Upwelling in the Baltic Sea — A review

Upwelling is a typical phenomenon of the Baltic Sea. Because the Baltic Sea is a semi- enclosed basin, winds from favorable directions blowing predominately parallel to the coast cause upwelling leading to vertical displacement of the water body and mixing. During the thermal stratified period, upwelling can lead to a strong sea-surface temperature drop of more than 10 °C changing drastically the thermal balance and stability conditions at the sea-surface. Upwelling can play a key role in replenishing the euphotic zone with the nutritional components necessary for biological productivity when the surface layer is depleted of nutrients. Consequently, it has been found out that in such areas where upwelling lifts phosphorus-rich deep water to the surface, the N/P ratio becomes low which favors the blooming of nitrogen-fixing blue-green algae. The rapid temperature decrease during such events was recognized and documented a long time ago when temperature measurements became available. Thus, the study of the upwelling process has a long tradition. However, although the importance of upwelling has generally been accepted for the Baltic Sea, no general review of upwelling exists. The objective of this paper is a comprehensive review of the upwelling process, its dynamics and reflections to ecosystem processes in the Baltic Sea using all relevant literature which will help to close the gaps of present knowledge and some recommendations for future work are outlined accordingly.