518 LOZANO AND KITCHELL 



TABLE 4 



DIFFERENTIAL MOVEMENT OF H. azteca BY TEMPERATURE 

 AND SIZE DURING 3-hr EXPERIMENTS* 



'N = 35 amphipods per experiment; 6 replicates at each temperature. 



DISCUSSION 



In Lake Columbia the distribution and growth of H. azteca are 

 restricted by the thermal characteristics of the cooling lake. High 

 summer temperatures exclude amphipods from 50% or more of the 

 lake; large numbers of amphipods were found only at the coolest 

 sampling site from March until November. Although growth rate and 

 duration of instar are accelerated by increased temperature (Cooper, 

 1965; Strong, 1973), length frequency distributions of samples show 

 greater growth at each instar at cooler temperatures in Lake 

 Columbia. This suggests that optimal temperatures for growth are 

 exceeded during summer months in Lake Columbia. 



There is also evidence from field and laboratory results that cool 

 water temperatures influence the activity of H. azteca. The marked 

 decline in amphipod abundance in the cooling lake in winter, when 

 water temperatures were below 7°C, could be a result of decreased 

 activity and could, therefore, result in slow colonization of samplers. 

 Winter mortality for amphipods is low (Cooper, 1965) and can be 

 discounted. Cool water temperatures were associated with lower 

 colonization rates in laboratory experiments. The colonization rate 

 at 13°C was still very high, however, with a median of 2.6 amphipods 

 per hour. This suggests that virtually all colonization of samplers 

 must cease at temperatures less than 7°C since samplers located near 

 the intake, where temperatures were less than 10°C, yielded an 

 average of one or less amphipod per station in January and 

 February. 



