Size‐Fractionated Primary Production Dynamics During the Decline Phase of the North Atlantic Spring Bloom.

The North Atlantic is a region of enhanced biogeochemical and climatological importance for the global ocean as it is the site of one of the largest seasonal phytoplankton blooms on the planet. However, there is a lack of understanding of how phytoplankton size influences bloom dynamics and associated nutrient utilization rates, particularly during the decline phase when export to the deep ocean is especially pronounced. Here, we evaluate trends in size‐fractionated carbon, nitrogen, and silicic acid uptake rates in conjunction with environmental parameters to assess these dynamics. In our study, the decline phase of the bloom continued to be highly productive with net primary production (NPP) ranging from 36.4 to 146.6 mmol C m−2 d−1 and approximately 54% of primary production being driven by large phytoplankton cells (≥5 μm) that were primarily utilizing nitrate (mean f‐ratio of 0.77). Entrainment of silicic acid related to deepening of the mixed layer caused by storms increased silicic acid uptake rates to 2.0–5.7 mmol Si m−2 d−1 without concomitant increases in NPP by large cells (silicic acid to carbon uptake ratios averaged 0.12). A companion study in the North Pacific allowed for paired evaluation of these regions. Our results suggest that in highly productive regions where phytoplankton biomass and productivity is distributed across a broad range of cell sizes, such as the North Atlantic, size itself has a stronger influence on nutrient cycling and potential carbon export relative to regions with lower production and a predominance of small (<5 μm) cells, such as the North Pacific. Plain Language Summary: The North Atlantic Ocean experiences a seasonal bloom of phytoplankton. This bloom represents one of the largest removals of anthropogenic carbon to the deep ocean on the planet. Here, we seek to better quantify the mechanisms of this removal by characterizing the amount and composition of phytoplankton present and how much primary production they are engaging in. We sampled during the decline phase of the spring bloom and found the region to be characterized by mostly large phytoplankton primarily utilizing nitrate. When compared to a companion study in the North Pacific, this study confirms the important role cell size and nutrient availability play in determining the degree of productivity in both high and low primary production oceanic regions. Key Points: The decline phase of the North Atlantic spring bloom exhibits transitions in the balance between small and large size‐fractionated primary production dynamicsDuring the bloom decline, diatoms contribute less to primary production but remain a substantial component of phytoplankton biomassPhytoplankton size is a substantial control on ecosystem dynamics in endmember oceanic productivity‐export systems [ABSTRACT FROM AUTHOR]