Algae report number 7, 2024

Type: Report
Author: Anders Torstensson
Published:

Summary

The analyses were conducted onboard the R/V Svea during the cruise, which began in Falkenberg, traversed the Baltic Sea, and concluded on the west coast. Surface samples were collected at all stations in the Baltic Sea specifically to monitor cyanobacteria blooms. Additionally, water from integrated depths or discrete depths was sampled, filtered through 10 µm filters, and analyzed with an upright microscope. This method primarily enables the identification of larger cells, while smaller cells are often harder to identify and more frequently overlooked. Species diversity was generally low in both the Skagerrak and the Kattegat areas. Centric diatoms dominated the phytoplankton community, particularly long, thin cells from the genera Proboscia and Rhizosolenia. At most stations, the larger cells were dominated by the diatom Proboscia alata. Species from the genus Tripos were very common. It is worth noting that at Anholt E in the Kattegat, the centric diatom genus Cyclotella was found in high cell numbers. Chlorophyll fluorescence maxima were noted at various depths and with varying intensity at most stations in the Kattegat and Skagerrak areas. These maxima were dominated by P. alata and Rhizosolenia imbricata. Surface accumulations of cyanobacteria were mainly observed in the southern and eastern Baltic Sea during this cruise, which coincided well with satellite analyses from the daily SMHI surveillance. The densest accumulations were found in the southeastern Baltic Sea, near station BCS III-10. Smaller accumulations were also noted in the northern Baltic Sea, near station BY29. The potentially toxic species Nodularia spumigena* was the most commonly occurring cyanobacterium in the southern and eastern Baltic Sea, while there was a more even distribution of groups in other parts of the Baltic Sea. At several stations, higher concentrations of filaments were noted in surface samples compared to integrated samples (0-10 m), indicating that the filaments were mainly concentrated near the surface. The absence of surface accumulations in the northern and western Baltic Proper was due to wind strength, but if the wind decreases, there is potential for surface accumulations in larger parts of the Baltic Proper.