Photosynthesis-irradiance (P-E) relationships and in situ measurements of primary production for 2 phytoplankton size fractions (> 2 mu m and 0.2 to 2 mu m) were used to evaluate variability in photophysiology over a 5 yr period (2004 to 2009) in the North Pacific Subtropical Gyre (NPSG). Picophytoplankton (0.2 to 2 mu m) were dominant contributors to euphotic zone chlorophyll a (chl a) concentrations (averaging 91 +/- 2% [mean +/- SD] of the 0 to 125 m depth-integrated inventories) and accounted for a major fraction (averaging 74 +/- 7%) of the in situ, depth-integrated, C-14-based primary production. Short-term in vitro P-E experiments were conducted to examine the photophysiology of both phytoplankton size classes. Results from these experiments demonstrated that in the well-lit ocean (0 to 45 m) chl a normalized maximum rates of photosynthesis, P-max(chl), were significantly greater among the larger phytoplankton size class than in the smaller size fraction (1-way ANOVA, p < 0.01), while in the dimly lit region (125 m) there were no significant size-dependent differences in P-max(chl) (1-way ANOVA, p > 0.05). Neither the initial slope of the P-E relationships, alpha, nor the light intensities required to saturate photosynthesis, E-k, varied significantly between the 2 size fractions. Although larger phytoplankton appear to constitute a relatively small fraction of phytoplankton biomass and production in this ecosystem, the photophysiological responses of plankton in this size class demonstrated considerable variability, suggesting these larger size phytoplankton experience time-variable changes in growth despite persistently oligotrophic habitat conditions.