Spatial and temporal variation in nitrogen fixation and its importance to phytoplankton in phosphorus‐rich lakes

1. 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.
2. 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.
3. 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 (S_NAM = –0.03 and S_Heterocyte = –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.
4. 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.