Seasonality in lake pelagic δ15N values:: patterns, possible explanations, and implications for food web baselines

We studied the seasonal delta N-15 patterns in particulate organic matter (POM < 48 mu m) and crustacean zooplankton collectect from a moderately eutrophic lake. Time series of POM and zooplankton showed consistent differences in delta N-15 Values between POM and different zooplankton taxa, but simular trends of steadily increasing delta N-15 values through summer with peak values in autumn. The observed annual range in delta(15) N was over 25 parts per thousand in POM and almost 17 parts per thousand in cladocera. We relate these marked seasonal changes in delta N-15 values to temporal variation in inorganic nitrogen source for pelagic primary production. Ammonium (NH4+) and nitrate (NO3-) showed distinct delta N-15 values, ammonium being considerably N-15-enriched (delta N-15 values similar to 20-30 parts per thousand) compared to nitrate (< 10 parts per thousand). The peak delta N-15 values in POM and zooplankton call hence be explained by I shift front nitrate uptake to ammonium uptake by phytoplankton, as ammonium concentrations in the Jyvasjarvi epilimnion increase after lake turnover in autumn.. However, such elevated delta N-15 Values were not observed during the high NH4+ concentrations after spring lake turnover, suggesting that the phytoplankton spring bloom was primarily Supported by nitrate. These apparent seasonal differences in nitrogen utilisation reflect variation in species composition of the phytoplankton community. We conclude that seasonal changes in nitrogen Source for phytoplankton can produce considerable differences in their delta N-15 values, which are transmitted up the food chain to crustacean zooplankton. This has important implications for many food web studies using nitrogen stable isotopes, especially when delta N-15 values have to be assigned to a trophic baseline.