614 CAMPBELL 



the base-line period and continued through the postoperational 

 years. Nannochloris has been observed to increase in dominance at 

 the expense of less-tolerant taxa after condenser passage (Knight, 

 1973). The similar perennial dominance of Peridmium inconspicuum 

 in year 6 suggests that it, too, may be a taxon more tolerant of these 

 conditions. 



Increases in the number of taxa occurred each year for the 

 important orders in the Chlorophyceae and for the class as a whole, 

 as well as for the Dinophyceae. These trends were established before 

 the postoperational period, as were the decreases in total density and 

 enrichment indexes and the increases in total biovolume and 

 diversity. The effects of power-plant operation, primarily that of 

 increased homogeneity, have been mainly superimposed on these 

 changes. Only the blue-green algae, which are noted for their 

 tolerance of high temperatures, exhibited a sharp rise in the number 

 of taxa exclusively between preoperational and postoperational 

 periods, but their abundances remained uniformly low. The general 

 long-term successional changes in the phytoplankton community 

 appear to have been only somewhat accelerated after power-plant 

 operation. 



In each of the 4 years, the two most important taxa by density 

 were small -celled organisms whose largest dimensions were imder 10 

 )um. Such "ultraplankton" cells have high surface-to-volume ratios, 

 high respiration-to-weight ratios, aiid, consequently, rapid turnover 

 rates. This makes them potentially highly productive taxa capable of 

 rapidly taking advantage of increases in limiting nutrients. By 

 biovolume, the trend was toward dominance by larger-celled dino- 

 flagellates. Members of this class produce algal toxins that encourage 

 rejection of the cells by grazers, and their larger size has adaptive 

 value both where there is pressure by filter-feeding zooplankton 

 capable of grazing only small algae and where the lower respiration 

 per unit weight of larger cells allows survival under high-temperature, 

 low-light, and low-nutrient conditions, although with consequently 

 slower turnover rates and lower productivity (Porter, 1977). Their 

 motility would also enable them to actively migrate vertically 

 throughout the thermocline to take advantage of the greater nutrient 

 concentrations available in the hypolimnion (Tailing, 1971; 

 Kamykowski and Zentara, 1977). Thus the increased dominance of 

 the dinoflagellates by biovolume is more likely to reflect their greater 

 resistance to grazing pressure and the stresses of condenser passage 

 and their greater capacity to deal with low nutrient levels than to 

 indicate their importance to the productivity of the aquatic food 

 web. 



