Marine carbonate system evolution during the EPOCA Arctic pelagic ecosystem experiment in the context of simulated Arctic ocean acidification

Abstract. A major, potential stressor of marine systems is the changing water chemistry following increasing seawater carbon dioxide concentration (CO₂), commonly termed ocean acidification. In order to understand how an Arctic pelagic ecosystem may respond to future CO₂, a deliberate ocean acidification and nutrient perturbation study was undertaken in an Arctic fjord. The initial setting and evolution of seawater carbonate chemistry were investigated. Additions of carbon dioxide resulted in a wide range of ocean acidification scenarios. This study documents the changes to the CO₂ system throughout the study following net biological consumption and gas exchange with the atmosphere. In light of the common practice of extrapolating results to cover regions away from experimental conditions, a modelling study was also performed to assess the representativeness, in the context of the simulated present and future carbonate system, of the experimental study region to both the near and wider Arctic region. The mesocosm experiment represented the range of simulated marine carbonate system for the coming century and beyond (pCO₂ to 1420 μatm) and thus extrapolations may be appropriate to ecosystems exhibiting similar levels of CO₂ system drivers. However, as the regional ocean acidification was very heterogenous and did not follow changes in atmospheric CO₂, care should be taken in extrapolating the mesocosm response to other regions based on atmospheric CO₂ scenarios.