Temporal variations in the surface primary productivity and their causes at a station F3 in the inner Tokyo Bay

To investigate temporal variations in primary productivity in Tokyo Bay, the surface water fixation rates of inorganic carbon by phytoplankton were measured using 13 C under simulated in situ conditions at approximately monthly intervals during 1999–2006 (Period I) and 2018–2021 (Period II) at the time-series observation station F3. The carbon-fixation rate showed a clear seasonal change, with higher rates during the stratified season (April–October) and lower rates during the mixing season (November–March). The mean rate during the stratified season in Period II (136.5 ± 151.1 μg C L −1 h −1 ) was 1.7 times higher than that in Period I (82.0 ± 95.5 μg C L −1 h −1 ), which was attributed to increases in chlorophyll a concentrations and assimilation number. The analysis of photosynthesis–irradiance (PE) curves, which were obtained from experiments with adjusted light intensity, indicated fewer cases of photosynthetic rate saturation under in situ light conditions during Period II than during Period I. The recent increase in chlorophyll a concentrations likely resulted from increased water temperature and reduced salinity, which have strengthened the stratification and allowed phytoplankton to accumulate in the surface water. The increase in the assimilation number and change in the shape of the PE curve may be due to the adaptation of phytoplankton to intense light in the surface water.