EFFECTS OF POWER-PLANT OPERATION ON ZOOPLANKTON 623 



averages of the replicate counts were transformed and used. The year 

 X month interaction term was very conservative owing to the large 

 and variable seasonal differences of the densities within a year. As a 

 result we thought that using this as the error term would not increase 

 the probability of a Type I error. 



Tests were also conducted to assess seasonal and spatial homoge- 

 neity. Spatial homogeneity refers to a similarity of taxon density 

 between stations in any given month within a year. Using a data set 

 of 19 taxa, 8 stations, and 3 years and using the station x month 

 interaction term of each year as the error term, we tested the 

 significance of station as a variable within each year. Seasonal 

 homogeneity refers to taxon density being similar between stations 

 during the same seasons of the year. The station x month interaction 

 term in ANOVA is a measure of the degree to which changing 

 densities at stations parallel one another over the year. We tested for 

 a significant decline in the station x month interaction by using the 

 year 6 station x month interaction as the error term and compared it 

 to the same term in year 4 and year 5. We partitioned the variance 

 appropriately within each year and then tested for a significant 

 decrease. Each taxon was tested separately. 



RESULTS AND DISCUSSIOIM 

 Changes in Environmental Parameters 



Temperature 



At Belews Lake the normal temperature pattern has been altered 

 by the release of heated effluents from the Belews Creek Steam 

 Station. Station 1904 did not stratify after power-plant operation 

 and was vertically isothermal or nearly so because of the mixing 

 effect of the discharged heated effluent. The heated effluent is 

 discharged near station 1907 from the connecting canal as a surface 

 plume about 5 m deep and dissipates its heat rapidly. A gradient of 

 less-heated water extends from station 1907 down lake toward the 

 dam station, 1118, and the intake station, 3. In the summer this 

 gradient (using surface temperatures from August, year 6) ranged 

 from 32.2 to 30.0 and 29.3°C, respectively. The average tempera- 

 tures that the zooplankton experience are less than this since the 

 heated water overlays cooler water deeper within the euphotic zone. 

 The lake is stratified by April and usually remains so until late 

 November. Under these conditions the average euphotic-zone tem- 

 peratures are some 4°C less than surface temperatures. The tempera- 



