NITRATE REDUCTASE ACTIVITY 407 



temperature. The assay used measures the amount of enzyme present 

 not its in vivo activity. The temperature shock of August through 

 October is, therefore, either inactivating the enzyme already present 

 or slowing or stopping the synthesis of the enzyme. This could be 

 either a generalized effect on all protein or a specific effect for 

 nitrate reductase. 



Nitrate reductase is an inducible enzyme, and, in an inducible 

 enzyme system, temperature can influence enzyme synthesis (Lang- 

 ridge and McWilliam, 1967). In some instances this effect may be 

 the result of a progressive loss of the response of the control system 

 to the inducer. Thus the temperature-related decrease in phytoplank- 

 ton nitrate reductase activity observed at the cut in August could be 

 the result of high-temperature impairment of the system controlling 

 production of the enzyme. 



In December 1973 and January 1974, ammonium concentrations 

 were high and nitrate reductase activities were undetectable at the 

 cut (Figs. 3 and 5). Also, as can be seen in Figs. 3 to 5, high 

 ammonium concentrations at the cut during two sampling dates in 

 the middle of September were accompanied by low nitrate reductase 

 activities (0 [ig atoms NOT" cell"' hr~' ) even though the intake and 

 cut temperatures were the same (18°C) because the plant was not 

 operating. Eppley, Coatsworth, and Solorzano (1969) found that 

 ammonium concentrations in the range of 5 to 15 ^Lig atoms /liter 

 inhibited nitrate reductase activity in sainples of tropical Pacific 

 phytoplankton. Therefore, from September through January, 

 ammonium concentration appears to be responsible for nitrate 

 reductase depression at the cut, whereas in August depression is 

 probably caused by temperature. 



To determine the correlation among nitrate reductase activity, 

 temperature, and ammonium statistically, we used a stepwise 

 regression analysis (the May 2, 1966, version of the Health Sciences 

 Computing Facility, UCLA at Connecticut College). Temperature 

 and nitrate reductase activity and ammonium and nitrate reductase 

 activity are significantly correlated. 



Ammonium concentrations exceeding 8 /^g atoms/liter observed 

 at the cut from September through January could be the result of 

 increased copepod excretion or decay in the depths of the effluent 

 pond. Carpenter, Peck, and Anderson (1974), working at this same 

 site, reported that live copepods sink relatively rapidly after passing 

 through the power plant. They found a large number (54 to 63%) 

 of dead copepods in the deep water of the effluent pond, in contrast 

 to a few dead (6 to 13%) at the surface. Conditions in the effluent 

 pond itself are not responsible for copepod mortality. Mortality is 



