Quantification of water, salt, oxygen and nutrient exchange of the Baltic Sea from observations in the Arkona Basin

Inflow of high-saline water of oceanic origin and diapycnal mixing force the vertical circulation of the Baltic Sea. The inflows of saline water are also enriched with oxygen and therefore the oxygen concentrations of the deep layers of the Baltic are strongly dependent on long-term inflow variations. This paper presents flux calculations for water, salt, nutrients and oxygen from the Arkona Basin towards the Baltic Sea. The calculation is based on an assumption of geostrophic flow and utilises historical hydrographical/hydrochemical profiles from two stations situated in the western (BY1) and eastern (BY2) part of the Arkona Basin. It is found that the long-term average flow is 21,000 and 22,400 m 3/s with salinities 14.6 and 14.0 psu at stations BY1 and BY2, respectively. Long-term average fluxes of nitrite, nitrate, ammonia, total nitrogen, silicate, phosphate, total phosphorus, and oxygen are calculated and compared with earlier estimates. Time-series are constructed from five-year mean fluxes of volume, salt, oxygen, phosphate and nitrate. It is found that the fluxes of volume, salt and oxygen were much lower in the 1980s than in the early 1970s. Both salinity and oxygen concentration co-varies with the magnitude of the volume flux, giving even larger relative variations in salt and oxygen fluxes than in volume flux. However, the fluxes of nitrate and phosphate did not decrease during the 1980s. It is shown that the concentrations of these nutrients in the inflowing water have drastically increased and this increase compensates for the reduced volume flow.