Abstract
The degree to which N 2 ‐fixation compensates for reduced external nitrogen loading has significant implications for eutrophication recovery. We analyzed 45 years of monitoring data from the shallow, polymictic Lake Müggelsee during which external P‐ and N‐loading declined by about 80% and there were extended periods of N‐limitation in summer. Contrary to expectations, decreasing N availability resulted in significant and prolonged reductions in the biomass of N 2 ‐fixing cyanobacteria of order Nostocales, leading to decreased total heterocyte concentration. Differences were observed within the Nostocales community, as periods of Aphanizomenon dominance were characterized by greater DIN and low light availability. Dolichospermum was more competitive in low DIN conditions, albeit at lower biomasses than previous Aphanizomenon blooms, indicating a significant growth penalty when utilizing N 2 ‐fixation. Increased light availability on account of biomass reductions appeared to facilitate heterocyte production, particularly by Dolichospermum , which produced more heterocytes under a narrower range of DIN concentrations compared to Aphanizomenon , which generally produced fewer heterocytes and formation often preceded growth limiting DIN concentrations. Nutrient supply conditions likely dictate the success of each strategy. Regression tree analyses indicated that light and temperature strongly regulate heterocyte production. Together, these results indicate that compensation of reduced external N‐loading by N 2 ‐fixation was low and heavily dependent on the lakes thermal and light regime, which in turn influences community composition.