Contribution of the open ocean to the nutrient and phytoplankton inventory in a semi-enclosed coastal sea

Abstract. The semi-enclosed coastal seas serve as a transition zone between land and the open ocean, and their environments are therefore affected by both. The influences of land were noticed but that of the open ocean were usually neglected. The Seto Inland Sea (SIS), which is connected to the Pacific Ocean, is a typical representative of semi-enclosed seas. To quantitatively assess the inventory of nutrients originating from land and the open ocean, as well as their supported phytoplankton in the SIS, we developed a three-dimensional coupled hydrodynamic–biogeochemical model and embedded a tracking technique in it. Model results showed that the open ocean contributes 61 % and 46 % to the annual inventory of dissolved inorganic nitrogen (DIN) and phytoplankton in the SIS, respectively. This proportion has apparent spatial variations, being highest near the boundary with the open ocean, decreasing from there towards the interior area of the SIS, and being lowest in the nearshore areas. The open ocean imports 799 mol s−1 of DIN to the SIS, 25 % of which is consumed by biogeochemical processes and 75 % of which is delivered again to the open ocean. Such a large amount of oceanic nutrient input and its large contribution to the inventory of DIN and phytoplankton suggest the necessity of considering the impact of open-ocean variabilities in the management of the land loading of nutrients for the semi-enclosed seas.