Seasonality in lake pelagic 15N values: patterns, possible explanations, and implications for food web baselines

Abstract

We studied the seasonal δ 15 N patterns in particulate organic matter (POM <48 μm) and crustacean zooplankton collected from a moderately eutrophic lake. Time series of POM and zooplankton showed consistent differences in δ 15 N values between POM and different zooplankton taxa, but similar trends of steadily increasing δ 15 N values through summer with peak values in autumn. The observed annual range in δ 15 N was over 25 %o in POM and almost 17 %o in cladocera. We relate these marked seasonal changes in δ 15 N values to temporal variation in inorganic nitrogen source for pelagic primary production. Ammonium (NH 4 + ) and nitrate (NO,) showed distinct δ 15 N values, ammonium being considerably 15 N-enriched (δ 15 N values ∼20-30%o) compared to nitrate (<10%o). The peak δ 15 N values in POM and zooplankton can hence be explained by a shift from nitrate uptake to ammonium uptake by phytoplankton, as ammonium concentrations in the Jyvasjarvi epilimnion increase after lake turnover in autumn. However, such elevated δ 15 N values were not observed during the high NH 4 + 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 δ 15 N 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 δ 15 N values have to be assigned to a trophic baseline.