THE STABLE NITROGEN ISOTOPE RATIO AS A MARKER OF FOOD-WEB INTERACTIONS AND FISH MIGRATION

We used stable nitrogen isotopes to describe the pelagic food-web stru cture of three coastal Baltic Sea areas, each of which was sampled twi ce. Two of the areas were influenced by N-15-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organ ic matter (<35 mu m, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M., relicta), sprat (Sprattus sprattus), smelt (Osmerus epe rlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased delta(15)N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic composi tions, consistent trophic food-web structures were observed on both oc casions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent t rophic fractionation, i.e., differences in delta(15)N between a consum er and its assumed food, averaged 2.4 parts per thousand with a standa rd error of +/-0.5 parts per thousand. Differences between areas in fi sh delta(15)N show young-of-the-year herring, sprat, smelt, and pikepe rch to be relatively non-migratory.