Description: Ocean acidification (OA) is one of the major symptoms of the current increase in atmospheric CO2. Here, we present results from a mesocosm experiment carried out during 113 days in the Gullmar Fjord, Skagerrak coast of Sweden, studying plankton responses to predicted end-of-century pCO2 levels. Abundances of both copepods (Pseudocalanus acuspes, Temora longicornis, and Oithona similis) and hydromedusae (Hybocodon prolifer and Aglantha digitale) responded to pCO2. Furthermore, stage-specific pCO2 sensitivities were detected in copepods, copepodites being the most sensitive stage.

Description: 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.

Description: In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2015) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2016-10-12.

Description: In the autumn of 2014, nine large mesocosms were deployed in the oligotrophic subtropical North-Atlantic coastal waters off Gran Canaria (Spain). Their deployment was designed to address the acidification effects of CO2 levels from 400 to 1,400 μatm, on a plankton community experiencing upwelling of nutrient-rich deep water. Among other parameters, chlorophyll a (chl-a), potential respiration (PHi), and biomass in terms of particulate protein (B) were measured in the microplankton community (0.7–50.0 μm) during an oligotrophic phase (Phase I), a phytoplankton-bloom phase (Phase II), and a post-bloom phase (Phase III). Here, we explore the use of the PHi/chl-a ratio in monitoring shifts in the microplankton community composition and its metabolism. PHi/chl-a values below 2.5 μL O2/h/ (μg chl-a) indicated a community dominated by photoautotrophs. When PHi/chl-a ranged higher, between 2.5 and 7.0 μL O2/h/ (μg chl-a), it indicated a mixed community of phytoplankton, microzooplankton and heterotrophic prokaryotes. When PHi/chl-a rose above 7.0 μL O2/h/ (μg chl-a), it indicated a community where microzooplankton proliferated (〉10.0 μL O2/h/ (μg chl-a)), because heterotrophic dinoflagellates bloomed. The first derivative of B, as a function of time (dB/dt), indicates the rate of protein build-up when positive and the rate of protein loss, when negative. It revealed that the maximum increase in particulate protein (biomass) occurred between 1 and 2 days before the chl-a peak. A day after this peak, the trough revealed the maximum net biomass loss. This analysis did not detect significant changes in particulate protein, neither in Phase I nor in Phase III. Integral analysis of PHi, chl-a and B, over the duration of each phase, for each mesocosm, reflected a positive relationship between PHi and pCO2 during Phase II [alpha = 230*10−5 μL O2/h/L/(μatm CO2)/(phase-day), R2 = 0.30] and between chl-a and pCO2 during Phase III [alpha= 100*10−5 μg chl-a/L/ (μ atmCO2)/ (phase-day), R2 = 0.84]. At the end of Phase II, a harmful algal species (HAS), Vicicitus globosus, bloomed in the high pCO2 mesocosms. In these mesocosms, microzooplankton did not proliferate, and chl-a retention time in the water column increased. In these V. globosus-disrupted communities, the PHi/chl-a ratio [4.1 +- 1.5 μL O2/h/(μg chl-a)] was more similar to the PHi/chl-a ratio in a mixed plankton community than to a photoautotroph-dominated one.

Description: Ocean acidification impacts many marine biota. Although evolutionary responses should occur during persisting environmental change, little is known about the adaptability of copepods. Therefore, we set up a 3½ yr long selection experiment, maintaining Acartia tonsa populations in seawater treated with 200 and 800 μatm CO2, and feeding them with algae cultured under corresponding CO2 conditions. In 3 reciprocal transplant experiments, roughly 1 yr apart, we measured developmental rates, C:N and C:P ratios, egg production and hatching rates of the different lines. In the transplant experiments, we observed significantly lower developmental rates in the high CO2 treatment independent of the selective history. Egg production and hatching success were unaffected by the experimental conditions, but we observed an earlier hatching of eggs from females with a high CO2 selective history. Over the experimental period, beneficial adaptations of the copepods cultured under high CO2 conditions of elevated seawater pCO2 and associated food quality were not detected. However, towards the end of the experiment, copepods cultured under elevated pCO2 and fed with high CO2 algae showed increased body mass and decreased prosome length.

Description: The acidification of the oceans could potentially alter marine plankton communities with consequences for ecosystem functioning. While several studies have investigated effects of ocean acidifications on communities using traditional methods, few have used genetic analyses. Here, we use community barcoding to assess the impact of ocean acidification on the composition of a coastal plankton community in a large scale, in situ, long-term mesocosm experiment. High-throughput sequencing resulted in the identification of a wide range of planktonic taxa (Alveolata, Cryptophyta, Haptophyceae, Fungi, Metazoa, Hydrozoa, Rhizaria, Straminipila, Chlorophyta). Analyses based on predicted operational taxonomical units as well as taxonomical compositions revealed no differences between communities in high CO2 mesocosms (~760 µatm) and those exposed to present day CO2 conditions. Observed shifts in the planktonic community composition were mainly related to seasonal changes in temperature and nutrients.

Description: Aquatic ecosystems face a multitude of environmental stressors, including warming and acidification. While warming is expected to have a pronounced effect on plankton communities, many components of the plankton seem fairly robust towards realistic end-of-century acidification conditions. However, interactions of the two stressors and the inclusion of further factors such as nutrient concentration and trophic interactions are expected to change this outcome. We investigated the effects of warming and high CO2 on a nutrient-deplete late summer plankton community from the Kiel Fjord, Baltic Sea, using a mesocosm setup crossing two temperatures with a gradient of CO2. Phytoplankton and microzooplankton (MZP) growth rates as well as biomass, taxonomic composition, and grazing rates of MZP were analysed. We observed effects of high CO2, warming, and their interactions on all measured parameters. The occurrence and direction of the effects were dependent on the phytoplankton or MZP community composition. In addition, the abundance of small-sized phytoplankton was identified as one of the most important factors in shaping the MZP community composition. Overall, our results indicate that an estuarine MZP community used to strong natural fluctuations in CO2 can still be affected by a moderate increase in CO2 if it occurs in combination with warming and during a nutrient-deplete post-bloom situation. This highlights the importance of including trophic interactions and seasonality aspects when assessing climate change effects on marine zooplankton communities.