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.

How biogenic polymers control surfactant dynamics in the surface microlayer: Insights from a coastal Baltic Sea study

“…3a). Explicitly, pico-NCBL numbers were elevated compared to the second cruise and may be interpreted as lasting imprint of the spring bloom in agreement with Zufia and Farnelid (2021). Relative to September, we observed a reduced pool of semi-labile DOC, which coincided with overall lower bacterial abundance and the dominance of LNA cells, commonly interpreted as inactive cells (Servais et al, 2003).…”

“…In comparison to June, pico-CBL cell abundance had increased tenfold and nano-NCBL cells also greatly increased, which aligns well with the expected high contribution of Synechococcus spp. and nano-flagellates (Zufia and Farnelid, 2021;Bunse et al, 2019). Therewith, the end of the first cruise overlapped with the transition into a highly productive summer bloom state, which collapsed just after the second cruise mid of September.…”

“…Phyto-and bacterioplankton dynamics in the Baltic Sea follow explicit seasonal cycles controlled mainly by abiotic factors such as water column stratification, nutrient supply, light and temperature (Wasmund et al, 2008;Bunse et al, 2019). During the spring bloom, diatoms dominate micro-phytoplankton abundance (Wasmund et al, 2008;Bunse et al, 2019;Zufia & Farnelid, 2021). However, eukaryotic pico-phytoplankton (summarized here as pico-NCBL) may substantially contribute to phytoplankton biomass at coastal sites (Zufia and Farnelid, 2021).…”

“…During the spring bloom, diatoms dominate micro-phytoplankton abundance (Wasmund et al, 2008;Bunse et al, 2019;Zufia & Farnelid, 2021). However, eukaryotic pico-phytoplankton (summarized here as pico-NCBL) may substantially contribute to phytoplankton biomass at coastal sites (Zufia and Farnelid, 2021). Heterotrophic bacteria production increases shortly after the spring bloom but declines to low levels simultaneously with phytoplankton biomass at the beginning of June (Bunse et al, 2019).…”

“…Synechococcus spp. may contribute as much as 27% to phytoplankton biomass (Zufia and Farnelid, 2021). The summer bloom period lasts approximately until the end of August (Wasmund et al, 2008).…”