Algae report number 3, 2025

Abstract

The spring bloom was ongoing in the Kattegat and Skagerrak areas with dominance of diatoms. The species Skeletonema marinoi and the genera Thalassiosira and Chaetoceros were the most abundant taxa. The highest species diversity and cell
numbers were found in the Kattegat, the lowest was found at Å17 in the open Skagerrak area. High chlorophyll concentrations were found around 15–20 m at several stations, at Släggö, the concentrations were high at 0–5 m. In spite of high individual concentrations, the integrated chlorophyll concentrations (0–10 m and 0–20 m) were within normal for this month at all stations.

Diversity and cell abundance of phytoplankton were generally very low in the Baltic Sea but slightly higher cell abundance was noted at the more northerly stations north of Gotland. The chain-forming diatom S. marinoi, common during the
spring bloom, was present at most stations but were more common at the more northerly stations. Also, Peridiniella catenata, a dinoflagellate also common in the spring bloom in the Baltic Proper was found in lower numbers at some stations. The
Imaging FlowCytobot, that samples on the way and take photos, recorded single filaments of Aphanizomenon flosaquae in the Baltic, but only in samples taken at BY5 filaments were seen in the microscope samples. The integrated chlorophyll
concentrations (0–10 m and 0–20 m) were within the normal range for this month at almost all stations.