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<p><strong>Abstract.</strong> Increasing atmospheric carbon dioxide (CO<sub>2</sub>) is changing seawater chemistry towards reduced pH, which consequently affects various properties of marine organisms. Coastal and brackish water communities are expected to be less affected by ocean acidification (OA) as these communities are typically adapted to high fluctuations in CO<sub>2</sub> and pH. Here we investigate the response of a coastal brackish water plankton community to increasing CO<sub>2</sub> levels as projected for the coming decades and the end of this century in terms of community and biochemical fatty acid (FA) composition. A Baltic Sea plankton community was enclosed in a set of off-shore mesocosms and subjected to a CO<sub>2</sub> gradient ranging from natural concentrations (~347 μatm <i>p</i>CO<sub>2</sub>) up to values projected for the year 2100 (~1333 μatm <i>p</i>CO<sub>2</sub>). We show that the phytoplankton community composition was resilient to CO<sub>2</sub> and did not diverge between the treatments. Seston FA composition was influenced by community composition, which in turn was driven by silicate and phosphate limitation in the mesocosms, and showed no difference between the CO<sub>2</sub> treatments. These results suggest that CO<sub>2</sub> effects are dampened in coastal communities that already experience high natural fluctuations in <i>p</i>CO<sub>2</sub>. Although this coastal plankton community was tolerant to high <i>p</i>CO<sub>2</sub> levels, hypoxia and CO<sub>2</sub> uptake by the sea can aggravate acidification and may lead to pH changes outside the currently experienced range for coastal organisms.</p>

Abstract. Ocean acidification is challenging phenotypic plasticity of individuals and populations. Calanoid copepods (zooplankton) are shown to be fairly plastic against altered pH conditions, and laboratory studies indicate that transgenerational effects are one mechanism behind this plasticity. We studied phenotypic plasticity of the copepod Acartia sp. in the course of a pelagic, large-volume mesocosm study that was conducted to investigate ecosystem and biogeochemical responses to ocean acidification. We measured copepod egg production rate, egg-hatching success, adult female size and adult female antioxidant capacity (ORAC) as a function of acidification (fCO2  ∼  365–1231 µatm) and as a function of quantity and quality of their diet. We used an egg transplant experiment to reveal whether transgenerational effects can alleviate the possible negative effects of ocean acidification on offspring development. We found significant negative effects of ocean acidification on adult female size. In addition, we found signs of a possible threshold at high fCO2, above which adaptive maternal effects cannot alleviate the negative effects of acidification on egg-hatching and nauplii development. We did not find support for the hypothesis that insufficient food quantity (total particulate carbon < 55 µm) or quality (C : N) weakens the transgenerational effects. However, females with high-ORAC-produced eggs with high hatching success. Overall, these results indicate that Acartia sp. could be affected by projected near-future CO2 levels.