ECOSUPPORT: A Pilot Study on Decision Support for Baltic Sea Environmental Management

Excerpt

The Baltic Sea is a semi-enclosed sea with limited water exchange with the world ocean and with a freshwater surplus resulting in a brackish water body with strong vertical and horizontal stratifications (e.g., Leppäranta and Myrberg 2009). The discharge of large rivers causes a wide range of sea surface salinities between 2 g kg⁻¹ in the north (Bothnian Bay) or east (Gulf of Finland) and 20 g kg⁻¹ in the south (Kattegat) (Fig. 1). Also, the vertical gradients of salinity are large. For instance, the vertical salinity gradient in the eastern Gotland Basin amounts to about 4 g kg⁻¹. This specific hydrography prevents a sufficient ventilation of the deep water with the consequence of oxygen depletion. In addition to this natural factor, the anthropogenic impact of 85 million people, living in the catchment area of the Baltic Sea, is significant. Inter alia, intensive agriculture and inadequately treated sewage water result in eutrophication, reinforcing oxygen depletion. The latter effect is today so pronounced that large areas of the sea bottom are covered by hypoxia or even anoxia, i.e. by bottom water with a dissolved oxygen concentration below ~2 ml O₂ l⁻¹, or completely depleted, respectively (Hansson et al. 2011).

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