Turbulence and planktonic nitrogen fixation: A mesocosm experiment

Abstract

Heterocystic species of N‐fixing cyanobacteria are rare in the plankton of most estuaries and coastal seas, and rates of N fixation tend to be concomitantly low in the water columns of these systems. High turbulence in estuaries and coastal seas is commonly cited as a factor excluding planktonic, heterocystic cyanobacteria. We investigated the effects of turbulence in a 7‐week‐long mesocosm experiment. Reciprocating grids generated turbulence in 3.0‐m3 tanks containing water and plankton from Oneida Lake. Four mesocosms received a high level of turbulence (mean values of 0.2 and 0.7 mW kg−1 as determined by two different methods); such turbulence is much higher than has been observed in lakes and is at the upper end of that found in estuaries strongly influenced by tides. Four other tanks received levels of turbulence that were higher yet (mean estimates of 1.8 and 3.8 mW kg−1 from the two methods); this higher level of turbulence probably exceeds that found in coastal waters except in areas influenced by surf or in shallow estuaries with very significant tides. Heterocystic, planktonic cyanobacteria bloomed and fixed N in all eight mesocosms, and turbulence had no significant effect on N fixation. Rates were quite high, indicating no major deleterious effect of even extremely high turbulence on this process. The turbulence in our mesocosms was great enough to have prevented the formation of anoxic microzones on the surface of cyanobacterial cells. Thus, our results indicate that such microzones are not essential for high rates of planktonic N fixation by heterocystic cyanobacteria and that the relative absence of N fixation by these organisms in coastal seas is not due to a lower probability of such microzones developing in marine systems than in freshwaters.