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.