2022 Volume 45 Issue 4 Pages 181-191
We investigated the effect of phosphorus concentration control in basin sewerage on the decline of the juvenile clam population in the Toyogawa River mouth area in eastern Mikawa Bay from October 2018 to March 2019. In FY2018, the chlorophyll a concentration in the tidal flat was higher than that in FY2016 before the management of nutrient supply in the autumn and winter. The survival rates of juvenile clams born before autumn in 2018 were higher than those in FY2016. Juvenile clam (S.L. ≦ 300 μm) settled from October 2018 to January 2019 and grew up toward March while maintaining the population density. The picoplankton and nanoplankton of the 0.2–10 μm size fraction proliferated, as inferred from the amount of phosphorus released from the sewer when the daily variation of phosphorus load from Toyogawa River decreased from November 2018 to March 2019. The presence densities of settled juvenile clam (S.L. ≦ 300 μm) and early juvenile clam (300 < S.L. ≦ 500 μm) correlated with the chlorophyll a concentration of the size fraction of 0.2–10 μm. Phosphorus supplied during the concentration control operation maintained a suitable feeding environment for juvenile clam through the proliferation of phytoplankton.
