Size Distributions and Carbon Biomass of Surface Picophytoplankton at Station ALOHA Using the SeaFlow Underway Flow Cytometer.

SeaFlow is a custom-built flow cytometer that samples continuously from the ship’s underway seawater system. SeaFlow has been deployed monthly at Station ALOHA since 2015 to better characterize the fluctuations of carbon produced by picoplankton, which dominate the phytoplankton community in oligotrophic waters. We estimated the equivalent spherical diameter using Mie theory and derived carbon quota for Prochlorococcus, Synechococcus, Crocosphaera (2-5 mm), and picoeukaryotic phytoplankton cells (< 5 mm). We found that Prochlorococcus comprised most of the picophytoplankton biomass in spring, fall, and winter, while picoeukaryotes and Crocosphaera were responsible for most of the carbon biomass in summer. These seasonal shifts in biomass are due to a summer upswing in picoeukaryote and Crocosphaera abundance. This summertime plenitude of N2-fixing Crocosphaera could play a significant role in the nitrogen cycle at Station ALOHA. Size structure in marine picophytoplankton communities has implications for nutrient uptake, trophic dynamics, and the biological carbon pump.