Molybdenum and sulfate as controls on the abundance of nitrogen‐fixing cyanobacteria in saline lakes in Alberta

We studied 13 saline lakes in Alberta to test the hypothesis that molybdenum availability influences the abundance of planktonic, N‐fixing cyanobacteria in saline ecosystems. Our earlier work in oxic seawater showed that the availability of Mo is controlled in part by the ratio of sulfate to molybdenum because sulfate inhibits the assimilation of molybdate. The SO42−: Mo ratio in seawater is very high relative to most freshwater lakes—a finding that is consistent with the scarcity of planktonic, N‐fixing cyanobacteria in coastal marine ecosystems. This ratio is constant in seawater, however, limiting a test of our hypothesis in marine systems. These Alberta salt lakes provide a more robust test in saline systems. The ratio of sulfate to molybdenum within any given saline lake was relatively constant over a summer season, but the ratio between lakes varied and ranged from values typical of freshwater lakes to values higher than in seawater. N‐fixing cyanobacteria are significant fractions of the plankton in six of the 13 lakes we studied and are rare or absent in the others. The SO42−: Mo ratio was a strong predictor of the abundance of planktonic, N‐fixing cyanobacteria. Sulfate or Mo concentrations alone, however, were not. This finding is consistent with our hypothesis that sulfate can control Mo availability in oxic waters. Phosphorus concentrations, and the ratio of N to P, were not good predictors of the abundance of N‐fixing cyanobacteria in these saline lakes, as they often are in freshwater lakes. The differences between predictions from a freshwater, P regression model and actual abundances of N‐fixing cyanobacteria in the saline lakes were best explained by the SO42−: Mo ratio.