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Stratification of the water column, consisting of the three layers (upper, intermediate, and deep layer) separated by the seasonal thermocline and the permanent halocline, respectively, is an important factor for the functioning of the brackish Baltic Sea. In the present work, changes in the vertical structure of temperature and salinity, as well as heat content, salt mass, and stratification conditions were estimated on the basis of in situ and remote sensing data in 1982-2016. The seasonal thermocline and the halocline have strengthened in most of the sea by a rate of 0.33-0.39 and 0.70-0.88 kg m −3 , respectively, during 35 years. The upper layer has warmed by 0.03-0.06 • C year −1 and sub-halocline deep layer 0.04-0.06 • C year −1 in most of the sea. The total warming trend in the whole Baltic has been 1.07 • C for 35 years, being approximately twice higher compared to the upper 100 m in the Atlantic Ocean. Average upper layer warming of the sea from May to September has been 0.07-0.08 • C year −1 while in winter, trends were mostly statistically not significant. More rapid warming during summers has occurred in shallower, closed-end areas of gulfs if compared to the rest of the sea. Possible reasons for high warming there might be shallow depths and limited water exchange, stronger stratification, and/or higher turbidity. Sea surface temperature trends estimated by in situ and satellite data agree well. Trends of freshening (−0.005 to −0.014 g kg −1 year −1 ) of the upper layer and increasing salinity (0.02 to 0.04 g kg −1 year −1 ) in the sub-halocline deep layer were detected. Increased salinity in the deep layer is likely caused by the increased lateral import of saltier water from the North Sea. Changes in the upper layer salinity might not be related to the accumulated river runoff only, but decadal changes of vertical salt flux might also contribute. The vertically distinct changes cancel each other and no significant trend in the mean salinity of the Baltic Sea was detected. No remarkable changes have occurred in the cold intermediate layer. In conclusion, different dominating processes have caused distinct long-term trends in the three layers of the Baltic Sea.

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Blooms of cyanobacteria are a recurrent phenomenon in the Baltic Sea, including the Gulf of Finland. The spatial extension, duration, intensity and species composition of these blooms varies widely between years. Alg@line data collected regularly from ferries as well as weather service and marine monitoring data from 1997 to 2005 are analysed to determine the main abiotic factors influencing the intensity and species composition of cyanobacterial blooms in the Gulf of Finland. It is demonstrated that the development of the Nodularia spumigena Mertens bloom is highly dependent on weather conditions such as photosynthetically active radiation and water temperature. Nutrient conditions, especially the surplus of phosphorus (according to Redfield ratio) related to the pre-bloom upwelling events in the Gulf, affect the intensity of Aphanizomenon sp. (L.) Ralfs blooms. Differences in bloom timing and duration indicate that, if the preconditions (like nutrient ratio/ concentration and weather conditions) for bloom formation are favourable, then the Aphanizomenon bloom starts earlier, the overall bloom period is longer and the Nodularia peak might appear in a wider time window.