Algae report number 6, 2025

Abstract

The analyses were carried out onboard the vessel R/V Svea during the course of the expedition. Surface samples were taken at all stations in the Baltic Sea to specifically monitor the cyanobacterial bloom. Water from integrated depths or discrete depths was sampled and filtered through 5 µm filters, after which the samples were analyzed using an upright microscope. This method allows for the identification of larger cells, while smaller cells are often harder to identify and are more likely to be overlooked.

The cyanobacterial bloom was very intense during this year’s July expedition. Surface accumulations of cyanobacteria were observed in large parts of the Baltic Proper. The most intense accumulations were found in the southeastern Baltic Sea, along the eastern coast of Öland, and in the Hanö Bay, where the potentially toxic species Nodularia spumigena* was very common. High concentrations of cyanobacteria and patchy surface accumulations were also present in the eastern and northern Baltic Sea. No surface accumulations were observed in the Arkona Basin, which was consistent with satellite analyses from SMHI’s monitoring. The bloom largely consisted of an even distribution of the potentially toxic species N. spumigena* together with Aphanizomenon flosaquae in the southern, eastern, and northern Baltic Sea, while Dolichospermum spp. became more common in the western Baltic Sea. At all stations, higher concentrations of filaments were noted in surface samples compared to integrated samples (0–10 m), indicating that the filaments were primarily concentrated near the surface.

Species diversity and total cell abundance were generally low in both Skagerrak and Kattegat. Centric diatoms dominated the phytoplankton community. At most stations, the larger cells were dominated by the diatom Proboscia alata. Species within the genus Tripos were also very common. It is worth noting that N. spumigena* was observed in low numbers at both stations in Kattegat, highlighting the intensity of the cyanobacterial bloom during the expedition. Chlorophyll fluorescence maxima were noted at various depths and intensities at several stations
in the Skagerrak-Kattegat region. These maxima were dominated by Leptocylindrus danicus and species within the genus Tripos.