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The method of estimating the source origin of the organic matter that consumes dissolved oxygen in an aquatic environment using carbon stable isotope (δC) of dissolved inorganic carbon (DIC) is effective because the method needs only a small portion of water and a brief duration. In an anoxic water mass in a low-lying depression in a port, δC-DIC decreases in accordance with the increase in the concentration of DIC (R=0.93); and hence, δC of the organic matter that consumes dissolved oxygen can be estimated with considerable accuracy using the relationship between the concentration of DIC and δC-DIC. We demonstrated that the method of using in situ δC-DIC, which had been applied to lakes, performs effectively under the condition of continually decreasing pH in an anoxic water mass formed at the bottom of the sea. The contribution ratio of the phytoplankton-derived and land-derived organic matter was estimated using their δC values estimated in previous studies. As a result, 59-88% of organic matter that consumes oxygen was estimated to originate from phytoplankton. Low-lying depressions in coastal seas, where the water exchange rate is significantly small, is suitable for examining remineralization processes of organic matter in coastal seas.

Estimation of Source Origins of Organic Matter Inducing Oxygen-Depleted Water in Coastal Seas Using Stable Isotope Ratios of Carbon