Abstract
Diurnal and semi-lunar variations in the light intensity, suspended sediment concentration (SSC), chlorophyll-a concentration, and pheophytin concentration in the Chikugo River estuary are presented, with an emphasis on the effect of the SSC on the light limitation during phytoplankton growth. Vertical profiles of the photosynthetically active radiation (PAR) intensity and SSC were determined, and the light attenuation coefficient (k) was expressed using the regression equation for the SSC. The temporal variation in the euphotic depth was calculated from the PAR and k. The SSC was 20 mg/l during neap tide, and the euphotic depth was 2 m. Since the average SSC was 270 mg/l during spring tide and ranged from 100 mg/l at slack low tide to 450 mg/l at flood tide, the euphotic depth decreased significantly to 0.2 m on average. Phytoplankton bloom was observed during neap tide, and it results from the positive response of phytoplankton to light enrichment. During spring tide, light limitation hampered phytoplankton growth; consequently, the bloom collapsed and an increase in detritus was observed.
