Hydrographic processes and changes in the Baltic Sea

The paper gives the system-conceptual basis of the hydrographic conditions in the Baltic Sea. The imposed boundary conditions, such as the bathymetry, river inflow, the oceanographic setting outside the Baltic Sea, etc., act through the laws of nature, and the resulting hydrographic conditions are considered the output of the Baltic system. The hydrographic conditions are described in terms of the salinity and temperature stratification and the advection and turbulent mixing of matter. The typical annual variability of stratification and transport rates are described. The changes since the glaciation and within the last century are also described, together with variations and their link to the boundary conditions. The large, long-term variations demonstrate that the Baltic hydrography is sensitive even to small changes in the boundary conditions. This calls for careful consideration of the man-made changes to the hydrographic boundary conditions, especially when the link between the hydrographic conditions and the biology is considered. This link is demonstrated by the dependency of the oxygen content in the lower layers of the Baltic Sea on the variability of the salt water inflows. Potentially, the most important anthropogenic impacts seem to be associated with the changes in the hydrological catchment area. Although there has been large scientific and public interest in the limited effects arising from the bridge projects in the Danish Straits, there has been little interest in impacts arising from changes within the catchment areas. The latter is an area requiring research.