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

Limnological theory posits that phosphorus (P) limits primary production in freshwater lakes, in part because fixation of atmospheric nitrogen (N ₂ ) can compensate for limitations in nitrogen (N) supply to phytoplankton. However, quantitative estimates of the degree to which N ₂ fixation satisfies planktonic N demand are rare. Here we used biweekly sampling during summer in seven lakes over 2 decades to estimate both planktonic N ₂ 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 N ₂ 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. 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 N ₂ -fixation, it does not appear to eliminate N limitation of phytoplankton growth in these P-rich hardwater lakes.