Impact of saltwater inflows on phosphorus cycling andeutrophication in the Baltic Sea: a 3D model study

摘要

The impact of dense saltwater inflows on the phosphorus dynamics in the Baltic Sea is studied from tracerexperiments with a three-dimensional physical model. Model simulations showed that the coasts of the NorthWest Gotland Basin and the Gulf of Finland, the Estonian coast in the East Gotland Basin are regions wheretracers from below the halocline are primarily lifted up above the halocline. After 1 yr tracers are accumulatedat the surface along the Swedish east coast and at the western and southern sides of Gotland. Elevatedconcentrations are also found east and southeast of Gotland, in the northern Bornholm Basin and in thecentral parts of the East Gotland Basin. The annual supplies of phosphorus from the deeper waters to theproductive surface layers are estimated to be of the same order of magnitude as the waterborne inputs ofphosphorus to the entire Baltic Sea. The model results suggest that regionally the impact of these nutrients maybe quite large, and the largest regional increases in surface concentrations are found after large inflows.However, the overall direct impact of major Baltic inflows on the annual uplift of nutrients from below thehalocline to the surface waters is small because vertical transports are comparably large also during periodswithout major inflows. Our model results suggest that phosphorus released from the sediments between 60 and100 m depth in the East Gotland Basin contributes to the eutrophication, especially in the coastal regions of theeastern Baltic Proper.