Direct and indirect effects of near-future pCO2 levels on zooplankton dynamics

Abstract

Ocean acidification has direct physiological effects on organisms, for example by dissolving the calcium carbonate structures of calcifying species. However, non-calcifiers may also be affected by changes in seawater chemistry. To disentangle the direct and indirect effects of ocean acidification on zooplankton growth, we undertook a study with two model organisms. Specifically, we investigated the individual effects of short-term exposure to high and low seawater pCO2, and different phytoplankton qualities as a result of different CO2 incubations on the growth of a heterotrophic dinoflagellate (Oxyrrhis marina) and a copepod species (Acartia tonsa). It was observed previously that higher CO2 concentrations can decrease phytoplankton food quality in terms of carbon:nutrient ratios. We therefore expected both seawater pCO2 (pH) and phytoplankton quality to result in decreased zooplankton growth. Although we expected lowest growth rates for all zooplankton under high seawater pCO2 and low algal quality, we found that direct pH effects on consumers seem to be of lesser importance than the associated decrease in algal quality. The decrease in the quality of primary producers under high pCO2 conditions negatively affected zooplankton growth, which may lead to lower availability of food for the next trophic level and thus potentially affect the recruitment of higher trophic levels.