In situ fast-repetition-rate fluorometric (FRRF) surveys were conducted in the North Pacific Subtropical Gyre (NPSG) at Station ALOHA (Sta. ALOHA, 22u459N, 158u009W), from September 2002 to December 2004, to assess temporal and vertical photosynthetic variability in relation to environmental conditions. The nighttime potential photosynthetic efficiency of the photoautotrophic microbial assemblage (given as the ratio of variable to maximal fluorescence, FV :F M) was low in the mixed layer and increased with depth. High FV :F M values were observed at and below the deep chlorophyll maximum layer (DCML); some values approached the theoretical maximum for prokaryotes grown under nutrient-replete conditions in the laboratory (0.60). In contrast, the absorption cross section of photosystem II (sPSII) was high at the surface and decreased with depth; minima (1,000 A ˚ 2 quanta21) occurred around the DCML. These vertical patterns suggest photosynthetic stress conditions in surface photoautotrophic populations. No significant seasonal cycles were found for FV :F M, but surface sPSII values peaked in winter and decreased during summer, suggesting that seasonal variations in light availability may influence the observed sPSII variability. A significant correlation was found among surface FV :F M, sPSII, and the distance from the mixed-layer depth (MLD) to the top of the nutricline. Neither nutrient nor light variations were significantly related to FV :F M and sPSII in the DCML. Within this layer, FV :F M variability was positively and negatively related to concentrations of chlorophyll b and zeaxanthin, respectively. Our results suggest that, at Sta. ALOHA, surface photosynthesis takes place under chronic nutrient limitation, while higher photosynthetic efficiency in the lower euphotic zone appears to be sensitive to community-structure changes.