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Spatial and temporal variation in nitrogen fixation and its importance to phytoplankton in phosphorus‐rich lakes
- Hayes, Nicole M., Patoine, Alain, Haig, Heather A., Simpson, Gavin L., Swarbrick, Vanessa J., Wiik, Emma, Leavitt, Peter R.
- Freshwater biology 2019 v.64 no.2 pp. 269-283
- Nostocales, basins, freshwater lakes, models, nitrogen, nitrogen content, nitrogen fixation, phosphorus, phytoplankton, primary productivity, spring, summer, temporal variation, trophic relationships
- Limnological theory posits that phosphorus (P) limits primary production in freshwater lakes, in part because fixation of atmospheric nitrogen (N₂) can compensate for limitations in nitrogen (N) supply to phytoplankton. However, quantitative estimates of the degree to which N₂ fixation satisfies planktonic N demand are rare. Here we used biweekly sampling during summer in seven lakes over 2 decades to estimate both planktonic N₂ fixation and phytoplankton N demand. We further assessed the ability of biologically fixed N to satisfy N needs of primary producers in productive hardwater lakes. Phytoplankton N requirements, derived from estimates of phytoplankton productivity and N content, were moderately synchronous (S = 0.41) among lakes (ca. 0.1–9.2 mg N m–³ hr–¹). In contrast, rates of N₂ fixation determined using isotopic natural abundance method (NAM; 0.002–3.2 mg N m–³ hr–¹), or heterocyte‐based calculations (0.10–1.78 mg N m–³ hr–¹), varied asynchronously (SNAM = –0.03 and SHₑₜₑᵣₒcyₜₑ = –0.11) among basins, accounted for a median of 3.5% (mean 11.3% ± 21.6) of phytoplankton demand, and were correlated to the abundance of Nostocales cyanobacteria when analysed using generalised additive models. Overall, the total mass of fixed N accounted for a median of only 3.0% of the spring standing stock of total dissolved N in study lakes (mean 7.5 ± 12.1%), with higher relative importance of fixed N in highly productive downstream lakes. Thus, while fixed N helps sustain primary productivity, particularly in years with high rates of N₂‐fixation, it does not appear to eliminate N limitation of phytoplankton growth in these P‐rich hardwater lakes.