Distribution and predictive occurrence model of charophytes in Estonian waters

Material collected during the years 1995–2011 was used to describe the distribution and environmental preferences of charophyte species in Estonian lakes and its coastal Baltic Sea. Altogether 22 species of charophytes were found in Estonian waters. Five taxa occurred in less than 10 localities and were classified as rare. Chara aspera and Tolypella nidifica were the most frequent and widespread species. The majority of species preferred shallow water less than 1 m in Estonian lakes and the coastal sea. Mud was the prevailing substrate on locations where charophytes were found, sandy substrate was characteristic for species which tolerate more exposed localities. Most of freshwater species preferred water alkalinity over 80 mg HCO3− l−1. A model was developed to predict the probability of the occurrence of Chara spp. in the extent of the whole Estonian marine waters based on several environmental variables. Boosted regression trees (BRT) was chosen as the modelling technique. Based on the model prediction, the vast majority of charophyte habitats are situated in the sea areas of the West Estonian Archipelago. That sea area is characterized by favourable conditions for charophytes: high proportion of shallow areas protected from wave exposure.