Lagoon, stable isotope analysis of mysids and mesozooplankton was conducted from July to October 2004. Nitrogen isotopic composition (δ15N) of both mysid bodies and dissected stomachs was analyzed and their δ15N values were compared to those in ambient mesozooplankton. We also evaluated changes in phyto- and mesozooplankton assemblages during the study period and related the community dominance structure to the isotopic composition of the mesozooplankton and mysids. Stable isotope data suggest that during the ontogenetic development, mysid diet changes to include a larger proportion of mesozooplankton and that mysids need to attain a threshold size of 8.7 ± 0.7 mm to feed on mesozooplankton. In adult mysids, δ15N values of the dissected stomachs and stomach content were indistinguishable from those of ambient mesozooplankton, indicating nearly complete reliance on mesozooplankton prey. A significant depletion of 15N in both mesozooplankton and mysids occurred during the study period, correlating with cyanobacteria abundance in the lagoon. These findings contribute to the understanding of P. lacustris' feeding biology, and the pelagic food web structure and functioning in the coastal environment.