conditions, this may have a significant impact on local fish populations. 

 Some adult fishes of many species suffer impingement, even though they can 

 swim against the flow of intake water (Marcy 1976) . 



Changes in zooplankton populations in large bodies of water receiving heated 

 effluents have not been observed in two intensive studies in Montsweag Bay 

 (McAlice et al. 1970-1977) and in Pennsylvania (Schuler 1970-1976). These 

 studies suggest that, at least in the above areas, temperature increases from 

 power plant discharges are too small to have significant effects on the 

 zooplankton population. This may be a result of proper design and location of 

 the power facilities and discharge plumes studied. 



According to Burton and coworkers (1976), the thermal effects on benthic 

 organisms either directly or indirectly are very localized and present little 

 change in species composition over large areas. Water temperature changes 

 (i.e., 2° to 5° C; 4° to 9° F) will have the most serious effect on species 

 that are living close to their thermal limit which is probably true of a few 

 Maine species (e.g., the quahog; Larsen, unpublished ) . Benthos, which are 

 normally exposed to greater temperature variations in the intertidal and 

 estuarine environments, are more resistant than subtidal marine organisms to 

 the harmful effects of temperature fluctuations (Prosser 1973). 



Benthic communities may be stressed by low dissolved oxygen levels caused by 

 high summer water temperatures and heated effluent. 



Marked changes in number and composition of species have been associated with 

 areas receiving water heated 6 to 10°C (11 to 18°F) higher than normal (Logan 

 and Maurer 1975). However, these effects on benthos are minimized, since 

 heated water is less dense and rises as it moves away from the source 

 (Warinner and Brehmer 1966) . 



Benthic studies of sedentary fauna in Montsweag Bay showed no evidence of 

 significant detrimental effects that could be attributed to the discharge of 

 thermal effluents from Maine Yankee Atomic Power Plant (Dean and Ewart 1978). 



No major adverse effects have been noted on larval polychaetes entrained in 

 the cooling effluent of Maine Yankee Atomic Power Company; however, effects on 

 the other major taxa have not been studied. 



PCBs . PCBs are long-lived organochlorines that are not soluble in water. 

 Once in a system, PCBs rapidly become bound to living or dead organic matter 

 or to bottom sediments. They are mostly industrial byproducts and still are 

 entering aquatic systems in the U.S. in large amounts (Ohlendorf et al. 1978). 

 Most PCBs enter the aquatic systems through industrial waste, sewage, sludge, 

 electrical transformers and conductors, and the burning of plastics. They 

 occur in highest concentrations around highly industrialized areas (Howe et 

 al. 1978). 



PCBs are toxic to aquatic invertebrates and vertebrates (Duke et al. 1970; and 

 Hanson et al. 1974) and can be transferred and accumulated in food webs, which 

 may include people (Sayler et al. 1978). (Bioaccumulation is the accumulation 

 of a substance in an animal at a greater concentration than in its food or the 

 next lower trophic level so that the members of the highest trophic level 

 within a food chain have the highest concentration.) PCBs have been reported 



3-32 



