The response of nitrogen fixing cyanobacteria to a reduction in nitrogen loading

Due to their competitive advantage when inorganic nitrogen (N) sources are scarce, it is widely assumed that the abundance and N₂-fixation rate of N₂-fixing cyanobacteria (Nostocales) would increase in response to reduced N loading. If realized, this response would render efforts to improve water quality by N reduction ineffective. To assess this, we performed a microcosm experiment using water from an N limited lake, Langer See, in which phosphorus loading was held constant, while a gradient of decreasing N loading was simulated. Nostocales biovolume increased over time in all microcosms, regardless of the N addition rate, so that no difference in Nostocales biovolume developed between high and low N microcosms. In contrast, the biovolumes of other taxa (especially non-fixing cyanobacteria) were lower at low N addition rates. N₂-fixation increased in low N microcosms, compensating for 36% of the difference in N addition by day 6. However, this compensation rate was achieved at Nostocales biovolumes far higher than those typical in the studied lake. At biovolumes typical for summer the compensation rate would only account for 7% of the omitted N. If these results were to hold over longer time scales, in shallow polymictic lakes like Langer See, reduced N loading may lower both in-lake N concentrations and biovolumes of non-fixing phytoplankton without significantly impacting Nostocales biovolume.