Species diversity and distribution of aquatic macrophytes in the Northern Quark, Baltic Sea

The occurrence of aquatic macrophytes was studied in a northern transition area of the Baltic Sea; the Northern Quark, Gulf of Bothnia. In the area there is a gradual, marked change in environmental conditions, the most prominent of which is a decrease in salinity from 5.0% in the Bothnian Sea to 3.5% in the Bothnian Bay. In all, 40 species of macrophytes were observed; 10 fucophyceans, 10 bangiophyceans, 8 chlorophyceans, 3 charophyceans, 1 tribophycean, 1 nostocophycean, 6 phanerogams and 1 water moss. 26 of the observed species were of marine and 14 of lacustrine origin. There was a clear change in species composition and community structure from south to north over the area. The vegetation at the southernmost localities had a marine character, with belt‐forming Fucus vesiculosus and a comparatively diverse flora of macroalgae. Further north, an ephemeral, lacustrine vegetation dominated by benthic diatoms and Cladophora aegagropila prevailed. The ratio marine: lacustrine species decreased from 4.2 to 1 when comparing a southern and a northern sub‐area of the Northern Quark. The species observed include 57 % of the marine macrophytes noted in the Aland and Archipelago Seas (N Baltic Proper) during the past two decades. Two marine species, Aglaothamnion roseum and Ahnfeltia plicata , are reported for the first time from the Northern Quark. This comprises a northern extension of their distribution limit with approximately 300 km.