2024 Volume 47 Issue 1 Pages 37-46
Even though total nitrogen (TN) and total phosphorus concentrations in semi-enclosed coastal seas have decreased over time, chemical oxygen demand (COD) has not decreased, and conversely, COD increase has been observed. In our previous reports, we showed a stoichiometric change in organic matter (increase in C:N ratio) owing to a decrease in TN in the sea area, suggesting a recalcitrancy of organic matter (hardly decomposed to inorganic matter) . In this report, a similar analysis was conducted in the Seto Inland Sea, and the COD increase due to the recalcitrancy of organic matter was also investigated. The following results were obtained from these studies. TN reduction causes a stoichiometric change and a recalcitrancy of organic matter. Both changes act in a way that does not lower COD. The stoichiometric change in organic matter causes a significant decrease in total organic nitrogen (TON) in the sea, but not in COD. Because of the recalcitrancy of COD, the concentration increases even if the amount of production does not increase. After 1990, the reduction of TN did not reduce the COD in any of the seas investigated, but in contrast, COD increased in highly enclosed waters.
