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

Abstract

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.

Hydrological fluctuations modulate phototrophic responses to nutrient fertilization in a large and shallow lake of Southwest China

“…Thereafter, this shallow (Table 2). This pattern suggests that non-nitrogen 571 fixers can bloom with varying N:P ratios when there is sufficient P supply, as is found in other large shallow lakes (Xie et al, 2003;Xu et al 2010;Hayes et al, 2019).…”

“…528large shallow lakes(Xie et al, 2003;Xu et al 2010;Hayes et al, 2019). 529Our comparative study highlights the increasing effects of hydrologic fluctuations and regional climate variability on variation in phototrophs over the last few decades.…”

Effects of lake warming on the seasonal risk of toxic cyanobacteria exposure

“…Microcystin concentrations were frequently below the limit of detections (337 of 604 samples), reflecting seasonal patterns in cyanobacteria abundance (Vogt et al 2018;Hayes et al 2019), variable cell quotas of microcystin (Donald et al 2011), and detection techniques with high LoD relative to biologicallyrelevant concentrations. A variety of statistical approaches have been used to account for the high frequency of samples below the LoD in microcystin studies including:…”

Seasonal variation in effects of urea and phosphorus on phytoplankton abundance and community composition in a hypereutrophic hardwater lake

“…increase during September (Donald et al., 2013). Heterocytes are not routinely enumerated in Wascana Lake, although they have been recorded for a decade in both Katepwa and Crooked lakes, downstream of Wascana Lake (Hayes et al., 2019). The pelagic invertebrate community is composed mainly of large‐bodied Daphnia spp.…”