Biogeographical patterns and environmental controls of phytoplankton communities from contrasting hydrographical zones of the Labrador Sea

The Labrador Sea is an important oceanic sink for atmospheric CO2 because of intensive convective mixingduring winter and extensive phytoplankton bloomsthat occur during spring and summer. Therefore, a broad-scale investigation of the responses of phytoplanktoncommunity composition to environmental forcing is essential for understanding planktonic food-web organisation and biogeochemical functioning in the Labrador Sea. Here, we investigated the phytoplanktoncommunity structure (>4 μm) from near surface blooms( 1.2 mg chla m−3) occurred on and near the shelves in May and in offshore waters of the central Labrador Sea in June due to haline- and thermal-stratification, respectively. Sea ice-related (Fragilariopsis cylindrusand F. oceanica) and Arctic diatoms (Fossula arctica, Bacterosira bathyomphala and Thalassiosira hyalina) dominated the relatively cold (<0 °C) and fresh (salinity < 33) waters over the Labrador shelf (e.g., on the southwestern side of the Labrador Sea), where sea-ice melt and Arctic outflow predominates. On the northeastern side of the Labrador Sea, intense bloomsof the colonial prymnesiophyte Phaeocystispouchetii and diatoms, such as Thalassiosira nordenskioeldii, Pseudo-nitzschiagranii and Chaetocerossocialis, occurred in the lower nutrient waters (nitrate < 3.6 μM) of the West Greenland Current. The central Labrador Sea bloomoccurred later in the season (June) and was dominated by Atlantic diatoms, such as Ephemeraplanamembranacea and Fragilariopsis atlantica. The data presented here demonstrate that the Labrador Sea spring and early summer bloomsare composed of contrasting phytoplanktoncommunities, for which taxonomic segregation appears to be controlled by the physical and biogeochemical characteristics of the dominant water masses.