Seasonal dynamics in picocyanobacterial abundance and clade composition at coastal and offshore stations in the Baltic Sea

Picocyanobacteria (< 2 µm in diameter) are significant contributors to total phytoplankton biomass. Due to the high diversity within this group, their seasonal dynamics and relationship with environmental parameters, especially in brackish waters, are largely unknown. In this study, the abundance and community composition of phycoerythrin rich (PE-rich) and phycocyanin rich (PC-rich) picocyanobacteria were monitored at a coastal (K-station) and at an offshore station (LMO; ~10 km from land) in the Baltic Sea over three years (2018–2020). Cell abundances of picocyanobacteria correlated positively to temperature and negatively to nitrate (NO 3 ) concentration. While PE-rich abundance correlated to the presence of nitrogen fixers, PC-rich abundance was linked to stratification/shallow waters. The picocyanobacterial targeted amplicon sequencing revealed an unprecedented diversity of 2169 picocyanobacterial amplicons sequence variants (ASVs). A unique assemblage of distinct picocyanobacterial clades across seasons was identified. Clade A/B dominated the picocyanobacterial community, except during summer when low NO 3 and high phosphate (PO 4 ) concentrations and warm temperatures promoted S5.2 dominance. This study, providing multiyear data, bridges the gap between Synechococcus/Cyanobium/Synechocystis phylogenetic classification and ecology. The difference in the response of the two functional groups and clades underscore the need for further high-resolution studies to understand their role in the ecosystem.