Effects of a phosphorus-increasing pilot operation at a sewage treatment plant on the color tone of nori and autumn-winter mortality of Manila (Asari) clam population in Mikawa Bay, Japan

Abstract

To recover fishery productivity in Isshiki tidal flat, located in the northwestern part of Mikawa Bay, an operational increase in phosphorus concentration in treated wastewater was implemented. This study investigated the effects of this operation on the color tone (L* value) of the fresh laver (nori) and the standing stock of Manila (Asari) clam. At survey sites near the purification center, nutrient concentrations were higher, and the L* values of nori were lower, indicating better color tone. In the month when the phosphorus-increasing operation began, the concentration of particulate organic matter, which was primarily composed of phytoplankton, increased. Compared to the fiscal year (FY) 2018–2019 period, when the phosphorus-increasing operation began in October, the FY2020–2021 period, when a similar operation began in September, was associated with reduced autumn and winter depletion of clams. These findings suggest that adjusting the phosphorus concentration in treated wastewater may increase the stock of clam populations.