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The NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was established to better quantify the pathways of the biological carbon pump in order to gain a more comprehensive understanding of global carbon export efficiency. The summer 2018 field campaign in the vicinity of Ocean Station Papa (Station P; 50°N, 145°W) in the Northeast Pacific Ocean yielded evidence of low phytoplankton biomass and primary productivity dominated by small cells (<5 µm) that are reliant on recycled nutrients. Using combined 13C/15N stable isotope incubations, we calculated an average depth-integrated dissolved inorganic carbon uptake (net primary production) rate of 23.1 mmol C m–2 d–1 throughout the euphotic zone with small cells contributing 88.9% of the total daily DIC uptake. Average depth-integrated NO3– uptake rates were 1.5 mmol N m–2 d–1 with small cells contributing 73.4% of the total daily NO3– uptake. Estimates of new and regenerated production fluctuated, with small cells continuing to dominate both forms of production. The daily mixed-layer f-ratio ranged from 0.17 to 0.38 for the whole community, consistent with previous studies, which indicates a predominance of regenerated production in this region, with small and large cells (≥5 μm) having average f-ratios of 0.28 and 0.82, respectively. Peak phytoplankton biomass, total primary productivity and new production occurred between Julian Days 238 and 242 of our observation period, driven primarily by an increase in carbon and nitrate assimilation rates without apparent substantial shifts in the phytoplankton size-class structure. Our findings demonstrate the importance of small cells in performing the majority of net primary production and new production and the modest productivity fluctuations that occur in this iron-limited region of the Northeast Pacific Ocean, driven by ephemeral increases in new production, which could have significant ramifications for carbon export over broad timescales.

The genus Phaeocystis is distributed globally throughout the world's oceans, forming blooms in European, Asian, Antarctic, Arctic, and North American waters and in tropical, polar and temperate systems. It often (but not always) releases dimethylsulfide into the surface layer, which has the potential for altering weather and climate patterns (Charlson et al., 1987; Stefels et al., 1995), and significantly influences the biogeochemistry of sulfur (Wang et al., 2016). In addition, Phaeocystis is considered to be largely "unpalatable" to zooplankton grazers (Nejstgaard et al., 2007). The ecology and physiology of Phaeocystis are relatively well studied, but given the variety of environments the genus can occupy, it is difficult to generalize about

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The second field campaign of the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was conducted in the late spring of 2021 within the vicinity of the Porcupine Abyssal Plain (49.0 degrees N, 16.5 degrees W) in the North Atlantic Ocean. Observations from EXPORTS support previous characterizations of this system as highly productive and organic matter rich, with the majority of primary production occurring in large cells (>5 microns) such as diatoms that are primarily utilizing nitrate. Rates of total euphotic zone depth-integrated net primary production ranged from 36.4 to 146.6 mmol C m-2 d-1, with an observational period average f-ratio of 0.74, indicating predominantly new production. Substantial variability in the contribution of small (<5 microns) and large cells occurred over the observation period, coinciding with the end of the annual spring phytoplankton bloom. Physical changes associated with storms appear to have impacted the integrated production rates substantially, enhancing rates by ~10%. These disturbances altered the balance between contributions of the different phytoplankton size fractions, thus highlighting the important role of mixed layer variability in nutrient entrainment into the upper water column and production dynamics. In diatoms, inputs of silicic acid related to deepening of the mixed layer increased silicic acid uptake rates yet concomitant increases in NPP in large cells was not observed. This campaign serves as the high productivity endmember within the EXPORTS program and as such, elucidates how nutrient concentrations and size class play key roles in both low and high productivity systems, but in differing ways.