Effects of copper on the competitive interactions of two species of cladocera

Abstract

The purpose of this study was to assess the influence of copper on the competitive interactions between Daphnia magna and Daphnia pulex. Prior studies indicated that D. magna could develop tolerance to the toxic effects of copper whereas D. pulex could not. Although D. pulex is generally a superior competitor over D. magna, it was hypothesised that the presence of copper may lend competitive superiority to D. magna, thus ensuring its survival under this environmental condition. D. magna and D. pulex were cocultured during this study: (1) in the absence of exogenous copper; (2) after temporary exposure to a sublethal copper concentration (10 μg litre −1); (3) after temporary exposure to a copper concentration lethal to a portion of the exposed population (30 μg litre −1); and (4) after temporary exposure to successive treatments of 10 and 30 μg litre −1 copper. D. pulex populations consistently exceeded D. magna populations when cocultured in the absence of copper or after temporary exposure to 10 μg litre −1 copper. Exposure to 30 μg litre −1 or sequential treatments of 10 and 30 μg litre −1 copper severely reduced the initial population growth of D. pulex whereas D. magna were unaffected. Although the D. magna populations were dominant by day 14 under these conditions, they did not suppress growth of the D. pulex populations. D. pulex subsequently gained population dominance and inhibited further growth of the D. magna populations. The reduced size of the D. magna suggested that D. pulex was outcompeting D. magna for available food. This study, in conjunction with others, suggests that D. magna population growth is augmented by the presence of low levels of copper. In addition, the inhibiting effect of copper on D. pulex population growth relieves D. magna of the competitive inhibition experienced when these species are together.