Simulated ocean acidification reveals winners and losers in coastal phytoplankton.

The oceans absorb ~25% of the annual anthropogenic CO2 emissions. This causes a shift in the marine carbonate chemistry termed ocean acidification (OA). OA is expected to influence metabolic processes in phytoplankton species but it is unclear how the combination of individual physiological changes alters the structure of entire phytoplankton communities. To investigate this, we deployed ten pelagic mesocosms (volume ~50 m3) for 113 days at the west coast of Sweden and simulated OA (pCO2 = 760 µatm) in five of them while the other five served as controls (380 µatm). We found: (1) Bulk chlorophyll a concentration and 10 out of 16 investigated phytoplankton groups were significantly and mostly positively affected by elevated CO2 concentrations. However, CO2 effects on abundance or biomass were generally subtle and present only during certain succession stages. (2) Some of the CO2-affected phytoplankton groups seemed to respond directly to altered carbonate chemistry (e.g. diatoms) while others (e.g. Synechococcus) were more likely to be indirectly affected through CO2 sensitive competitors or grazers. (3) Picoeukaryotic phytoplankton (0.2-2 µm) showed the clearest and relatively strong positive CO2 responses during several succession stages. We attribute this not only to a CO2 fertilization of their photosynthetic apparatus but also to an increased nutrient competitiveness under acidified (i.e. low pH) conditions. The stimulating influence of high CO2/low pH on picoeukaryote abundance observed in this experiment is strikingly consistent with results from previous studies, suggesting that picoeukaryotes are among the winners in a future ocean.

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.