Algae report number 9, 2026

Abstract

Along the Skagerrak and Kattegat, phytoplankton diversity was generally moderate to high, with total cell numbers and biovolume lowest at Å17 and highest at Släggö. Diatoms and dinoflagellates dominated most communities, with Pseudosolenia calcar-avis and other large centric diatoms contributing substantially at Släggö, Anholt E and N14 Falkenberg. Dinoflagellates such as Heterocapsa rotundata were common, and smaller cells were represented mainly by cryptophytes and other small flagellates. Several potentially harmful taxa were observed at low to moderate levels, including Pseudo-nitzschia spp.*, Pseudochattonella spp.*, Prorocentrum cordatum*, Dinophysis acuta*, Dinophysis acuminata* and Phalacroma rotundatum*. Chlorophyll concentration was normal for the season and in good agreement with the biovolume recorded by the IFCB.

In the Baltic Sea, species richness was moderate and total cell numbers were relatively high at most stations, with the highest biovolume at BY29, BY31 and BY39. The communities were dominated by filamentous and colonial cyanobacteria, cryptophytes and other small flagellates, with diatoms contributing more strongly at BY2, BY29 and BY31. Potentially harmful cyanobacteria Aphanizomenon flos-aquae* and Nodularia spumigena* occurred at all Baltic stations, and Dolichospermum spp. and Aphanothece parallelliformis were important at BY39. The potentially harmful dinoflagellate P. cordatum* was present at low to moderate levels throughout the Baltic. Chlorophyll concentration was highest in the northern Baltic Proper, matching the elevated cyanobacteria and small-flagellate biovolume, while BY39 showed lower chl concentration despite high cyanobacteria biovolume, indicating a community dominated by colonial forms (A. parallelliformis och Cyanophyceae), which yield high biovolume estimates when assessed using imaging.