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

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

Source: Fundamental and Applied Limnology / Archiv für Hydrobiologie, Volume 172, Number 3, August 2008 , pp. 255-262(8)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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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: AMMONIUM; NITRATE; NITROGEN UPTAKE; PHYTOPLANKTON; STABLE ISOTOPES; ZOOPLANKTON

Document Type: Research article

DOI: http://dx.doi.org/10.1127/1863-9135/2008/0172-0255

Publication date: 2008-08-01

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