Algae report number 6, 2026

Abstract

In the Skagerrak and Kattegat, phytoplankton diversity was moderate to high with generally low to moderate total cell numbers. At Släggö, the community was dominated by diatoms such as Guinardia flaccida, Cerataulina pelagica and Dactyliosolen fragilissimus, with dinoflagellates and various small flagellates contributing to the biovolume; chlorophyll concentration here was moderate and in good agreement with the elevated diatom biovolume. At Å17, Anholt E and N14 Falkenberg, the community was instead dominated by small flagellates and other small cells, with diatoms and dinoflagellates contributing less to the total biovolume; chlorophyll concentration at these stations was similar to or slightly higher than at Släggö and broadly consistent with the IFCB biovolume patterns. Several potentially harmful taxa were observed at low to moderate levels along the West Coast, including Pseudo-nitzschia spp.*, Pseudochattonella spp.*, Alexandrium pseudogonyaulax* and Dinophysis acuminata*, but no clear dominance of these taxa was detected.

In the Baltic Sea, a pronounced cyanobacterial bloom was observed at the offshore stations, with cyanobacteria clearly dominating the phytoplankton community and chlorophyll concentration markedly elevated. At BY15, BY29 and BY31, cyanobacteria, in particular Aphanizomenon flos-aquae* together with Nodularia spumigena*, dominated the biovolume, accompanied by dinoflagellates (including Gymnodiniales and other dinoflagellates) and various small flagellates; chlorophyll concentration at these stations was high and consistent with the very high biovolume measured by the IFCB. At BCS III-10, cyanobacteria and dinoflagellates co-dominated, again with elevated chlorophyll concentration in line with the increased biovolume. The more southern and northern Baltic stations (BY2, BY5 and BY39) showed lower biovolume and chlorophyll concentration, with communities composed of mixtures of cyanobacteria, dinoflagellates, cryptophytes and other small flagellates. Throughout the Baltic Sea, potentially harmful taxa were present, including A. flos-aquae*, N. spumigena*, D. acuminata*, Dinophysis norvegica* and Pseudo-nitzschia spp.*, but no warning level exceedances were indicated in the available data.