516 LOZANO AND KITCHELL 



tion that differences in size are a result of differential movement of 

 larger amphipods and are independent of temperature. 



Growth is also temperature dependent. Amphipods grew larger 

 per instar during the cooler months in fall (Table 2); in November 

 each instar was at least 19% larger than the corresponding instar 

 collected in August. Sizes of instars at different dates were found to 

 be significantly different for all age groups (Kruskal— Wallis H test). 



We compared the seasonal abundance of amphipods at station 9, 

 near the intake, with that at station 10, in the settling basin and, 

 thus, not influenced by the thermal effluent from power-plant 

 operations (Fig. 3). Both populations of amphipods exhibited a 

 bimodal shift in seasonal abundance, but with different seasonal 

 maxima. Young instars (amphipods with 16 or fewer antennal 

 segments) were found in the cooling lake in the fall (September 

 through October) and spring (March through May) but were absent 

 from samples collected during the warm summer months. In the 

 settling basin young instars were present throughout the summer and 

 early fall but were proportionately most abundant in June. 



TABLE 2 



SEASONAL SIZE DIFFERENCES OF H. azteca IN 

 LAKE COLUMBIA FROM ALL STATIONS (1 TO 9) 



Median head length,* mm 



*P < 0.05 for differences among all dates. 



Laboratory Results 



Amphipods rapidly colonized samplers in the laboratory during 

 12-hr tests (Table 3). Although neither temperature nor density 

 influenced colonization rate during the 12-hr experiments, we did 

 find a significant difference (P < 0.05) in colonization rate at two 

 temperatures (13 and 21°C) in the 3-hr experiment (Mann— Whitney 

 U test. Table 4). Amphipods that colonized the samplers were larger 

 than those remaining outside the samplers (P < 0.05). 



