Seasonal variations in phytoplankton productivity in a shallow cove in the eastern Seto Inland Sea, Japan

We examined phytoplankton productivity in Shido Bay, an active oyster farming area in the eastern Seto Inland Sea, Japan, by an in situ 13C method from March 2016 to January 2017. The depth-integrated daily phytoplankton production ranged from 0.13 to 1.61 g C m−2 day−1, and the estimated annual production was 218 g C m−2 year−1. The daily production peaked in the rainy season and autumn, when phytoplankton blooms were observed, and production during the bloom events largely contributed to the annual production. There was a strong correlation between daily production and phytoplankton biomass (chlorophyll a) throughout the study period. In the study area, due to the shallow depth (6 m) and moderate light attenuation coefficient of the water column (0.38 ± 0.07 m−1), the mean light intensity in the water column was maintained at high levels (36 ± 5% of incident photosynthetically active radiation) throughout the study period. In contrast, the dissolved inorganic nitrogen concentration of the water was persistently low (< 1.5 µM) from late winter to summer. As a consequence, an  increase in nitrogen supply seemed to be the key factor causing the increases in daily production and directly triggering the phytoplankton blooms in the bay.