Algae report number 7, 2025

Abstract

found at N14 Falkenberg at the Kattegat coast. Filamentous cyanobacteria were found at all stations except Å17 in the open Skagerrak. Dolichospermum sp. * and Nodularia spumigena *were the cyanobacteria that were observed and they were rather numerous, foremost in the Kattegat. The filamentous cyanobacteria have been found along the coast and into the fjords of Bohuslän in other monitoring
programmes as well. Interestingly, the phosphate concentrations have been high which give the cyanobacteria an advantage compared to other phytoplankton which need both inorganic phosphate and nitrogen to grow. These cyanobacteria are able to utilize atmospheric nitrogen, which is always available. The integrated chlorophyll concentrations (0–20 m) in the Skagerrak were within normal whereas the ones in the Kattegat were below normal for this month. All of the integrated concentrations from 0–10 m were within normal in both areas.

The diversity and cell abundances of phytoplankton were generally low in the Baltic Sea. In the southern part of the Baltic Sea filamentous cyanobacteria were almost completely absent. East of Gotland and north of Gotland quite a few filaments of Aphanizomenon flosaquae were found but only occasional filament of Nodularia spumigena*. The filamentous genus Dolichospermum* were noted at one station
between Gotland and Öland. The stations in the middle and north of the Baltic Proper had chlorophyll a concentration above what is normal for the month in the upper layer of the water column (0-10m).