Impacts of Copper on Daphnia Populations Through the Disturbance of Predator–Prey Interactions

Abstract

The kairomone released by larvae of phantom midges in the genus Chaoborus induces a defensive morphology in Daphnia pulex. This morphology effectively reduces predation pressure and prevents a sharp decline in prey population density. Copper, an anthropogenic pollutant, inhibits the induction of defense mechanisms. However, the consequences of this inhibition at the population level are unclear. In the present study, we confirmed the inhibitory effect of copper on morphological defense using an individual-level experiment (Exp. 1) and further evaluated the effects in a bi-trophic population-level experiment (Exp. 2). In Exp. 1, the induction of neckteeth was inhibited by copper at a concentration of 53.9 nM (Cu²⁺ activity of 0.26 nM). In Exp. 2, no inhibition was detected at a total concentration of 100 nM (on day 12); however, inhibitory effects were observed as Cu²⁺ activity increased. These results indicate that the inhibition of morphological changes cannot be explained by the total copper concentration but by the concentration of free copper ions. The copper-induced suppression of morphological defense resulted in a decrease in the abundance of juvenile Daphnia due to increased predation pressure from Chaoborus larvae. In conclusion, these findings emphasize the potential for environmental copper pollution to alter population dynamics and prey–predator relationships in aquatic taxa.