Effects of power-plant cooling systems on marine phytoplankton

The large quattities of marine phytoplankton passing through the cooling systems of two Southern California coastal power plants were found to be greatly reduced in numbers (41.7%) and in volume (33.7%). The biomass killed from June, 1972 to May, 1973 amounted to approximately 1,700 tons of organic carbon. Phytoplankton mortalities were most pronounced from October to December when intake waters of 17° to 20°C were subjected to temperature elevations of 9 to 11C°, and were lowest from January to March when cooler ambient temperatures prevailed. There was no apparent reduction in phytoplankton stocks when the intake water was cooler than 15°C. Surviving cells in 25° and 26.5°C effluent waters were growing three times faster than influent populations, which suggests that power-plant effects on phytoplankton stocks are often short-lived. However, entrainment effects appear very disruptive, in changing the structure of phytoplankton communities and in constantly reducing species diversity (H′). Passage through the condenser tubes affected algal species differentially, killing diatoms in greater numbers (45.7%) than dinoflagellates (32.8%), and reinforcing the dominance of the two major species, Asterionella japonica and Gonyaulax polyedra, that were the most tolerant. The severity of the impact appears to be controlled by two interacting factors: intake water-temperature and magnitude of temperature increase. On this basis, use by coastal power plants of deep-sea water for cooling is strongly advocated.