|Beschreibung:||For decades, ecologists have studied trophic interaction in aquatic systems, and described the food web structure of dominant ecological groups based on gut content analyses. The conception of these interactions may, however, be biased by the lack of couplings to the microbial food web and direct errors in diet analyses (e.g. differences in digestion rate between food types). In this thesis, I examined the planktonic food web by analyzing the trophic structure (i.e. trophic levels) with an alternative technique, the abundances and fractionation of stable isotope. I used natural abundances of stable isotopes of nitrogen (delta15N) and carbon (delta13C) to describe the food web structure of mesozooplankton communities in the Central Baltic Sea (Bornholm Basin, Gdansk deep and Gotland Basin) and Southern North Sea (German Bight). I focused on assessing spatial and seasonal isotopic variation with respect to trophic levels of the dominant mesozooplankton species in both marine systems. The stable isotopes of nitrogen and carbon provide powerful tools for assessing trophic levels of and carbon flow to consumers in the food web.