RESPONSE OF A MOBILE INVERTEBRATE 515 



RESULTS 



Field Results 



Seasonal distribution and abundance of H. azteca, presented in 

 Fig. 2, are related to seasonal and spatial differences of water 

 temperature measured in the outfall and intake channels. Only in 

 July 1976 were amphipods found at sampling sites that had water 

 temperatures above 33°C, which is the upper lethal temperature, as 

 determined in laboratory studies (Sprague, 1963; Bovee, 1949). We 

 found no amphipods at 50% of the sampling sites during the warmest 

 months of the year. As temperatures declined, either from seasonal 

 variations or power-plant shutdovms, amphipods began to colonize a 

 greater proportion of the cooling lake. We concluded that amphipods 

 moved from two sites nearest the intake to station 2, which is near 

 the outfall. We assumed that any amphipods in water pumped over 

 the power-plant condensers would die because of the large rise in 

 temperature. Since entrainment can be discounted as a mechanism of 

 dispersal, the population of amphipods in the cooling lake recolo- 

 nized over 3000 m in 2 months. 



Amphipod movement was found to be related to size; larger 

 H. azteca moved more rapidly during the fall migration. There was a 

 significant difference between median head lengths of animals 

 collected at stations 1 to 5 and 6 to 9 in the fall of 1975 

 (Mann— Whitney U test, Table 1). Analyses of collections from 

 October, a period of homothermal conditions, support the observa- 



TABLE 1 



MEDIAN HEAD LENGTHS OF AMPHIPODS, H. azteca, IN 



NEWLY COLONIZED, OUTFALL AREAS (STATIONS 1 TO 5) 



OF LAKE COLUMBIA AS COMPARED TO INTAKE AREAS 



(STATIONS 6 TO 9)* 



*Intake water temperatures are equivalent to or slightly greater than 

 ambient for nearby lakes, and temperatures near the outfall are 5 to 10 C 

 above ambient. 



fP < 0.05 for all size differences between the two groups of stations. 



