Turbulence and planktonic nitrogen fixation: A mesocosm experiment

Heterocystic species of N-fixing cyanobacteria arc rare in the plankton of most estuaries and coastal seas, and rates of N fixation tend to bc 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. Rccip-rotating grids gencratcd turbulence in 3.0-m ” 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- ’ as determined by two dilTcrent 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- I from the two methods); this higher level of turbulence probably cxcecds 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 mcsocosms, 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 microzone