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Nitrogen fixation by filamentous cyanobacteria supplies significant amountsof new nitrogen (N) to the Baltic Sea. This balances N loss processes suchas denitrification and anammox, and forms an important N source supportingprimary and secondary production in N-limited post-spring bloom planktoncommunities. Laboratory studies suggest that filamentous diazotrophiccyanobacteria growth and N2-fixation rates are sensitive to oceanacidification, with potential implications for new N supply to theBaltic Sea. In this study, our aim was to assess the effect of oceanacidification on diazotroph growth and activity as well as the contributionof diazotrophically fixed N to N supply in a natural plankton assemblage. Weenclosed a natural plankton community in a summer season in the Baltic Seanear the entrance to the Gulf of Finland in six large-scale mesocosms(volume ∼ 55 m3) and manipulated fCO2 over a rangerelevant for projected ocean acidification by the end of this century(average treatment fCO2: 365–1231 µatm). The direct responseof diazotroph growth and activity was followed in the mesocosms over a 47 day study period during N-limited growth in the summer plankton community.Diazotrophic filamentous cyanobacteria abundance throughout the study periodand N2-fixation rates (determined only until day 21 due to subsequentuse of contaminated commercial 15N-N2 gas stocks) remained low.Thus estimated new N inputs from diazotrophy were too low to relieve Nlimitation and stimulate a summer phytoplankton bloom. Instead, regenerationof organic N sources likely sustained growth in the plankton community. Wecould not detect significant CO2-related differences in neither inorganic nororganic N pool sizes, or particulate matter N : P stoichiometry.Additionally, no significant effect of elevated CO2 on diazotrophactivity was observed. Therefore, ocean acidification had no observableimpact on N cycling or biogeochemistry in this N-limited, post-spring bloomplankton assemblage in the Baltic Sea

No observed effect of ocean acidification on nitrogen biogeochemistry in a summer Baltic Sea plankton community