Air–sea CO2 exchange in the Gulf of Bothnia, Baltic Sea

The surface water CO2 partial pressure, pCO2, was measured during nine cruises in the Gulf of Bothnia at different seasons between 1999 and 2010, with continuous recording during most of the cruises using a bubble-type equilibrator and infrared CO2 detection. The data were used to construct the mean seasonality of the pCO2 in characteristic sub-areas. The low pCO2 measured in spring/summer and the high pCO2 measured in fall/winter indicated that biological production, stratification, mineralization and deep mixing, controlled the seasonal pCO2 cycle in both the Bothnian Bay and the Bothnian Sea. To calculate CO2 exchange with the atmosphere, the gas exchange transfer velocity was computed as a function of wind speed according to Wanninkhof et al. (2009). Based on the mean seasonal pCO2 distribution, CO2 fluxes were calculated, taking into account the differing wind conditions and the varying ice coverage. The resulting mean annual flux of +140 mmol m−2 yr−1 in the Bothnian Bay indicated that this area is a weak source for atmospheric CO2. In contrast, the Bothnian Sea was found to act as a sink for atmospheric CO2, with a mean flux of −730 mmol m−2 yr−1.