The highly fragmented Archipelago Sea in the northern Baltic Sea forms part of a sill area between two large sea basins. In addition to the water exchange between the basins, its waters are influenced by runoff, and thus the sea area has both sill and estuarine characteristics. We studied surface layer salinity gradients and their applicability in defining water exchange patterns through and within the region. A broad scale salinity pattern was detected during two sequential years. The spreading of fresh water in the spring was succeeded by a gradual increase in salinity during the summer. Long term data revealed a non-seasonal salinity fluctuation and diminished salinity stratification in the central and northern parts of the study area. We concluded that temporally unrepresentative mean values of salinity alone are inadequate for the purposes of coastal management in this region. In addition, both the range of variation and persistence of the conditions define the character of the transitional and coastal waters.
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