EFFECTS OF POWER-PLANT OPERATION 611 



each year, as did the desmid-containing order Zygnematales, the 

 flagellate-celled order Volvocales, and the classes Dinophyceae and 

 Cryptophyceae. Other classes fluctuated in numbers, but none 

 exhibited any notable decrease over more than a 1-year period. The 

 only class to exhibit a sharp change exclusively between the 

 preoperational and postoperational periods was the Cyanophyceae, 

 which more than doubled in number of taxa with this change. 



DISCUSSION 



In comparison with other North Carolina lakes, the water of 

 Belews Lake is very clear, with a green tint similar to that found in 

 the state's mountain reservoirs. Phytoplankton abund£inces were too 

 low to have any important effect on light penetration. Seasonally, 

 water turbidity was lowest in the warmer months, the period when 

 phytoplankton abundance was normtilly greatest. 



Inorganic nitrogen and soluble phosphorus levels were low. 

 Soluble phosphorus values were always less than 10 /jg/liter. In 

 Belews Lake, phosphorus was determined to be the critical limiting 

 element responsible for inhibiting phytoplankton productivity (Weiss 

 et al., 1978a). Mean inorganic nitrogen and soluble phosphorus levels 

 both decreased gradually through the 4 years of study, as did mean 

 total phytoplankton density and enrichment index levels. These 

 tendencies were all indicative of a trend toward increasing oligo- 

 trophy in the lake. Shannon— Weaver diversity indexes exhibited a 

 general increase in values throughout the study period, a further 

 reflection of this trend. A low nutrient flux would lead to higher 

 diversity because of the greater overlap between successive species 

 populations caused by low grovd;h rates (Moss, 1973). Also contrib- 

 uting to the increasingly higher diversity values was the steady yearly 

 increase in the number of taxa present; this suggests a gradual 

 maturing of the system. 



The lowest individual Shannon— Weaver diversity index and the 

 largest density, biovolume, and enrichment index values occurred in 

 the first month of the study, while the lake was still filling and 

 receiving nutrients washed and leached from newly flooded topsoils. 

 Filling of the lake was completed in the following spring, and by the 

 second summer only reduced levels of the blue-green alga were 

 responsible for the first summer's bloom. Enrichment indexes at all 

 other times were very low and indicated a lake free of any organic 

 enrichment problems. Mean values for Secchi depth, total phos- 

 phorus, total phytoplankton density, and biovolume and enrichment 

 indexes were together within the range indicative of an oligomeso- 



