Size-fractionated phytoplankton production and biomass during the decline of the northwest Atlantic spring bloom

The size structure of phytoplankton production and biomass was surveyed during a 7-day Lagrangian study of the decline of the spring phytoplankton bloom in the northwest Atlantic and during post-bloom conditions. Well-stratified and nutrient-depleted surface waters led to nitrogen limitation of phytoplankton production and to the decline of one of the most productive blooms ever reported for the open North Atlantic Ocean. During the Lagrangian study period, large centric diatoms (>= 5 mu m) dominated both the phytoplankton production (>= 50%) and chlorophyll a (chl a) biomass (>= 82%) in the euphotic zone. During post-bloom conditions, small flagellates (0.7-5 mu m) represented up to 79% of the total phytoplankton production and 45% of total chl a biomass. Throughout the study period, a deep chl a maximum (DCM), corresponding to a maximum in phytoplankton biomass and abundance, developed at the base of the euphotic zone, near the nutricline, where photo-acclimated cells maintained low sinking velocities. The analysis of the relative contribution of small and large phytoplankton cells to total primary production and biomass revealed that small phytoplankton were probably grazed by microzooplankton, whereas large cells accumulated in the euphotic zone at the DCM. The decline of the bloom was thus not associated with the rapid sinking of large phytoplankton cells out of the euphotic zone during the study period.