and zooplankton during the warmer months. Unfortunately, the results of the 

 experiments do not make it dear if the turbulence effect is felt directly by 

 both populations or if the enhancement of phytoplankton growth is the result 

 of lower zooplankton grazing pressure in the more turbulent tanks. The lack of 

 a significant turbulence effect on phytoplankton during the colder months may 

 result from the fact that the phytoplankton and zooplankton virtually do not 

 interact at low temperatures when feeding rates and excretion approach zero 

 (Heinle and Vargo, 1978). During the warmer months there is evidence from 

 some of our other microcosm experiments that the zooplankton are more 

 effective at cropping down phytoplankton tlian a 60 percent artificial level of 

 cropping imposed biweekly (Oviatt et al in press). In some cases, such as the 

 April run, it appeared that the lower grazing pressure might be due to less 

 effective zooplankton feeding as well as to a higher zooplankton mortality in 

 the well mixed microcosms. At this point, however, it is still not clear if this 

 increased zooplankton mortality was the result of a real physiological or 

 behavioral response to the turbulent field or if it was a simple mechanical 

 artifact resulting from the manner in which turbulence was generated. 



Not only is the physical basis of turbulence confusing, but, at least at this 

 point, so are its ecological consequences. The experiments described here are 

 among the first ever reported on this problem, and it is not surprising that so 

 much remains obscure. The results demonstrate the potential significance of 

 turbulence as an ecological factor in pelagic systems and illustrate the 

 importance of carrying out relatively long term (15-30 day) experiments at 

 different times of the year, or at least at different temperatures, when studying 

 the problem. It is also important to explore different ways of generating 

 turbulence as well as the effects of its intensity in experimental ecosystems. 



ACKNOWLEDGEMENTS 



We are grateful to Ken Perez, Peter Murphy and DonWinslowofthe U.S. 

 Environmental Protection Agency, Narragansett, R.I., for their help in 

 maintaining tire microcosms and in the collection of plankton data. Mark 

 Wimbush, Randy Watts, Diego Alonso and Michael Pilson of the Graduate 

 School of Oceanography at the University of Rhode Island struggled with us 

 over the matter of turbulence and its measurement and meaning. Patrick 

 Roques and Dana Kester at the Graduate School of Oceanography contributed 

 to the gas exchange measurements. The research was supported in part by grant 

 No. R-803143 from the U.S.-E.P.A. to the University of Rliode Island. 



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