Carbon fluxes across the halocline in the eastern Gotland Sea

The fluxes of carbon associated with sinking particles and due to mixing across the permanent halocline in the eastern Gotland Sea were investigated. Mean salinity profiles in the surface water were determined during different seasons in 1997 and used to estimate salt fluxes and cross-halocline exchange coefficients. To convert the salt fluxes to the exchange of total CO2 (Ct) and dissolved organic carbon (DOC), relationships between Ct, DOC and salinity were established. An annual mean Ct input rate into the surface water of 5.3 mmol/m2 d was obtained. The DOC flux was one order of magnitude smaller (−0.44 mmol/m2 d) and directed into the deep water. The mean vertical flux of particulate organic carbon (POC) determined by a sediment trap was −1.4 mmol/m2 d. Relating this value to the carbon exchange by mixing, indicates that a net carbon flux occurs from the deep water to the surface water. However, a Ct increase was observed in the deep water. Therefore, a considerable lateral input of POC and subsequent mineralization must be assumed to balance the deep water budget. Our estimate indicates that the biological production in the surface water contributes only about 13–26% of the total POC input and, hence, to the oxygen consumption in the deep water.