Algae report number 8, 2026

Abstract

Along the Skagerrak and Kattegat coast, phytoplankton diversity was generally moderate to high while total biovolume remained relatively low. Communities were mainly composed of dinoflagellates and various small flagellates, with diatoms contributing less. Potentially harmful taxa were present at several stations, including Pseudochattonella spp.*, Dinophysis acuminata*, Alexandrium pseudogonyaulax*, Gonyaulax spp.*, Pseudo-nitzschia spp.* and low levels of filamentous cyanobacteria such as Nodularia spumigena* and Aphanizomenon flos-aquae*. Chlorophyll-a concentration was lowest at the Kattegat stations and somewhat higher in the Skagerrak, broadly consistent with the relatively modest biovolume measured by the IFCB.

In the Baltic Sea, a pronounced cyanobacterial bloom was observed. Filamentous cyanobacteria dominated the communities at most stations, with N. spumigena* and A. flos-aquae* together with Dolichospermum spp. forming dense surface accumulations at BY15, BY29, BY31, BY39 and BY2. At BY2, diatoms and dinoflagellates also contributed substantially to the total biovolume. Potentially harmful taxa were widespread, including N. spumigena*, A. flos-aquae*, Prorocentrum cordatum*, D. acuminata*, Dinophysis norvegica*, Alexandrium spp.* and Pseudo-nitzschia spp.*. Chlorophyll-a concentration was clearly elevated at the central Baltic stations, and highest at BY2 and BY31, in good agreement with the high cyanobacterial biovolume recorded by the IFCB and confirming that a cyanobacterial bloom was ongoing in the Baltic Sea during the cruise.