Alterations of physiological energetics, growth and reproduction of Daphnia magna under toxicant stress.

The study investigates the relationship between changes in physiological energetics of organisms and alterations of growth, development and reproduction of Daphnia magna. Groups of primiparous daphnids were subjected to 8-day exposures to the heavy metals cadmium and copper or to the cationic surfactant, cetyltrimethylammonium bromide (CTAB). Energetic alterations were estimated from the measurement of oxygen consumption and feeding activity which was performed during the last 3 days of the exposure period and from the calculation of simplified carbon balances. The physiological effects were compared to effects on organismal growth and reproduction as obtained from 17-day exposure experiments. Toxicant exposure reduced weight and body length of daphnids indicating an impaired growth rate, but effects on total metabolic costs measured as weight-specific oxygen consumption could not be detected. Net carbon gain of individuals decreased in a concentration-dependent way for the tested chemicals reflecting effects on biomass of daphnids. In the case of cadmium and copper, reproduction ( summation operatormx: number of offspring per female of age x born during the time interval x-1 to x, summarised over the entire exposure period) and the estimate for the intrinsic rate of natural increase, derived from the 17-day exposure-experiment, were affected at concentrations comparable to the effect levels as observed for growth. In the case of copper, the concentrations affecting growth and reproduction were close to the 17-day LC(50) value. CTAB caused a reduction in body length of primiparous daphnids whereas a decrease in the reproductive performance was not apparent. In conclusion, the chemicals did not change metabolic costs of exposed daphnids as it would be expected as a consequence of resistance or repair mechanisms, however, they induced alterations of SFG, growth, reproduction and intrinsic rate of natural increase. These alterations were chemical-specific. The fact that toxicant-related effects on growth and reproduction could not be linked to an elevated metabolic rate of daphnids may indicate that demand side effects occurred early during exposure - before the start of respirometric measurements - or that effects on growth were caused by an altered energy uptake. The results illustrate the importance of trade-off processes in regulating the distribution of energy among growth and reproduction of daphnids.