The influence of river runoff on deep water conditions of the Baltic Sea

The effect of variations in river runoff on abiotic environmental conditions in the deep water of semi-enclosed stratified sea areas in humid climatic zones in temperate latitudes was investigated using the Baltic Sea as an example. Runoff has an indirect effect on deep water conditions, where it acts through two mechanisms. First, in the shallow transition area, the inflowing highly saline water is transformed by mixing with the outflowing low saline Baltic water diluted by runoff. The transformed water which crosses the sills penetrates into the central basins, thereby additionally mixing with the ambient water along its path. Second, is the impact of runoff variations on the occurrence of major inflow events. Variations in the annual runoff might contribute to the strength of inflows. Reduced runoff increases the probability of inflow events. Since the mid-seventies, the frequency and intensity of major Baltic inflows has changed, only a few major events having occurred since then. Drastic changes in environmental conditions in the central Baltic deep water can be explained by increased zonal circulation linked with more intensive precipitation in the Baltic region and increased river runoff into the Baltic. There are signs that anthropogenic changes in runoff due to river regulation may be causing changes in the frequency of major inflows.