The CO2 System of the Baltic Sea: Biogeochemical Control and Impact of Anthropogenic CO2

Abstract

The marine CO2 system is controlled by chemical equilibria between the dissolved CO2 species (CO2 (pCO2), H2CO3, HCO3−, CO32−) and the hydrogen ion concentration (pH). By gas exchange the system interacts with the atmospheric CO2 and by dissolution/precipitation with solid CaCO3. A major variable of the system is the alkalinity which determines the large scale distribution of the background total CO2 concentration. The background concentrations are modulated by the consumption/production of CO2 by biological production/decomposition of organic matter. The biological production is reflected in the seasonality of the surface water CO2 partial pressure, pCO2. Two pCO2 minima are observed in the central Baltic Sea which are due to the CO2 uptake during the spring bloom and during the mid-summer production fuelled by nitrogen fixation. The pH is closely related to the pCO2 and shows two maxima which are coinciding with the pCO2 minima. Due to the increasing atmospheric CO2 the mean pH in the central Baltic Sea will decrease from currently 8.07 to 7.91 during the next approximately 100 years. The ecological consequences are not yet foreseeable, but may be severe since many biochemical reactions are controlled by the pH. At the same time the CaCO3 saturation will decrease by a factor of almost two and the Baltic Sea surface water will be below saturation for aragonite almost throughout the year. This may affect the abundance of calcifying organisms because their growth is impeded at low CaCO3 saturation.