The population dynamics and energetics of two populations of Diaptomus gracilis have been studied in two Thames Valley reservoirs. The seasonal changes in the standing crop biomass varied from a low of 0.3 Cal/m2 to a high of 7 Cal/m2. Although the population overwintered primarily as adults, reproduction was continuous throughout the year. From the mortality rate the production has been estimated to be 52 and 48 Cal/m2 for the Queen Elizabeth II and King George VI reservoirs, respectively. Feeding and respiratory rates of D. gracilis were measured both in the laboratory and in the field simultaneously with the population dynamics in the reservoir. The feeding rate and the rate of assimilation varied with the species of phytoplankton used as food. Filtering rates varied from 0.1 ml copepod—1 day—1 on bacteria to a high of 2.54 on Diplosphaeria, the latter also having an assimilation efficiency of 78% by D. gracilis. Respiratory rates varied with the season. When the metabolic rate was plotted against body weight, evidence of three regression lines during the year was found, the lowest slope in the winter and the highest in the late spring and early summer. When the regression of oxygen consumption on weight and temperature was calculated, it was necessary to calculate two lines, one for the fall—winter period and one for the spring—summer.

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Daphnia rosea were cultured at 12C for a prolonged period; filtering rates then determined over a range of temperature were significantly different from those of animals cultured at 20C.

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