Thermal Effects of a Nuclear Power Plant on the Mississippi River at Monticello, Minnesota

The thermal effects of the nuclear power plant at Monticello, Minnesota were assessed by comparing postoperational (1971-1972) data to baseline data (1968-1971) on the Mississippi River. Macroinvertebrate samples from artificial substrates placed upstream were compared to those from the heated zone downstream. Trichopteran biomass increased, ephemeropto heated water were observed for Trichoptera in the July-October period, Ephemeroptera were most affected in the May-August period. Dipterans declined significantly during September through November. The number of species was not affected by the heat. Growth rates of macroinvertebrate groups in upstream and downstream areas were not significantly different in four of seven 35-day cycles observed. The trichopteran seasonal cycle showed higher peaks of numbers and depressed peaks of weights in the heated zone, but the overall pattern was the same in heated and and unheated areas. Cycles of ephemeropterans and dipterans had depressed summer peaks in the heated zone and the general pattern was disrupted, especially in the spring and early summer. Flow characteristics caused the heated water to remain along the bank from the point of discharge to about 3 miles downstream. A comparison of the number of fish caught by electrofishing along the opposite sides of the river showed that black crappies, smallmouth bass, and walleyes were more numerous on the heated side, and were often attracted to the discharge canal from adjacent areas of the river. Rough fish were generally indifferent to the heated water in most areas, but carp were more attracted by the discharge canal than other species. Normal movements of minnows and other population characteristics seemed unaffected by the temperature rise. Food studies of the shorthead, carp, and the minnows and darters showed that the carp ate Cladophora when it was available, but were more opportunistic than the other species which were generally insectivourous. Fish in the heated water had a lower percentage of stomachs containing food than the same species in unheated areas, but the sample size was insufficiently large to test the significance of the difference. Effects of the power plant upon other organisms, such as birds have been minor.Water Resources Research Cente