Long-term changes in a northern Baltic macrophyte community

Over the last 45 years significant changes in the floristic composition, distribution pattern, and dominance structure of phytobenthos were observed in Haapsalu Bay. Although the species composition of macroalgae did not change much over the course of this study, we observed notable changes in the community structure, i.e. algal shares. Some species, e.g. Tolypella nidifica, disappeared, other species such as Chara connivens and Chara baltica appeared for the first time in the study area. Besides, many prevailing species were relocated within the observed area over the course of the study. Another distinct feature of the recent years is that the vegetation had no dominant species. In general decadal variability exceeded yearly variability. There was no single abiotic variable that exerted major influence on phytobenthic communities. Instead we observed a combined effect of multiple environmental variables on phytobenthos communities. This study indicated that the changes in phytobenthic communities in Haapsalu Bay over the last 45 years were mainly due to large-scale weather patterns that determined regional salinity and ice conditions. Salinity sets the dominance pattern of phytobenthic species of freshwater and marine origin within communities. The intensity of ice scrape in turn created new unvegetated substrate and determined the overall phytobenthic cover in the study area. Within these large-scale patterns exposure, depth, and spatial salinity gradients contributed to the variability of small-scale patterns of macrophyte communities. Regional nutrient loading had weak effects on macrophyte communities, mainly in interactions with local abiotic variables and regional weather patterns.