Temporal and vertical variability in picophytoplankton primary productivity in the North Pacific Subtropical Gyre

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) pico plankton cells. Primary productivity by picophytoplankton comprised ∼68 to 83% of total (>0.2 μm) particulate ¹⁴C-based productivity, and was lowest be tween September and December and highest between March and August. Group-specific rates of production by Pro chlorococcus, Synechococcus, and photosynthetic pico eukaryotes (PPE) averaged ∼39, ∼2, and ∼11% of the total ¹⁴C-productivity, respectively. Average cellspecific 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 lownutrient ecosystem, highlighting the importance of cell loss processes responsible for mediating organic matter cycling in the euphotic zone of the NPSG.