Effect of ocean acidification on the structure and fatty acid composition of a natural plankton community in the Baltic Sea

Abstract. Increasing atmospheric carbon dioxide (CO₂) is changing seawater chemistry towards reduced pH, which 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 CO₂ and pH. Here we investigate the response of a coastal brackish water plankton community to increasing CO₂ 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 offshore mesocosms and subjected to a CO₂ gradient ranging from natural concentrations ( ∼  347 µatm fCO₂) up to values projected for the year 2100 ( ∼  1333 µatm fCO₂). We show that the phytoplankton community composition was resilient to CO₂ 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 CO₂ treatments. These results suggest that CO₂ effects are dampened in coastal communities that already experience high natural fluctuations in pCO₂. Although this coastal plankton community was tolerant of high pCO₂ levels, hypoxia and CO₂ uptake by the sea can aggravate acidification and may lead to pH changes outside the currently experienced range for coastal organisms.