408 PECK AND WARREN 



apparently related to the mechanical effects of passage through the 

 power plant. 



The failure of the heat-depressed cut samples to recover any 

 nitrate reductase activity after 24 hr of incubation suggests that the 

 ability of this phytoplankton population to synthesize nitrate 

 reductase after heat shock was seriously altered. During incubation at 

 intake temperatures, cells were contained within dialysis tubing, and 

 it is possible that this containment influenced the nitrate reductase 

 activity of the cells. Ammonium concentrations in August did not 

 exceed 5 [ig atoms/liter. However, the complicating effect of 

 ammonium on changes in the nitrate reductase activity of contained 

 cells must be recognized when interpreting any failure of nitrate 

 reductase activity to recover. Vargo, Hargraves, and Johnson (1975) 

 reported that detrital deposition and epiphytic growth on dialysis 

 membranes at summer temperatures may decrease the membrane 

 area available for nutrient flux in less than 3 days. This would tend 

 to decrease the nitrate reductase present by limiting the nitrate 

 entering the dialysis membrane. In our study this was not the case. 

 Intake controls held at ambient intake temperature for 24 hr show 

 no change in nitrate reductase activity. 



ACKNOWLEDGMENTS 



We thank E. J. Carpenter of the Marine Sciences Research 

 Center, State University of New York, Stony Brook, Long Island, 

 and Ian Morris and T. T. Packard of the Bigelow Laboratory for 

 Ocean Sciences, West Boothbay Harbor, Me., for their comments on 

 the original manuscript. Gratitude is also expressed to Jerry Lamb of 

 the Naval Underwater Systems Center, New London, Conn., for 

 statistical advice. 



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