Net primary production in the Labrador Sea between 2014 and 2022 derived from ocean colour remote sensing based on ecological regimes

Phytoplankton play a major role in carbon export and storage in the ocean interior through remineralization of particulate carbon into dissolved inorganic carbon (DIC), and represent the “gain” side of the biological carbon pump. Shifts in phytoplankton community structure and species succession impact primary production, quality of food for zooplankton consumers and the fate of organic matter in marine ecosystems.

In the Labrador Sea (LS), a sub-arctic environment, the emergence of large blooms of Phaeocystis spp. in spring at the expenses of diatoms may disrupt phytoplankton species succession with drastic consequences on the carbon cycle and the functioning of the marine ecosystem as these small flagellates aggregate in colonies of up to several millimeters, embedded in gelatinous matrices, that modify elemental stoichiometry, sinking rates and transfer of energy to higher trophic levels. In this study, we develop an ecological approach to estimate primary production due to Phaeocystis sp. in the LS from satellite remote sensing data. We used a regionally-tuned primary production model to assign phytoplankton photosynthesis efficiency as a function of oceanographic regimes defined by phytoplankton community structure and biomass, and sea-surface temperature. We found that four oceanographic regimes corresponded to broad phytoplankton taxonomic assemblages and environmental factors in the LS: the diatom-dominated Shelf, the low chlorophyll-a Basin, the mesotrophic Basin regimes and a last oceanographic regime within the Basin, where the flagellated prymnesiophyte Phaeocystis spp. likely dominated the assemblage. Annual primary production in the Labrador Sea varied between 200 and 300 Tg of carbon between 2014 and 2022 in agreement with previous studies. While Phaeocystis spp. contributed about 10 % of the annual production, two unusual blooms that occurred in 2015 and 2022 contributed about 14 and 20 % of total production, respectively. During these two events Phaeocystis sp. contributed 40 % and 60 % to the May production and extended over more than half the Labrador Sea. Spring blooms dominated by Phaeocystis may occur more frequently due to climate change and have the potential to impact the fate of carbon and alter the functioning of the LS ecosystem.