Behavior of river-supplied substances in the coastal sea near the estuary of the Mukawa river during snowmelt season

摘要

The target area of this survey was the estuary of the Mukawa River that is located at the northern part of Japan and faces the Pacific Ocean. The suspended solid (SS) and nutrient concentrations of the river water increased as the runoff increased. The concentrations of particulate nitrogen and particulate phosphorus accounted for over 80% that of total nitrogen and almost all of that of total phosphorus, respectively. This indicates that nutrient was absorbed and transported mainly by SS during floods. During the floods, at near the estuary, the SS and particulate nutrient concentrations of the seawater increased, which clearly reflected the effects of the floods in the river. The concentration of chlorophyll-a behave the almost same as that of particulate total phosphorus. The nutrient contents of the SS in the river water and seawater, and of sediment on the sea bottom increased as the grain diameters became small. Because, the smaller the grain diameter, the lager surface area per unit weight of the grains encourage absorption of nutrient. The fine-grained sediment, which was discharged from the river and contained much nutrient, was deposited even in the offing more than 10 km from the estuary after floods. The river-supplied SS has considerable effects on the bottom environment in large area.