The Stable Nitrogen Isotope Ratio as a Marker of Food-Web Interactions and Fish Migration

Abstract

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (<35 μm, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased δ15N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in δ15N between a consumer and its assumed food, averaged 2.4‰ with a standard error of ±0.5‰. Differences between areas in fish δ15N show young-of-the-year herring, sprat, smelt, and pikeperch to be relatively non-migratory.