Patterns of phytoplankton composition in coastal lakes differed by connectivity with the Baltic Sea.

The study was aimed to analyse reactions of the major groups of phytoplankton to the mixing regime of fresh and brackish waters in coastal lakes and the associated changes in physicochemical properties of lake waters. For this purpose, on the basis of data collected from 6 coastal lakes located along the southern coast of the Baltic Sea, differing in intensity of intrusion of brackish sea water: limnetic, L (2), limnetic/oligohaline, L/O (2), and oligohaline, O (2), we assessed the associations of major phytoplankton groups with environmental conditions. Statistical analyses revealed that the structure of phytoplankton groups significantly differed among three lakes types, and the variation in these variables was best explained by water temperature, total phosphorus, salinity, PO43-, transparency, dissolved oxygen, and NO3-. Relative phytoplankton biomass showed significant differences among the O-L/O-L lake types and formed the following proportion 1:2:3.5. Cyanobacteria constituted a dominant algae group in the lakes, showing the decreasing trend from 86.5% in the L to 69.3% in the O lakes. The indicator value analysis showed that all the studied lakes were dominated by algae preferring freshwater habitats, among which Cyanobacteria and Cryptophyta were indicator groups. Redundancy analysis showed that increasing salinity has got a negative effect on Cyanobacteria and Bacillariophyta biomass, and did not stimulate the development of any algal group. In the coastal lakes we observed 2 distinct stable states (limnetic and oligohaline) as well as transitional phases between them: (1) seaward drift (limnetic-oligohaline) with increasing salinity, and (2) landward drift (oligohaline-limnetic) with decreasing salinity. Algal communities showed the most distinct differences in biomass in limnetic and oligohaline states. These observations suggest that the structure and biomass of phytoplankton may respond gradually on the level of hydrological connectivity, or may respond abruptly creating two alternative stable states: limnetic and oligohaline.