ABSTRACT:

Picophytoplankton (≤3 µm) are major contributors to plankton biomass and primary productivity in the subtropical oceans. We examined vertical and temporal variability of picophytoplankton primary productivity at near-monthly time scales (May 2012–May 2013) in the North Pacific Subtropical Gyre (NPSG) based on filter size-fractionated and flow cytometric sorting of radiolabeled (¹⁴C) picoplankton cells. Primary productivity by picophytoplankton comprised ~68 to 83% of total (>0.2 µm) particulate ¹⁴C-based productivity, and was lowest between September and December and highest between March and August. Group-specific rates of production by Prochlorococcus, Synechococcus, and photosynthetic picoeukaryotes (PPE) averaged ~39, ~2, and ~11% of the total ¹⁴C-productivity, respectively. Average cell-specific rates of production by PPE (15.2 fmol C cell^-1 d^-1) were 25- to 90-fold greater than Prochlorococcus (0.36 fmol C cell^-1 d^-1) and Synechococcus (1.56 fmol C cell^-1 d^-1). Prochlorococcus dominated (61-78%) the summed picophytoplankton biomass, while PPE and Synechococcus contributed 21-36% and 2-8%, respectively. Rates of production normalized to biomass were nearly equivalent amongst Prochlorococcus, Synechococcus, and PPE, averaging 0.6, 0.5, and 0.4 d^-1, respectively. Over our study period, Prochlorococcus and PPE production varied 4- to 5-fold, while biomass varied ~3-fold. In contrast, Synechococcus production varied ~30-fold, with peak rates in March 2013 accompanied by ~13-fold increase in biomass. Combined, our results provide evidence for rapid growth by picophytoplankton in this persistently low-nutrient ecosystem, highlighting the importance of cell loss processes responsible for mediating organic matter cycling in the euphotic zone of the NPSG.