Time-series of surface water CO 2 and oxygen measurements on a platform in the central Arkona Sea (Baltic Sea): Seasonality of uptake and release

Abstract

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 CO 2 partial pressure ( pCO 2 w) and oxygen (O 2) concentration were mounted in 7 m depth on a moored platform which is used for hydrographic and meteorological monitoring. The pCO 2 w data were obtained in half hour intervals and O 2 was measured each hour as an average of a 10 min measurement. To check the performance of the sensors, pCO 2 w and O 2 were determined by shipboard measurements on a research vessel which visited the site in 1–2 month intervals. In addition, pCO 2 w 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 pCO 2 w were observed at the time of the spring bloom and a cyanobacteria bloom in mid-summer. During winter the pCO 2 w was mostly close to equilibrium with the atmosphere but maxima of 430 to 530 μatm were also observed. The seasonality of oxygen and pCO 2 w showed an opposing pattern. From a multiple regression analysis, we concluded that two processes primarily controlled pCO 2 w during our study: biological turnover and mixing. A parameterization, based on apparent oxygen utilisation (AOU) and salinity ( S) only ( pCO 2 w = 1.23 AOU + 43 S), reproduced the seasonality of pCO 2 w in surface water reasonably well. Based on our pCO 2, salinity, and temperature data set, we attempted to separate processes changing total inorganic carbon concentrations ( C T) by using an alkalinity–salinity relation for the area. The contribution of CO 2 gas exchange and mixing were calculated and from this the biological turnover was deduced to reveal the calculated C T changes. The net annual uptake of CO 2 in the central Arkona Sea was estimated to be about 1.5 Tg (1.5·10 12 g) which was approximately balanced by a net oxygen release considering the uncertainties of the flux calculations. Near-coast CO 2 emission due to episodic upwelling partly compensated the uptake of the central part of the Arkona Sea reducing the overall magnitude of the CO 2 uptake.