EFFECTS OF THERMAL ALTERATION 577 



summer the snail population was smaller, but channel 2 again had the 

 highest average, 400 snails/m*^ , compared with 200 for channel 1 and 

 280 for channel 3. Throughout the 27-month study, channel 3 

 generally had the lowest population of snails of the three channels. 

 Physa was the most prevalent snail, but Fenissia, Gyraulus, and 

 Lymnaea were also found. 



Bivalves 



Corbiciila manilensis (Philippi), the Asiatic clam, dominated the 

 clam population in all channels. Channel 2 had many more Corbicula 

 than did channel 1 or 3. 



Mayflies 



The highest population of mayflies for the plate samplers was in 

 channel 1, with an intermediate population in channel 2 and the 

 smallest in channel 3. The dredge data indicated similar values, 

 except that during the winter of 1976 channel 2 had more mayflies 

 than channel 1. 



Chironomids 



Dredge samples from channel 3 had fewer chironomid larvae than 

 did either of the other channels. Except for the initial periods when 

 the chironomids were becoming established in both channels 1 and 3, 

 there was only one period, spring of 1976, when the population of 

 chironomids in channel 1 fell below that of channel 3. The 

 predaceous chironomids were more numerous in channel 3 than 

 channel 1. Predaceous chironomid larvae include the genera 

 Clinotanypus, Procladiiis, Psectrotanypus, and Ablabesmyia (Darby, 

 1962). 



Ce ra topogon ids 



Ceratopogonids (Diptera) were more prevalent in the plate 

 samplers during summer months in channels 2 and 3 than in 

 channel 1. No significant trends for the nine periods could be 

 established for the dredge samples. 



DISCUSSION 



The decrease in water temperature the winter of 1976 probably 

 caused the lower D's foimd in all channels for the spring of 1976. 

 The high temperatures in the simimer of 1976 seemed to affect the 



