In the subarctic Pacific and its adjacent waters, the photochemical quantum yield (F-v/F-m) and the functional absorption cross section (sigma(PSN)) of photosystem II for surface phytoplankton were continuously measured during the early summer of 1999 using a fast repetition rate fluorometer. Concentrations of total dissolvable iron (TD-Fe) were also determined at each sampling station. The maximum value (0.61) of F-v/F-m was observed off the Aleutian Peninsula, where TD-Fe concentrations were relatively high. The Western Subarctic Gyre (WSG) and the Alaskan Gyre (AG), located in the northwest and northeast subarctic Pacific, respectively, were in high-nutrient, low-chlorophyll (HNLC) conditions. Surface TD-Fe generally remained in the WSG but was depleted (<0.01 nmol L-1) in the AG. Nighttime F-v/F-m and sigma(PSII) in the WSG were significantly higher (p < 0.01) and lower (p < 0.01), respectively, than in the AG. Iron or nitrogen limitations generally lead to a decrease in F,IF and an increase in sigma(PSII). These results suggested that there was an east-west gradient (WSG > AG) in the photosynthetic competence of phytoplankton in the subarctic Pacific and that the difference was probably caused by iron levels in seawater. Indeed, our iron enrichment experiment in the AG revealed that F-v/F-m increased from 0.27 to 0.49 and sigma(PSII) decreased from 496 X 10(-20) to 365 X 10(-20) m(2) photon(-1) after a 0.8 nmol L-1 iron addition. At the same time, a dramatic floristic shift from phytoflagellates to diatoms was found by pigment signatures. Iron could principally control the photosynthetic physiology of phytoplankton in the whole subarctic Pacific.