Elevated dissolved carbon dioxide and associated acidification delays maturation and decreases calcification and survival in the freshwater crustacean Daphnia magna

Abstract Increasing pCO 2 in freshwaters across the globe will likely be accompanied by acidification. Although this has been linked to reduced calcification, growth, and survival in several marine species, similar responses in freshwater organisms remain largely unexplored. Here, we investigated the direct effects of elevated pCO 2 and associated acidification on the water flea Daphnia magna . As a highly efficient filter feeder, this crustacean is a dominant primary consumer in freshwater zooplankton assemblages, capable of controlling algal growth. It has a high calcium content compared to other zooplankton species, which might make it particularly sensitive since calcification may be impaired under elevated pCO 2 . We exposed newly hatched individuals of four clonal lineages to 27,000 ppm pCO 2 corresponding to a pH of 6.7 in a controlled laboratory experiment. Both survival and calcium content were reduced and juveniles grew slower and matured later under elevated pCO 2 . Adult growth rate was not affected by pCO 2 suggesting that surviving adults are less sensitive or may acclimate over time but at the cost of reduced offspring size. We hypothesize that the combination of elevated pCO 2 and low pH interferes with the calcification process, reflected in slower growth, smaller body size and reduced calcification in D . magna . In turn, this might lead to less efficient control of phytoplankton as an ecosystem service and reduce their competitive advantage over other zooplankters with less calcified bodies.