Phenology of phytoplankton was investigated at a coastal station in the western Baltic Sea from 1988 to 2017 by means of microscopically determined biomass and chlorophyll a (chl a) data. The prolongation of the growing season in this marine area is much stronger than that known from terrestrial areas. The growing season, defined by biomass or chl a thresholds, increased by 125 or 129 d, respectively, and extends recently from February to December. The spring bloom started earlier at a rate of 1.4 d yr −1 and the end of the autumn bloom was delayed by 3.1 d yr −1. The duration of the growing season increased at a rate of 4.5 d yr −1. The earlier start of the growing season was correlated with a slight increase in sunshine duration during spring, whereas the later end of the growing season was correlated with a strong increase in water temperature in autumn. The period with sea surface temperature >10°C shifted towards the end of November. Correlations of the duration of the growing season with the phosphate and nitrate concentrations were probably not causative. The shifts in the spring and autumn blooms led to a prolongation of the summer biomass minimum. The earlier spring bloom was caused, among other factors, by a shift of the biomass maximum of the dominant diatom Skeletonema marinoi from May to February/March. The delay in the autumn bloom was induced by a retardation of dominant dinoflagellates and diatoms, such as Ceratium spp. and Dactyliosolen fragilissimus.

During autumn 2007, an unusual increase in an algal species belonging to the order Prymnesiales was observed throughout the Baltic Sea Proper during routine national monitoring. Electron microscopical examination of the blooming species showed two types of flat scales -small and large -that resembled those of the alternate stage of Prymnesium polylepis. No spinebearing scales were found. The 18S rDNA sequence data (n ¼ 20, c. 1500 bp) verified the species identification as P. polylepis. There was up to 0.5% (7 bp) variability in the P. polylepis partial 18 S rDNA sequences from the Baltic Sea. These environmental sequences differed by 0-0.35% (0-4 bp) from cultured P. polylepis (isolate UIO036), and by 1.0-3.7% from other available Prymnesium sequences. The number of cells assumed to be P. polylepis began to increase in October 2007 coincidently with significantly calm and dry weather, and at their maximum the cells accounted for over 80% of the total phytoplankton biovolume in December-January. During February-April 2008, 95% of the Prymnesiales cells were in the size class of P. polylepis (46 mm). The species attained bloom concentrations (41 Â 10 6 cells l -1 ) from March to May 2008. The species was observed throughout the Baltic Sea, except the Bothnian Bay, Gulf of Riga and the Kattegat. No toxic effects of the bloom were observed.

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Dissolved and particulate organic carbon (DOC, POC) and nitrogen (DON, PON) constitute essential nutrient and energy sources to heterotrophic microbes in aquatic systems. Especially in the shallow coastal ocean, the concentrations are highly variable on short timescales, and cycling is heavily affected by different sources and environmental drivers. We analyzed surface water organic carbon and nitrogen concentrations determined weekly from 2010 to 2020 in the nearshore southwestern Baltic Sea (Heiligendamm, Germany) in relation to physical, chemical and biological parameters available since 1988. Mixing of low-DOC North Sea water with high-DOC Baltic Sea water, as well as in situ primary production, were confirmed as the main drivers of organic carbon and nitrogen concentrations. Tight coupling between POC, PON, chlorophyll a and phytoplankton carbon with DON seasonal dynamics corroborated the close relationship between phytoplankton production and degradation of organic nutrients with preferential remineralization of nitrogen. Significant changes in air and water temperature, salinity, and inorganic nutrients over time indicated effects of climate change and improved water quality management in the eutrophic Baltic Sea. Bulk organic nutrient concentrations did not change over time, while the salinity-corrected fraction of the DOC increased by about 0.6 μmol L–1yr–1. Concurrently, chlorophyll a and Bacillariophyceae and Cryptophyceae carbon increased, denoting a potential link to primary productivity. The high variability of the shallow system exacerbates the detection of trends, but our results emphasize the value of these extended samplings to understand coupled biogeochemical cycling of organic matter fractions and to detect trends in these important carbon reservoirs.