Numerical Simulation of Dissolved Iron Production and Transport in the Amur River and the Sea of Okhotsk

Abstract

Wetlands in the Amur River basin play an important role in producing dissolved iron. A significant part of this dissolved iron is removed at the estuary zone. However, the mouth of the river is coincidentally located over the northwestern shelf where dense shelf water (DSW) is vigorously produced. Therefore, dissolved iron from the Amur River should be transported by DSW to the Sea of Okhotsk where iron is the limiting factor in primary production. During the last century, large areas of wetland within the Amur River catchment have been cultivated, which, in turn, may have had a great impact on the production of dissolved iron. To assess the impact of this conversion in land cover on the production of ­dissolved iron—and thereby the primary production in the Sea of Okhotsk—we constructed a numerical model to simulate dissolved iron production and transport in the Amur River and the Sea of Okhotsk.The developed model consists of terrestrial and marine components. The terrestrial part of the model successfully simulates the discharge and dissolved iron flux of the basin. The marine part of the model also effectively simulates realistic DSW, which flows along the coast of Sakhalin through an intermediate (200–500 m deep) layer, and then experiences strong tidal mixing adjacent to the Kuril Islands in the southern Okhotsk Sea. Thus, our model can simulate dissolved iron production and transport in the Amur River basin and the Sea of Okhotsk.Using the terrestrial part of the model, we simulated the impact of various land cover changes on the dissolved iron productivity of the Amur River basin. Two typical land cover change scenarios were proposed as possible situations in the basin. One involves the conversion of wetland to agricultural land (paddy fields and dry land); the other is forest fire. Results indicated that the conversion of wetlands to agricultural land has a significant impact on the dissolved iron flux. Further research is needed to determine how wetland decrease might affect primary production in the Sea of Okhotsk. This paper consists of two parts. The first part addresses the results of the terrestrial simulation, and the second part deals with the results of the marine simulation.