Patterns of macroalgal distribution in the Kattegat-Baltic region

Abstract

The decline of species diversity of benthic macroalgae in estuaries has been explained mainly by the physiological restrictions imposed by reduced salinity, although a complex of factors is likely to influence the distribution. A comprehensive distributional index for the 422 species of benthic marine macroalgae in 22 districts of the Kattegat-Baltic brackish region provided an opportunity to analyze in detail the factors regulating species number and composition. Kattegat is the source area for the predominantly marine benthic algal flora of the Baltic Sea. The narrow Danish straits are the main barriers to exchange of species and water between Kattegat and the Baltic, and patterns of species distribution suggest that the wider and deeper strait (Great Belt) has formed the least stressful and most important dispersal route. Species number and composition of benthic macroalgae across the entire region were closely related to both salinity and distance from the source area. Though these two factors are correlated, we found support for the separate influence of declining salinity by the steeper reduction of species number among the predominantly marine groups of red and brown algae than among the mixed marine–freshwater green algae and by the profound reduction of species number over short distances into Øresund. The influence of dispersal barriers was supported by the much stronger numerical reduction from Kattegat to the Baltic of nondominant rather than dominant marine species and by the continued reduction of species number over long distances, but small salinity gradients, across the Baltic Sea. Within the restricted salinity range of the Baltic Sea, districts with rocky coasts suitable for attachment and survival of benthic macroalgae supported more species than districts with moraine coasts, along which benthic macroalgae are confined to scattered stones surrounded by soft-bottom sediments.