Impact of Nutrient Reduction on Organic Matter (COD and TOC) in Coastal Embayments: Effect of the Reduction and Its Working Mechanism

Abstract

Because of the reduction of nutrients in marine areas nationwide, the rates of achieving total nitrogen (TN) and total phosphorus (TP) water quality standards have reached 96% and 95%, respectively. Nevertheless, the achievement rate of the chemical oxygen demand (COD) standard has remained almost unchanged over the last 30 years. The cause of this discrepancy was investigated in Osaka Bay and the following was found. Eutrophication measures such as “TN reduction to suppress the occurrence of high-concentration phytoplankton blooms (red tide) ” are working effectively. This was seen in the water quality indicators associated with particulate organic matter. In contrast, the amount of dissolved organic matter has not decreased. The reduction of nutrients has resulted in a rise in the C: N ratio of organic matter, caused by the constancy of the organic carbon concentration and the decrease in organic nitrogen concentration. A discrepancy between COD and TOC trends has also occurred. It was common to Tokyo Bay and Ise Bay that COD has not decreased because nutrient reduction induces a change in the quality of organic matter.