The ability of the benthic cyanobacterium Lyngbya wollei to fix nitrogen was studied using field samples and axenic cultures. L. wollei was collected and isolated from Lake Okeechobee, Florida, where it forms extensive mats. Rates of acetylene reduction up to 39.1 nmol mg dry wt-' h -' were observed for field samples. The maximum observed rate of acetylene reduction in axenic laboratory cultures was 200 nmol mg dry wt -' h -'. Aerobic conditions limited nitrogen fixation activity, but darkllight cycles promoted the development of activity. Reduced oxygen levels appeared to be required for the development of significant levels of nitrogenase activity. The level of irradiance also had a significant impact on the level of activity. The potential significance of nitrogen fixation to Lyngbya production is discussed.

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SynopsisTemperatures preferred by four groups of adult mountain whitefish, Prosopium williarnsoni, collected in October before spawning, in December after spawning, in late winter, and in spring were determined in the laboratory in a horizontal gradient. Acute temperature preference based on fish tested soon after capture and final preferendum estimates were 12.8 and 17.7"C, respectively (pre-spawning), 9.6 and 11.9"C (post-spawning), 10.8 and 9.9"C (win'ter), and 16.4 and 16.3"C (spring). Seasonal influence on temperature preference was evident on the basis of differences in final preferenda, covariance analysis of responses of laboratory-acclimated fish, and temperature preference of fish held at ambient river temperatures. Fish of the post-spawning and winter groups preferred lower temperatures than did those of pre-spawning and spring groups. Temperatures preferred by pre-spawning fish were too high for embryo survival. Caution is necessary in predicting thermal preference on the basis of a sample collected at one time of year or stage of sexual development.