ALBERT

Description: The Labrador Sea is an ideal region to study the biogeographical, physiological and biogeochemical implications of phytoplankton communities due to sharp transitions of distinct water masses across its shelves and the central basin, intense nutrient delivery due to deep vertical mixing during winters and continual inflow of Arctic, Greenland melt and Atlantic waters. In this study, we provide a decadal assessment (2005–2014) of late spring/early summer phytoplankton communities from surface waters of the Labrador Sea based on pigment markers and CHEMTAX analysis, and their physiological and biogeochemical signatures. Diatoms were the most abundant group, blooming first in shallow mixed layers of haline-stratified Arctic shelf waters. Along with diatoms, chlorophytes co-dominated at the western end of the section (particularly in the polar waters of the Labrador Current (LC)), whilst Phaeocystis co-dominated in the east (modified polar waters of the West Greenland Current (WGC)). Pre-bloom conditions occurred in deeper mixed layers of the central Labrador Sea in May, where a mixed assemblage of flagellates (dinoflagellates, prasinophytes, prymnesiophytes, particularly coccolithophores, and chrysophytes/pelagophytes) occurred in low chlorophyll areas, succeeding to blooms of diatoms and dinoflagellates in thermally-stratified Atlantic waters in June. Light-saturated photosynthetic rates and saturation irradiance levels were higher at stations where diatoms were the dominant phytoplankton group (〉 70 %), as opposed to stations where flagellates were more abundant (from 40 % up to 70 %). Phytoplankton communities from the WGC (Phaeocystis and diatoms) had lower light-limited photosynthetic rates, with little evidence of photo-inhibition, indicating greater tolerance to a high light environment. By contrast, communities from the central Labrador Sea (dinoflagellates and diatoms), which bloomed later in the season (June), appeared to be more sensitive to high light levels. Ratios of accessory pigments (AP) to total chlorophyll a (TChl a) varied according to phytoplankton community composition, with polar phytoplankton (cold-water related) having lower AP to TChl a ratios. Phytoplankton communities associated with polar waters (LC and WGC) also had higher and more variable particulate organic carbon (POC) to particulate organic nitrogen (PON) ratios, suggesting the influence of detritus from freshwater input, derived from riverine, glacial and/or sea-ice meltwater. Long-term observational shifts in phytoplankton communities were not assessed in this study due to the short temporal frame (May to June) of the data. Nevertheless, these results have provided a baseline of current distributions and an evaluation of the biogeochemical role of spring phytoplankton communities in the Labrador Sea, which will improve our understanding of potential long-term responses of phytoplankton communities in high-latitude oceans to a changing climate.