Time-series of surface water CO2 and oxygen measurements on a platform in the central Arkona Sea (Baltic Sea): Seasonality of uptake and release

High resolution measurements of carbon dioxide and oxygen were made in surface waters of the central Arkona Sea (Baltic Sea) from May 2003 to September 2004. Sensors for CO2 partial pressure (pCO2w) and oxygen (O2) concentration were mounted in 7 m depth on a moored platform which is used for hydrographic and meteorological monitoring. The pCO2w data were obtained in half hour intervals and O2 was measured each hour as an average of a 10 min measurement. To check the performance of the sensors, pCO2w and O2 were determined by shipboard measurements on a research vessel which visited the site in 1–2 month intervals. In addition, pCO2w was measured on a “volunteer observing ship” (VOS) passing the platform each second day at a distance of about 25 km. Minima of 220 to 250 μatm of pCO2w were observed at the time of the spring bloom and a cyanobacteria bloom in mid-summer. During winter the pCO2w was mostly close to equilibrium with the atmosphere but maxima of 430 to 530 μatm were also observed. The seasonality of oxygen and pCO2w showed an opposing pattern. From a multiple regression analysis, we concluded that two processes primarily controlled pCO2w during our study: biological turnover and mixing. A parameterization, based on apparent oxygen utilisation (AOU) and salinity (S) only (pCO2w = 1.23 AOU + 43 S), reproduced the seasonality of pCO2w in surface water reasonably well. Based on our pCO2, salinity, and temperature data set, we attempted to separate processes changing total inorganic carbon concentrations (CT) by using an alkalinity–salinity relation for the area. The contribution of CO2 gas exchange and mixing were calculated and from this the biological turnover was deduced to reveal the calculated CT changes. The net annual uptake of CO2 in the central Arkona Sea was estimated to be about 1.5 Tg (1.5•1012 g) which was approximately balanced by a net oxygen release considering the uncertainties of the flux calculations. Near-coast CO2 emission due to episodic upwelling partly compensated the uptake of the central part of the Arkona Sea reducing the overall magnitude of the CO2 uptake.