Algae report number 7, 2026

Abstract

In the Skagerrak and Kattegat areas, phytoplankton diversity was moderate to high but total cell numbers and biovolume were low. At Anholt E on both visits, the community was dominated by diatoms and other flagellates, including Guinardia spp. and Chaetoceros spp., with dinoflagellates contributing less to the total biovolume. At N14 Falkenberg, flagellates and Gymnodiniales dominated, with only minor contributions from diatoms and cyanobacteria. The potentially harmful taxon Prorocentrum cordatum* was detected at both N14 Falkenberg and Anholt E, but only in low abundances. Chlorophyll fluorescence was similar and relatively low at all West Coast stations, in agreement with the low IFCB biovolume. There was however some fluorescence peaks dominated by dinoflagellates in the Skagerrak. At Anholt E in the Kattegat, two peaks were observed; at 25 meters dinoflagellates dominated and close to the bottom a sinking bloom of diatoms was found.

In the Baltic Sea, phytoplankton communities were clearly dominated by filamentous cyanobacteria, particularly Dolichospermum spp.*, Nodularia spumigena* and Aphanizomenon flos-aquae. This was most pronounced at BY2 and BY5, where cyanobacteria accounted for the majority of the biovolume and total cell numbers were high. At the more offshore stations (BCS III-10, BY15, BY29, BY31 and BY39), cyanobacteria still dominated but cryptophytes and other small flagellates also contributed notably to the community. Several potentially harmful taxa were present across the Baltic stations, including N. spumigena*, Dolichospermum spp.*, Dinophysis acuminata*, D. norvegica* and P. cordatum*, but none were very abundant in the IFCB data. Chlorophyll fluorescence was highest at BY15 and BY2, consistent with the elevated cyanobacterial biovolume there, while somewhat lower fluorescence at the remaining Baltic stations reflected their slightly lower biovolume.