Charophytes in the estuarine Curonian Lagoon: Have the changes in diversity, abundance and distribution occurred since the late 1940s?

The results of the recent (2014-2015) inventory and three historical datasets (1949-1959, 1960-1980 and 1997-2007) were analyzed in order to track the long-term changes of charophytes in the largest estuarine lagoon of the Baltic Sea. The present species composition of charophytes in the estuarine part of the Curonian Lagoon consisted of 7 species, including tolerant to salinity (Chara aspera, C. contraria, C. globularis and Nitellopsis obtusa) and typical brackish-water species (Chara baltica, C. canescens and Tolypella nidifica). The highest congruence of species was between the 1997-2007 and 2014-2015 datasets, which covered respectively eutrophication and post-eutrophication periods. The 1949-1959 dataset (closest to the reference conditions) differed by the absence of typical brackish-water species. The 1960-1980 dataset (the major period of eutrophication) was relatively poor in study sites and species. During the last 6 decades, only tolerant to salinity freshwater species were constant and abundant; only Nitella mucronata can be considered as extinct. Recently, C. contraria became dominant and widespread. The changes in the charophyte species composition, abundance and distribution can be explained by different intensity of surveys and/or density of study sites, but also by the increased exposure to brackish waters since 1980s and/or recently reduced effect of eutrophication.