NITRATE REDUCTASE ACTIVITY 395 



water cools by only about 0.5 to 1.0° C in passage through the pond 

 to the end of the cut. 



Sampling 



Sampling was conducted to assure, as much as possible, that the 

 same water masses (intake, discharge, and cut) were sampled from 

 each station. All samples were collected around noon because 

 maximum solar radiation occurred near noon, and maximum nitrate 

 reductase activities have been reported about noon. Commencement 

 of sampling deviated by a maximum of 30 min before or after noon 

 during the entire study period. Twelve casts were made to a depth of 

 1 m with a 1-liter Kemmerer sampler at the intake, discharge, and cut 

 (Fig. 1). Intake samples were the controls. Discharge samples were 

 collected within 15 min after collection of intake samples. Cut 

 samples were collected no more than 1.5 hr after collection of 

 discharge samples. 



All treatments, with and without temperature and chlorine 

 additions, were randomized and replicated throughout the 1-year 

 study, and samples discussed here represent trends observed. Samples 

 were collected five times monthly from April through September 

 1973, three times in October 1973, twice monthly in February, 

 March, and November 1973, once monthly in January and December 

 1973, and once in January 1974. On several occasions samples were 

 collected from three depths in the middle of the effluent pond (1, 

 15, and 30 m) to determine nitrate reductase activity and primary 

 productivity in the water column of the pond. Also at each station 

 (intake, discharge, and cut) salinity and temperature were measured 

 with an induction salinometer. Percent surface illumination at each 

 depth was calculated according to Holmes (1970) from Secchi disk 

 lowerings and was validated by a submarine photometer. 



Chlorine 



Chlorine was added at the plant intake at rates of 95 to 1100 

 g/hr to give 0.1 to 1.2 ppm chlorine. Free residual chlorine was 

 measured with an orf/io-tolidine chlorometric method at a 1-m depth 

 at intake, discharge, and cut. This method is not sensitive enough to 

 measure residual chlorine below 0.05 ppm. 



Nitrate reductase activity and primary productivity were mea- 

 sured at various levels of continuous chlorine applications ranging 

 from 0.0 to 1.2 ppm. Continuous chlorination was begun at least 12 

 hr before measurements were made. Samples without chlorination 

 were taken no less than 12 hr after chlorine applications ceased. This 



