The Omuta City North Sewage Treatment Plant (STP) conducts seasonal operations to release nutrients into the Ariake Sea. However, it is unclear how much of the nutrient load from STP is supplied to the Ariake Sea via the Domen River. Therefore, in this study, we calculated the nutrient load from the STP and the nutrient load upstream of the Domen River, and investigated the effect of seasonal operation on the nutrient load supplied from the Domen River to the Ariake Sea. As a result, the ratio of the T-N load from the STP to the T-N load supplied from the Domen River to the Ariake Sea accounts for 82 to 98% in the nitrification control period and 61 to 94% in the nitrification accelerated period. The average annual T-N load from the STP, which increased due to seasonal operation, was 23 tons.