Picophytoplankton seasonal dynamics in the Baltic Sea

Abstract: Picophytoplankton (<2 μm diameter) is a diverse group ofpicocyanobacterial and photosynthetic picoeukaryotes (PPE).Picophytoplankton contribute significantly to total phytoplankton biomassand can dominate primary production in oceans, lakes and estuaries. In theestuarine Baltic Sea, the composition of picophytoplankton is linked to thenorth to south salinity gradient but knowledge of the seasonal dynamics interms of abundance, biomass and diversity is largely unknown. This thesisinvestigated the in situ dynamics, bottom up and top down controls ofpicocyanobacteria (SYN; consisting of primarily Synechococcus andCyanobium among other genuses) and PPE at two sampling stations, onecoastal and one offshore. Monitoring data over three years (2018-2020)showed high biomass contribution across all seasons. Picocyanobacterialpeak abundances occurred from spring to summer at the coastal station andin late-summer to autumn at the offshore station (up to 4.7 × 105 cells mL-1).Differentiation of pigment populations showed that phycoerythrin rich(PE)-SYN was the main contributor to SYN abundances except at the coastalstation during summer, when PE-SYN and phycocyanin rich (PC)-SYN hadequal contributions. PPE peak abundances occurred during late summer toautumn (up to 1.1 × 105 cells mL-1 cells ml-1). Temperature was linked topicophytoplankton growth and abundance, with PE-SYN, PCSYN and PPEadapted to different temperature ranges. Temperature also affected SYNnitrogen preference: SYN was nitrogen limited during early summer and at>15°C there was a preference for ammonium over nitrate. Clade A/Bdominated the SYN community, except during summer at the coastalstation when low nitrate and warm temperatures promoted S5.2dominance. Grazing was observed to control SYN and PPE abundances andhad an effect on the SYN community structure. Identification andlaboratory experiments of key Synechococcus strains using a range of salinity,temperature and light conditions provided important insights into thephysiological diversity of co-occurring ecotypes and links to the SYNdynamics that were observed in the field. In summary, this thesis providednovel information of picophytoplankton dynamics and communitystructure in the Baltic Sea. The results show that picophytoplankton play arelevant role in Baltic Sea and shows the importance of monitoringprograms to understand picophytoplankton dynamics.