Effects of Increased Water Temperature on Daphnia Pulex

Techniques were developed to study the effects of increased water temperature on certain zooplankters; specific studies were conducted on Daphnia pulex, an abundant and important zooplankter of the lower Columbia River. Study methods simulated prolonged exposure to constant high tempera· tures in thermal discharges and short exposures to increased temperatures in condensers of cooling systems. Effects were evaluated on the basis of survival and reproduction for periods ranging from 34 to 90 days. The time to death of 50% of the D. pulex, both mature and young, was less than 24 h at temperatures above 27°C. Temperatures of 27°C and below required an exposure of at least 192 h to cause 50% mortality. The young females were more tolerant of temperature increases than older females. The greatest reproduction by older females was at the control temperature (15°C), whereas reproduction by the young females was low at lower temperatures. No reproduction occurred above 27°C. Two groups of D. pulex (one from the Seattle, Wash., area and the other from the Columbia River) studied at increased temperatures for prolonged periods revealed similar patterns of survival and reproduction, but the Columbia River group appeared less tolerant of increased temperatures. A short exposure (15 min) to increased temperatures up to 30°C had little effect on survival and reproduction. It was concluded that temperatures should not exceed 26° or 27°C for prolonged periods or 30°C for more than 15 min to protect D. pulex populations in the river. The lower reaches of the Columbia River (below Portland, Oreg.) support extensive and valuable commercial and sport fisheries as well as other types of recreational activities. This section of the river is also becoming increasingly industrialized. Associated with the industrialization is 1) the extensive use of river water for cooling purposes and 2) the discharge ofheated cooling water back into the river. This increasing use of the river for industrial cooling has created concern that the aquatic biota is endangered by thermal pollution. North and Adams (1969) have described thermal conditions at outfalls and in condenser cooling systems of some California plants. They pointed out that increases of +100F (5.6°C) above normal are considered significant biologically at all seasons of the year. Coutant (1970) presented a diagram of the hypothetical timecourse of acute thermal shock to any organism entrained in condenser cooling water systems that indicates they could be exposed to the maximum increase (10.8°C) for at least 9 min in diffuser systems and to substantial increases 'Northwest Fisheries Center, National Marine Fisheries Service, NOAA, 2725 Montlake Boulevard East, Seattle, WA 98112. Manuscript accepted October 1975. FISHERY BULLETIN: VOL. 74, NO.2, 1976. from 12 to 20 min in the discharge canal system. He also noted the average temperature rise reported is about 10.SOC but may be as great as 16°C. I studied the effect of increased water temperatures on one of the abundant cladocerans of the area, Daphnia pulex. It has been found to be important in the diet of valuable stocks of juvenile chinook salmon, Oncorhynchus tshawytscha, in certain seasons of the year (Craddock et al.2). Cladocerans may be thermally affected by a thermal nuclear power plant where, along with other zooplankton, they may be entrained with intake cooling water and pass through the condenser cooling system encountering sudden and sizable temperature increases. Increased cooling water use by industrial and power plants may increase the temperature of certain areas of the river (bays and eddies) for extended periods and also affect zooplankton. The specific objectives of the study were: 1) to develop techniques for laboratory study of thermal effects on zooplankton and 2) to assess the 'Craddock, D. R., T. A. Blahm, and W. D. Parente,1974. Occurrence and utilization of zooplankton by juvenile chinook salmon in the lower Columbia River. UnpubI. manuscr. Northwest Fish. Cent., Nat!. Mar. Fish. Serv., NOAA, Seattle, Wash.