The juvenile growth rate of Daphnia:: A short-term alternative to measuring the per capita rate of increase in ecotoxicology?

The juvenile growth rate (g(j)) of Daphnia is a good predictor of the per capita rate of increase (r), which is considered the ecologically most meaningful parameter. The hypothesis is tested that g(j) may be used as a surrogate for r in short-term ecotoxicological tests to replace the time-consuming population-based testing. The effects of 3,4-dichloroaniline (DCA) on various reproductive parameters of Daphnia magna, including r, net reproductive rate in 21 days (R21), age at first reproduction (AFR), adult survival, and egg viability, were measured simultaneously with g(j). Reproductive parameters were not affected below a concentration of 9 mug/L DCA, but all of them responded strongly above this concentration. DCA reduced the total number of eggs produced only moderately, but induced strong egg mortality in particular in the first brood. Because egg mortality is an important factor, measures of sublethal toxic effects must be based on viable eggs. Although (g) is a good predictor of individual fitness of Daphnia under variable natural conditions when egg mortality is negligible, it is not a good predictor of the effects of xenobiotic stress; hence it cannot replace r. Short-term tests based solely on either somatic growth or egg viability are not sufficiently correlated with r to give ecologically meaningful results, but protocols based on the number of viable eggs in the first clutch and the time to release of the first neonates may be an alternative to the 21-day test or longer life-table experiments.