Low CO2 sensitivity of microzooplankton communities in the Gullmar Fjord, Skagerrak: evidence from a long-term mesocosm study

Ocean acidification is considered as a crucial stressor for marine communities. In this study, we tested the effects of the IPCC RPC6.0 end-of-century acidification scenario on a natural plankton community in the Gullmar Fjord, Sweden, during a long-term mesocosm experiment from a spring bloom to a mid-summer situation. The focus of this study was on microzooplankton and its interactions with phytoplankton and mesozooplankton. The microzooplankton community was dominated by ciliates, especially small Strombidium sp., with the exception of the last days when heterotrophic dinoflagellates increased in abundance. We did not observe any effects of high CO₂ on the community composition and diversity of microzooplankton. While ciliate abundance, biomass and growth rate were not affected by elevated CO₂, we observed a positive effect of elevated CO₂ on dinoflagellate abundances. Additionally, growth rates of dinoflagellates were significantly higher in the high CO₂ treatments. Given the higher Chlorophyll a content measured under high CO₂, our results point at mainly indirect effects of CO₂ on microzooplankton caused by changes in phytoplankton standing stocks, in this case most likely an increase in small-sized phytoplankton of <8 μm. Overall, the results from the present study covering the most important part of the growing season indicate that coastal microzooplankton communities are rather robust towards realistic acidification scenarios.