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

The study investigates the relationship between changes in physiological en ergetics of organisms and alterations of growth, development and reproducti on of Daphnia magna. Groups of primiparous daphnids were subjected to 8-day exposures to the heavy metals cadmium and copper or to the cationic surfac tant, cetyltrimethylammonium bromide (CTAB). Energetic alterations were est imated from the measurement of oxygen consumption and feeding activity whic h was performed during the last 3 days of the exposure period and from the calculation of simplified carbon balances. The physiological effects were c ompared to effects on organismal growth and reproduction as obtained from 1 7-day exposure experiments. Toxicant exposure reduced weight and body lengt h of daphnids indicating an impaired growth rate, but effects on total meta bolic costs measured as weight-specific oxygen consumption could not be det ected. Net carbon gain of individuals decreased in a concentration-dependen t way for the tested chemicals reflecting effects on biomass of daphnids. I n the case of cadmium and copper, reproduction (Sigma mx: number of offspri ng per female of age a born during the time interval x - 1 to x, summarised over the entire exposure period) and the estimate for the intrinsic rate o f natural increase, derived from the 17-day exposure-experiment, were affec ted at concentrations comparable to the effect levels as observed for growt h. In the case of copper, the concentrations affecting growth and reproduct ion were close to the 17-day LC,, value. CTAB caused a reduction in body le ngth of primiparous daphnids whereas a decrease in the reproductive perform ance was not apparent. In conclusion. the chemicals did not change metaboli c costs of exposed daphnids as it would be expected as a consequence of res istance or repair mechanisms, however, they induced alterations of SFG, gro wth, reproduction and intrinsic rate of natural increase. These alterations were chemical-specific. The fact that toxicant-related effects on growth a nd reproduction could not be linked to an elevated metabolic rate of daphni ds may indicate that demand side effects occurred early during exposure - b efore the start of respirometric measurements - or that effects on growth w ere caused by an altered energy uptake. The results illustrate the importan ce of trade-off processes in regulating the distribution of energy among gr owth and reproduction of daphnids. (C) 2001 Elsevier Science B.V. All right s reserved.