Seasonal changes in phytoplankton community composition and primary production in the southern Baltic Sea based on monitoring, ocean glider and satellite data

We present year-long, vertically-resolved, high-resolution observations of phytoplankton community and primary productivity in the Bornholm Basin of the Baltic Sea. Sustained high-resolution monitoring reveals how the small-scale environmental dynamics regulate the phytoplankton community composition and primary productivity in the Bornholm Basin. We combine data from the Swedish National Monitoring Program, collecting phytoplankton (abundance, biomass and productivity) and environmental  (e.g. nutrients concentration) samples with remote sensing data and sustained observations from an Ocean Glider Observatory from March 2021 to 2022.

The Ocean Glider Observatory continuously collected high-resolution physical (e.g.temperature, salinity) and biochemical (chlorophyll a, dissolved oxygen) data (vertical resolutions at 10-centimetre scale and vertical profiles with a mean temporal resolution of ~37 min). Glider data are calibrated against remote sensing data, while in situ measurements of photosynthetic yield and community composition allow us to model community composition and primary productivity across the dataset.

Accurate estimation of primary productivity reveals the importance of short term and rapid changes in near-surface stratification and the deep chlorophyll maximum for the Bornholm ecosystem. Vertically resolved glider estimates also highlight limitations of remote sensing methods in this cloud covered region. As phytoplankton plays a crucial role in the transfer of energy through the food web via zooplankton up to fish, the importance of sustained, continuous high-resolution observations is required to fully capture the magnitude and variability of ecosystem processes and consequences to productivity.