Using stable nitrogen isotopes to evaluate the relative importance of external and internal nitrogen loadings on phytoplankton production in a shallow eutrophic lake (Lake Mikata, Japan)

Ryo Sugimoto, Takahisa Sato, Takehito Yoshida and Osamu Tominaga

Limnol. Oceanogr., 59(1), 2014, 37-47 | DOI: 10.4319/10.2014.59.1.0037

ABSTRACT: The seasonal variability of the stable nitrogen isotope (δ15N) in phytoplankton and dissolved inorganic nitrogen (DIN) in the shallow, eutrophic Lake Mikata was examined to assess the relative importance of external and internal DIN supply on phytoplankton production. δ15N values of phytoplankton varied seasonally from 2.1‰ in the early summer to 8.0‰ in autumn with an annual average of 3.9‰, which was similar to the annually averaged δ15N of DIN in a river entering the lake. The estimated DIN flux from the river was always higher than that from the adjacent Lake Suigetsu, suggesting that DIN from the river was a primary N source. The seasonality of phytoplankton δ15N was largely determined by the relative strengths of external DIN fluxes (river and adjacent lake) and internal N cycling. A decrease of δ15N in phytoplankton during summer suggested an assimilation of low δ15N of regenerated within the lake. In contrast, an increasing δ15N trend in autumn suggested the assimilation of high δ15N of from Lake Suigetsu. provided 39–62% of the N assimilated by phytoplankton in the autumn, with isotopic discrimination ranging from 8.3‰ to 12.6‰. These results show the importance of intra-annual variability in the concentrations and δ15N of major nutrient sources on the δ15N of phytoplankton.

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