PRIMARY PRODUCTIVITY AND PHYTOPLANKTON COMMUNITY STRUCTURE IN A TEXAS COASTAL COOLING RESERVOIR
WELCH, MARGARET OGDEN
Doctor of Philosophy
Primary productivity was investigated for 24 months in a 404 ha cooling reservoir of a 530 MW electrical generating plant located on the Texas coastal plain. Carbon fixation was estimated in situ at 0.5 meters from the surface and in the laboratory by carbon-14 techniques. For 18 months phytoplankton community structure was analyzed for biomass, density and species diversity. Increased temperature stimulated the rate of carbon fixation. Average overall carbon fixation for the two-year study for Station 1 was 34 mg m('-3) hr('-1) and 24 mg m('-3) hr('-1) for Station 2. There was no significant difference between these means but for 66% of the sampling dates, carbon fixation was significantly higher at the heated station than at the control. Four out of seven in situ experiments in which water samples from Station 1 and Station 2 were reversed and incubated at the opposite station indicated that the productivity difference was due to temperature. In laboratory experiments where natural phytoplankton from each station were incubated at different temperatures, the phytoplankton community of each station responded to temperature similarly. However, the difference in primary productivity ((DELTA)P) between stations was not a simple function of the difference in temperature ((DELTA)T) between stations. Nutrients were low, perhaps due to removal from the water column by the primary producers, but did not appear to be limiting. Values for primary productivity were independent of light intensity and photosynthesis was probably light saturated at the 0.5 m incubation depth. Community structure was similar at both stations. The phytoplankton at both stations also responded similarly to temperature, increasing in activity up to an optimum range of about 24-28(DEGREES)C and decreasing in activity above 30(DEGREES)C. Community structure changed from greens to bluegreens when the temperature optimum was exceeded. Peaks of primary productivity occurred in the spring and in the fall and coincided with the occurrence of optimum water temperatures. Hence, it is suggested that temperature was responsible for the seasonal variation in carbon fixation.