Algae report number 3, 2026

Abstract

Species diversity was generally low in the Skagerrak and Kattegat, while cell abundance was relatively high due to a bloom of Pseudochattonella*. Elevated concentrations of Pseudochattonella* were observed throughout the Skagerrak and Kattegat, from the Kullen Peninsula to Släggö, exceeding 500 000 cells/L according to the Imaging FlowCytobot (IFCB). Traces of the spring bloom were present at all stations, with low densities of chain-forming diatoms. Cryptomonads were very common in Skälderviken, together with Pseudochattonella*, where an additional sample was collected. The integrated chlorophyll concentrations (0-10 m and 0-20 m) were within the normal range for the month at all stations in Skagerrak, but lower than normal at Anholt E.

Diversity and cell abundances of phytoplankton were generally very low, with the only exception being BY2 Arkona where a spring bloom situation at some stage was occurring. The vertical CTD profile from this station showed a complex situation with several fluorescens peaks. The higher total cell numbers at BY2 compared to the rest of the Baltic Proper was obvious as the area around BY2 Arkona had many more pictures than the rest of the sampled areas in the Baltic. In general, there were diatoms of the species Skeletonema marinoi, cryptomonads and ciliates such as Mesodinium rubrum that were found in the Baltic Proper. The integrated chlorophyll concentrations were within the normal range for this month at all stations except BY2 0-20 m where they were higher than normal probably due to a large fluorescence peak just below 10 m.