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

Picophytoplankton (<= 3 mu m) 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 (C-14) pico plankton cells. Primary productivity by picophytoplankton comprised similar to 68 to 83% of total (> 0.2 mu m) particulate C-14-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 similar to 39, similar to 2, and similar to 11% of the total C-14-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 similar to 3-fold. In contrast, Synechococcus production varied similar to 30-fold, with peak rates in March 2013 accompanied by similar to 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.