592 LENAT 



for polycentropid larvae (which includes Cyrnellus) is at the 

 periphery of macrophyte beds. Sandberg (1969) found that most 

 Orthocladinae (a group including Eukiefferella) are associated with 

 vegetation zones. It is, therefore, reasonable to suggest that the 

 distribution of these organisms is controlled by the distribution of 

 macrophyte growths. The low densities of the organisms in the 

 discharge basin may result from inhibition of macrophytes by 

 power-plant effluents. However, increases in temperature may have 

 been equally important. Macrophytes (Potamogeton diuersif alias) 

 became abundant at discharge stations in year 7, but the densities of 

 E. nigricans and C. marginalis remained very low (Lenat, manuscript 

 in preparation). 



Changes in community structure were assessed by the use of 

 percent similarity coefficients (PSC), which vary from to 100%. 

 The PSC values were computed for all possible station pairs with 

 each quarterly sampling. This process generated 12 matrixes, which 

 were reduced to three matrixes by the computation of yearly 

 averages. These data were further reduced by the computation of 

 wiihin-group and between-group averages for M and D stations 

 (Table 4). 



TABLE 4 



AVERAGE BETWEEN-GROUP AND WITHIN-GROUP 

 SIMILARITY COEFFICIENTS 



Within-group comparisons for M stations show a slight increase 

 from year 4 to year 6, indicating an increase in similarity at control 

 stations. This is probably caused by the spread of macrophytes in 

 this area. At D stations, however, a decline in similarity was noted 

 during the same period. The imposition of a thermal gradient in this 

 area appears to have induced a greater biological gradient. Note, also, 

 that between-group similarities declined slightly from year 4 to year 

 6. Since it is not possible to evaluate these data statistically, only the 

 general trends are discussed here. 



Other summary statistics are presented in Table 5 — total num- 

 bers of organisms collected, taxa richness. Shannon— Weiner diver- 



