Certain biological effects of thermocline shifts

Summary

Under the influence of variable climatic conditions in the spring and due to the peculiar shape of its basin, Lake West Okoboji, Iowa, showed considerable variations in mid-summer temperatures from year to year. The lake either became stratified early in June or stayed mixed until some time in August. When the thermocline formed early between 10–12 m., as usual, it left a deep hypolimnion of relatively small volume. In such years, yellow perch and other fish had weak diel movements from 12 to 5 m. and back. When, in other years, the epilimnion was deep and ample oxygen extended to 20 m, the same fish showed more pronounced on- and off-shore migrations and spent part of the day at the 20 m. contour where they foraged on the tendipedid larvae which reached greatest abundance there. In years of early stratification, however, when the midge larvae were fewer and showed some concentration between 10 and 12 m., they were not a preferred item in the diet of the perch.

The behavior of the fish is discussed with physiological limitations in mind and their ability to contract an oxygen debt as well as the adjustment of swim bladder volume are considered as critical factors. Some speculation is made about the possible role of density-dependent water movements on the distribution of midge larvae; the concentration zones of Tendipes larvae coincided closely with the depth to which the wind-driven circulation extended and where water density shifted most pronouncedly.