Resolving the interannual variability of phytoplankton community composition in Fram strait using ship-based high-frequency spectrophotometric measurements from 2015 to 2024

The Arctic is warming more than four times as fast as any other region of our planet. This warming has led a dramatic decline in seasonal sea-ice coverage and thickness and to a poleward extent of the Atlantic water. The Fram Strait is the only deep gateway of the Arctic Ocean where the West Spitsbergen Current transports the Atlantic water northward and the East Greenland Current the polar water southward. This is a highly dynamic oceanographic regime in the zone (central Fram Strait) between the two currents with its mixed water in combination with the semi-permanent sea-ice edge and large horizontal density gradients observed in the marginal ice zone. Long-term high-spatial resolution data on phytoplankton distribution and its community composition help to assess the impact of these physical changes on the biological processes in the surface waters of Fram Strait. In this study, we resolve the implications of changes in sea-ice conditions and physical hydrographic properties in Fram Strait to the chlorophyll-a concentration (Chl-a) of the whole phytoplankton community and of the major phytoplankton groups (PGs) contributing to its biomass. We extend the formerly collected total Chl-a (TChl-a) and PG Chl-a data sets of High-Performance Liquid Chromatography and of hyperspectral particulate absorption data sets from high-frequency underway spectrophotometry measurements to include the five recent East Greenland Sea expeditions from 2019 and 2021 to 2024. We adapt well-established methods to retrieve from the hyperspectral data as baseline for more high-spatial resolution TChl-a and PG Chl-a retrievals. Cross validation and independent validation confirm the robustness of our 2015 to 2024 data sets. We analyse the ship-based high-spatial resolution (around 300 m) continuous Fram Strait time series data along the ship transect of the eight expeditions. We observe diatoms and haptophytes, the key functional groups of the marine ecosystem, together with chlorophytes to represent most of the phytoplankton community. We identify a shift in spring blooms’ phytoplankton community composition responding to the distinct sea-ice thickness conditions in different years. We observe in the West Spitsbergen Current that the mid-summer TChl-a and the contribution of haptophytes to the phytoplankton community are much lower during the latest years (2022 and 2024) identified as cold as opposed to the warm (2015 to 2017) temperature anomaly years. We recommend continuing the underway spectrophotometry measurements in future and combine these long-term high-spatial resolution time series on phytoplankton community composition with satellite measurements to track the effects of the changes of the physical environment to global warming on the main primary producer in the Arctic Ocean. • High-spatial resolution Chl-a data, including phytoplankton groups’ contribution. • Spring bloom phytoplankton composition responds to sea-ice thickness conditions. • Chl-a is lower in mid-summer during cold opposed to warm temperature anomaly years. • Haptophytes contribute less during cold as opposed to warm anomaly years.