Summary
Chlorophyll fluorescence maxima and some high measured chlorophyll concentrations in the Kattegat and Skagerrak areas were mainly caused by the flagellate Pseudochattonella spp.*, potentially harmful for fish. The flagellate has been observed during the two previous cruises as well. The cells were smaller and rounder now compared to the earlier observations which indicates the end of the bloom phase. The diatom Chaetoceros similis was numerous at most stations and at Släggö in the Skagerrak, diatoms were more common compared to the other stations. A chlorophyll fluorescence peak at BY1 was caused by the sinking bloom of the flagellate Pseudochattonella spp.*. The other fluorescence peaks in the Baltic Sea were caused by the spring bloom in different stages. The dinoflagellate Peridiniella catenata which is a typical Baltic spring bloom species, was abundant. Rather high cell numbers of the filamentous cyanobacterium Aphanizomenon flos-aquae was found in the Eastern Proper Baltic. Spring bloom was observed in the Gulf of Finland and dominated by diatoms, the dinoflagellate P. catenata and Aphanizomenon flos-aquae.