Original paper

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

Syväranta, Jari; Tiirola, Marja; Jones, Roger I.

Abstract

We studied the seasonal δ15N 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 δ15N values between POM and different zooplankton taxa, but similar trends of steadily increasing δ15N values through summer with peak values in autumn. The observed annual range in δ15N was over 25 ‰ in POM and almost 17 ‰ in cladocera. We relate these marked seasonal changes in δ15N values to temporal variation in inorganic nitrogen source for pelagic primary production. Ammonium (NH4+) and nitrate (NO3-) showed distinct δ15N values, ammonium being considerably 15N-enriched (δ15N values ~20-30 ‰) compared to nitrate (< 10 ‰). the peak δ15N 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 Jyväsjärvi epilimnion increase after lake turnover in autumn. However, such elevated δ15N 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 δ15N 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 δ15N values have to be assigned to a trophic baseline.

Keywords

ammoniumnitratenitrogen uptakephytoplanktonstable isotopeszooplankton