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An annual temperature cycle exists in fresh waters. This cycle, associated with periods of winter and summer stagnation, and spring and fall turnover, is governed by the addition and loss of heat. The amount of heat (in gram-calories) needed to raise the water temperature from its winter minimum to its summer maximum, is known as the "annual energy" or "heat budget." The heat-gain occurs in the spring, and effects spring-turnover; the heat-loss occurs in the fall, and effects fall-turnover. Over a period of years, the heat gained equals the heat lost (Allee 1939.) Plankters in their life cycles also exhibit seasonal cycles closely correlated with the temperature cycle. There are spring-and autumn maxima and winter-and summer minima in populations.

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