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Underwater light environment and photosynthetic accessory pigments were investigated in Ariake Bay in order to understand how change of the pigments occurs in response to the tidal-induced changes in underwater light conditions. We hypothesize that phytoplankton increases photo-protective pigments and decreases light-harvesting pigments under higher light condition in the mixed layer caused by tidal cycle. Contribution rates of non-phytoplankton particles ( a(400-700)) for light attenuation coefficient ( K) was highest (32-85%), and those of phytoplankton particles ( a(400-700)), dissolved organic matter ( a(400-700)) and water were 6-32, 6-21 and 5-23%, respectively. Mean K was higher during the spring tide (0.55 ± 0.23 m) than the neap tide (0.44 ± 0.16 m), and the K difference was caused by the substances resuspension due to the tidal current. In contrast, ratios of photo-protective pigments (diadinoxanthin and diatoxanthin) per chlorophyll a ((DD+DT)/Chl a) were higher during the neap tide (0.10 ± 0.03 mg mg-Chl a) than the spring tide (0.08 ± 0.03 mg mg-Chl a). And there was significant positive correlation between (DD+DT)/Chl a and mean relative PAR in the mixed layer ( $$ \overline {I_{mix} } $$). Moreover, there was significant negative correlation between ratios of light-harvesting pigments (fucoxanthin) per Chl a (Fuco/Chl a) and $$ \overline {I_{mix} } $$. These results suggested that phytoplankton in Ariake Bay increase photo-protective pigments and decrease light-harvesting pigments in the higher light condition of less turbid, shallower mixed layer during neap tide than spring tide. [ABSTRACT FROM AUTHOR]