and zooplankton during the warmer months. Unfortunately, the results of the 
experiments do not make it clear 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 than 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 Peter Murphy and Don Winslow of the U.S. 
Environmental Protection Agency, Narragansett, R.I., for their help in 
maintaining the microcosms and in the collection of plankton data. Mark 
Wimbush, Randy Watts, Diego Alonso and Michael Prison 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 Rhode Island. 
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