RESPONSE OF A MOBILE INVERTEBRATE 



513 



configuration and shallow depth, vertical mixing is easily accom- 

 plished with very little thermal and chemical stratification. Except 

 for temperature, there are no consistent horizontal gradients of 

 physical and chemical parameters ( Andren et al., 1976). Seasonal and 

 spatial variation in water temperature between outfall and intake are 

 shown in Fig. 2. During periods of power-plant shutdown (October 

 and June), homothermal conditions existed in Lake Columbia. Water 

 temperatures measured in the settling basin were 5 to 6°C cooler 

 than intake temperatures. 



40 



(J 



LU 



tr 30 



< 



?r. 20 



1- 10 



1 



2 



z 



O 3 

 H 



5 4 

 9 5 



< 



C/3 



^•--\ 





N--'\ _ 



i 



I 



500 

 number/sampler 



J I I I I I I 1 1 1 1 1 L. 



AUG. SEPT. OCT. NOV. DEC. 

 1975 



JAN. FEB. MAR. APR. MAY JUNE JULY AUG. 

 1976 

 DATE 



Fig. 2 Seasonal changes in distribution and abundance of H. azteca 

 compared with the annual temperature regime in Lake Columbia. 



METHODS AIMD MATERIALS 



Field Techniques 



Ten sampling sites were chosen at distances 300 to 700 m apart 

 along the west and center dikes of Lake Columbia (Fig. 1). Three 



