ABSTRACT: Variations in photosynthetic parameters of phytoplankton during the spring bloom were investigated in the western subarctic Pacific. Light-saturated maximum photosynthesis rate (P^B_max) and light-limited slope (α^B) in the photosynthesis-irradiance (P-E) curve were almost constant in the first half of the bloom and then concurrently increased by 5 times in the latter half with a constant light-saturation index (E_k = P^B_max/α^B). In contrast, the maximum photochemical quantum efficiency (F_v/F_m) of the algal photosystem II increased in the first half and thereafter decreased. Water temperature significantly affected the covariation in P^B_max and α^B (i.e. E_k-independent variability), suggesting that dark reaction processes in photosynthesis mainly regulated the changes in the P-E parameters. Although influences of nitrate, silicic acid, and dissolved iron on the P-E parameters were not observed, the elevated ammonium levels found in the latter half of the bloom could contribute to the E_k-independent variability. There was no significant relationship among algal net growth rate, P^B_max, and α^B throughout the observations. Since most ecosystem models have used a constant α^B, parallel changes in P^B_max and α^B should be taken into account in models reproducing ecosystem dynamics during spring diatom blooms.