Seasonality of Coastal Picophytoplankton Growth, Nutrient Limitation and Biomass Contribution
Abstract Picophytoplankton in the Baltic Sea includes picocyanobacteria (Synechococcus/Cyanobium) and photosynthetic picoeukaryotes (PPE). Picophytoplankton are thought to be a key component of the phytoplankton community but their seasonal dynamics and relationships with nutrients and temperature are largely unknown. We monitored pico- and larger phytoplankton at a coastal site in Kalmar Sound (K-Station) weekly during 2018. Among the picocyanobacteria, phycoerythrin-rich Synechococcus (PE-rich) dominated in spring and summer while phycocyanin-rich Synechococcus (PC-rich) dominated during autumn. PE-rich and PC-rich abundances peaked during summer (1.1x105 and 2.0x105 cells mL− 1) while PPE reached highest abundances in spring (1.1x105 cells mL− 1). PPE was the main contributor to the total phytoplankton biomass (3–73%). To assess nutrient limitation, bioassays with combinations of nitrogen (NO3 or NH4) and phosphorus additions were performed. PE-rich and PC-rich growth was mainly limited by nitrogen, with a preference for NH4 at 15–19°C. The three groups had distinct seasonal dynamics and optimal temperatures for growth were 10°C and 17–19°C for PE-rich, 13–16°C for PC-rich and 11–15°C for PPE. We conclude that picophytoplankton contribute significantly to the carbon cycle in the coastal Baltic Sea and underscore the importance of investigating functional groups to assess the consequences of the combination of high temperature and NH4 in a future climate.