Effects of an organochlorine pesticide on different levels of biological organization in Daphnia

The toxicity of the organochlorine pesticide, endosulfan, to Daphnia carinata was measured at three levels of biological organization: allocation of resources to reproduction, growth and reproductive rates, and population dynamics. The mechanisms by which responses at one level of organization manifest at higher levels are discussed. Two experiments were performed: in the first experiment, groups of Daphnia were exposed to endosulfan at one of three concentrations (0, 40, or 80 μg/liter) and two algal food levels (Selanastrum capricornatum) (1 x 10⁵ or 5 x 10⁴ cells/ml). Animals were subsampled daily and length, dry weight, egg number, total egg mass, mean egg weight, and timing of reproduction measured. The fraction of available resources that were allocated to reproduction was calculated. In the second experiment, 12 populations of Daphnia were established in flowthrough culture systems. Populations were allowed to grow for 45 days before addition of endosulfan at 0, 40, 80, or 160 μg/liter. Population density, the number of egg-bearing females, and the chlorophyll a concentration in each culture were measured at weekly intervals. The effects of endosulfan on length, dry weight, brood size, and total egg mass were greater at the high food level; however, the timing of reproduction was significantly delayed for the low- food endosulfan-exposed animals. A model is proposed whereby the low-food animals increased the intermolt period to partially offset the costs of endosulfan toxicity. The fraction of available resources allocated to reproduction increased with each instar, although no endosulfan-induced changes could be detected. Daphnia population densities fluctuated in a cyclic manner. Chlorophyll a concentration also fluctuated with peaks coinciding with depressions in the daphnia population density. The effect of endosulfan was to dampen the amplitude of the cycles.