Impact of Nutrient Reduction on Organic Matter (COD and TOC) in Coastal Seas

Abstract

Nutrient load reduction has been conducted in many semi-enclosed coastal seas in Japan for restoration from eutrophication. The nutrient concentrations in these seas have significantly decreased; however, the expected reductions were not seen in the organic matter indices (e.g., chemical oxygen demand (COD) ) . The causes of these phenomena were analyzed and the following was found. (1) The decrease in total nitrogen (TN) concentration caused a reduction in organic nitrogen, but no or lesser reductions in total organic carbon (TOC) and COD. This phenomenon is caused by the increase in C:N ratio in the organic matter in the sea. (2) The increase in C:N ratio caused by the nitrogen deficit is also seen in seaweeds (macroalgae) . (3) This plant characteristic suggests that nutrient availability controls the quality (e.g., C:N ratio) of autochthonous organic matter in estuaries and coastal seas. (4) The relationships between COD and TOC have changed spatially and temporally. (5) Nutrient reduction has changed not the quantity but the quality of the organic matter.