Description: 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.

Description: Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes–summarized by the term ocean acidification (OA)–could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (~380 μatm pCO2), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (~760 μatm pCO2). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a “long-term mesocosm” approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.

Description: Among the most important stressors for aquatic ecosystems in the future are global warming and ocean acidification. To investigate their direct and indirect effects on a near-natural plankton community, a large-scale approach is needed. Therefore, four mesocosm experiments were conducted within the BIOACID II framework, using plankton communities from the Baltic Sea, North Sea and Atlantic Ocean. The focus of this project was on the pivotal role of microzooplankton (MZP) as trophic intermediary between the microbial loop and higher trophic levels. At the base of the food web, MZP has a strong impact on phytoplankton standing stocks due to high growth and grazing rates, leading to dietary competition with mesozooplankton. Thus, data on MZP abundance, biomass and taxonomic composition was analysed with emphasis on phytoplankton-MZP-mesozooplankton interactions. In conclusion, warming can be expected to directly affect MZP communities and enhance their growth and grazing pressure. In contrast, the results point at more complex responses of MZP to an increase in pCO2. While direct effects on the MZP community could not be observed, the present data points at predominately indirect effects via e.g. changes in phytoplankton community composition and/or standing stocks. Such indirect alterations might, however, be compensated on an ecosystem level.

Description: In order to assess the effects of ocean acidification (OA) on natural plankton communities, a large-scale mesocosm experiment was conducted in Gullmar Fjord (North Sea) from a spring bloom to a mid-summer situation. Here, the emphasis was on the pivotal role of microzooplankton (MZP) as trophic intermediary between the microbial loop and higher trophic levels. Due to its high specific growth and grazing rates, MZP has a strong impact on phytoplankton standing stocks which can lead to dietary competition between MZP and mesozooplankton. Furthermore, the ability of MZP to act as trophic upgraders for mesozooplankton by buffering nutritional imbalances of algae might gain importance given the expected decrease in algal food quality with OA. We present data on MZP communities (abundance, biomass, taxonomic composition) with emphasis on phytoplankton-MZP-mesozooplankton interactions. Overall, two phytoplankton peaks occurred (1st peak: around day 30, 2nd peak: around day 50). While there was no direct numerical response of MZP biomass to increases in phytoplankton biomass during the 1st peak, a clear numerical response of MZP to the 2nd peak was observed. However, no significant differences in MZP biomass with regard to the different CO2 scenarios occurred. In general, the ciliate community was dominated by small Strombidiids and no clear CO2 impacts on the ciliate community composition could be detected. Additional grazing experiments conducted during the 1st phytoplankton peak showed negative phytoplankton growth rates throughout. MZP grazing could only be detected in some of the low pCO2 treatments, pointing towards complex responses of MZP communities to OA.

Description: Microzooplankton (MZP) plays an important role as intermediary between the microbial loop and higher trophic levels. Due to high growth and grazing rates, MZP has a strong impact on phytoplankton biomass and community composition thus leading to dietary competition with mesozooplankton. Ocean acidification (OA) is anticipated to cause food quality alterations by changing algal stoichiometry in disfavor of grazers and the ability of MZP to buffer nutritional imbalances for higher trophic levels is expected to gain importance. Within the framework of an outdoor mesocosm experiment in the North Sea (BIOACID II), we investigated the effects of elevated CO2 on natural plankton communities. From a spring to a mid-summer situation, MZP abundance, biomass and species composition were analyzed. Grazing experiments provided additional information on the MZP grazing potential. Results show a predominance of small Strombidiids and a lower MZP biomass at high CO2 during the bloom. This finding is contradictory to the hypothesis that MZP growth will increase at high CO2 due to enhanced phytoplankton growth, thus pointing at more complex responses of MZP to ocean acidification.