RESPONSE OF A MOBILE INVERTEBRATE 

 TO HETEROTHERMAL CONDITIONS 



S. J. LOZANO and J. F. KITCHELL 



Laboratory of Limnology, University of Wisconsin, Madison, Wisconsin 



ABSTRACT 



Distribution and abundance of Hyalella azteca, a freshwater amphipod, were 

 studied to determine responses to temperature of an organism that spends its 

 entire life cycle in a spatially heterothermal system. The study site, a 190-ha 

 power-plant cooling lake, has a stable 15 C horizontal temperature gradient 

 throughout the year. Seasonal temperatures range from 4 C at the cool end of 

 the lake in winter to 45 C at the warm end in summer. High water temperatures 

 restricted the distribution and abundance of H. azteca. At other seasons, lower 

 temperatures allowed wider distribution of amphipods in the lake. Reproduction 

 and growth were also influenced by the temporal and spatial heterothermal 

 conditions. Reproduction occurred only during short time intervals in spring and 

 fall and differed from the continuous summer reproduction evident in 

 surrounding waters. Field data on distribution, reproduction, and growth, plus 

 laboratory studies, show evidence for a modified life history and temperature- 

 directed movement in H. azteca. 



Temperature is one of the most important environmental factors 

 influencing the life history and distribution of aquatic invertebrates 

 (Jensen et al., 1969; Hutchison, 1976). Growth rate, fecundity, and 

 survivorship are temperature-dependent population features that 

 change seasonally in natural environments. Changes in distribution 

 dxe mediated through behavioral and/or population responses. Motile 

 organisms are capable of moving to more-favorable habitats in 

 response to changes in water temperature. 



Cooper (1965) and Mathias (1971), analyzing the population and 

 production biology of Hyalella azteca, a freshwater amphipod, in 

 lakes with typical annual temperature cycles, found that growth, 

 onset of reproduction, and fecundity in this organism are tempera- 



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