Tolerance of benthic marine macroalgae to low salinities is believed to be a main determinant of their vertical distribution across the marine coastline and their horizontal distribution from oceanic regions into low-saline bays, fjords and estuaries. Salt tolerance should also be important for the distribution of the nearly 400 macroalgal species across the large salinity gradient in the Kattegat-Baltic Sea area ranging from 30 psu at the entrance to 3 psu in the inner parts 1500 km away. Previous evaluations of interspecific differences in salt tolerance in relation to horizontal distribution patterns have been hampered by examination of only a few species in each study and differences in methods among studies. To permit interspecific comparisons and statistical evaluations of relations to distribution patterns we studied the ability of 44 macroalgal species from North Zealand, South Kattegat, Denmark (salinity: 16 psu, tide 10 cm) to maintain photosynthesis after 4 days exposure to declining salinities (16 to 0 psu). Overall, the algal community had a high short-term tolerance to low salinities because 35 of 44 species maintained more than half the photosynthetic capacity at salinities lower than 3.7 psu and eight species of high vertical zonation maintained this capacity below 1.45 psu. A tolerance index, accounting for photosynthetic performance at all low salinities, permitted reproducible ranking of the 44 species. There was no discrete break between those species tolerant of low salinities and those that require high salinity, although species at the ends of the continuum have markedly and significantly different tolerances. Interspecific differences in tolerance to low salinity were highly significantly correlated to vertical zonation in Kattegat (r: 0.73) and weakly to horizontal distribution in the inner Baltic Sea (r: 0.32). A broad tolerance and high abundance of species in the Kattegat were significantly related to the contemporary distribution of Baltic macroalgae. Nonetheless, a large proportion of the interspecific variability was unaccounted for, presumably because a range of other biological features influence distribution patterns over historical-evolutionary time scales.