Analyzing the impacts of elevated-CO2 levels on the development of a subtropical zooplankton community during oligotrophic conditions and simulated upwelling

Ocean acidification (OA) is affecting marine ecosystems through changes in carbonate chemistry that may influence consumers of phytoplankton, often via trophic pathways. Using a mesocosm approach, we investigated OA effects on a subtropical zooplankton community during oligotrophic, bloom, and post-bloom phases under a range of different pCO₂ levels (from ∼400 to ∼1480 μatm). Furthermore, we simulated an upwelling event by adding 650 m-depth nutrient-rich water to the mesocosms, which initiated a phytoplankton bloom. No effects of pCO₂ on the zooplankton community were visible in the oligotrophic conditions before the bloom. The zooplankton community responded to phytoplankton bloom by increased abundances in all treatments, although the response was delayed under high-pCO₂ conditions. Microzooplankton was dominated by small dinoflagellates and aloricate ciliates, which were more abundant under medium- to high-pCO₂ conditions. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO₂ treatments during the oligotrophic phase of the experiment but were found in higher abundance under medium- and high-pCO₂ conditions toward the end of the experiment, most likely as a response to increased phyto- and microzooplankton standing stocks. The second most abundant mesozooplankton taxon were appendicularians, which did not show a response to the different pCO₂ treatments. Overall, CO₂ effects on zooplankton seemed to be primarily transmitted through significant CO₂ effects on phytoplankton and therefore indirect pathways. We conclude that elevated pCO₂ can change trophic cascades with significant effects on zooplankton, what might ultimately affect higher trophic levels in the future.