Effect of sublethal concentrations of chlorpyrifos on three successive generations of Daphnia carinata.

Effects of sublethal concentrations of chlorpyrifos (ranging from 0.005 (0.01 LC(50)) to 0.500 microg/L (1 LC(50))) on population characteristics of individual cultures of Daphnia carinata were investigated over 21 days with subsequent testing of the two next generations. The endpoints for the first and second generations observed were survival, fecundity, time to first brood, and number of offspring per female. The results were incorporated into the computation of the intrinsic rate of natural increase for daphnids in each of the treatments. Exposure to chlorpyrifos affected survival and fecundity of animals in the first generation. In the second generation, the most affected endpoint was time to the first brood with an indication of hormesis. The LC(50) tests were then conducted using animals of the third generation from each of the exposures in individual tests. Despite the absence of a negative effect of chlorpyrifos in the second generation, results of testing the third generation showed constant significant decline in LC(50) from control daphnids through to 0.1 LC(50) preexposed daphnids (0.1 LC(50), 0.05 microg/L, being the highest concentration in which animals survived exposure to the toxicant in the second generation).