Contribution of 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 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 open ocean, and 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 73 % and 60 % 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 SIS, and being lowest in the nearshore areas. The open ocean imports 797 mol s−1 of DIN to the SIS, 25 % of which is consumed by biogeochemical processes, and 75 % 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 to consider the impact of the open ocean variabilities in the management of land loading of nutrients for the semi-enclosed seas.
Qian Leng et al.
Status: open (until 13 Jun 2023)
- RC1: 'Comment on egusphere-2023-753', Anonymous Referee #1, 19 May 2023 reply
- RC2: 'Comment on egusphere-2023-753', Hagen Radtke, 22 May 2023 reply
Qian Leng et al.
Qian Leng et al.
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This is a numerical model study to identify the contribution of three sources of nitrogen, which is from the land, from the open sea, and from the seafloor, to DIN in a semi-enclosed coastal sea (Seto Inland Sea: SIS). Although this information is important for the prevention of eutrophication in the sea, the calculation method has some problems as shown below.
Unlike the case of phosphorus, nitrogen leached from the seafloor is the result of mineralization of “new” sediments, so nitrogen originating from the seafloor may be included in the open sea nitrogen and land nitrogen.
Dissolved organic nitrogen, which accounts for about 90% of the total nitrogen in SIS, is not included in the calculation. The Ministry of the Environment's total load reduction for SIS is also based on total nitrogen in its calculations.
The boundary condition is that the open sea origin DIN is zero at the seafloor and landward.
If there are no biochemical processes in SIS, no DIN supply from land or seafloor, only physical diffusion, then the DIN concentration in SIS is equal to the open boundary DIN (DIN from the open sea) and SIS is filled with DIN from the open sea. In other words, DIN = 0 does not occur on the seafloor surface or on the landward shore.
The terrestrial nitrogen load for SIS is published every five years by the Ministry of the Environment of Japan (MEJ). It is necessary to state the values of the terrestrial load by MEJ and the terrestrial load in this report.
In SIS, which experienced eutrophication in the 1970s, the majority of domestic and industrial wastewater is treated at treatment facilities on the waterfront and discharged directly into the sea in recent years. Therefore, there is a large difference between the DIN flow via rivers and the total nitrogen flow actually entering the sea (especially in the eastern Seto Inland Sea).
In SIS, river discharge is significantly lower in winter, resulting in large seasonal variations in DIN flow from rivers, whereas there is little seasonal variation in DIN flow from domestic and industrial sources. This affects the seasonal variation of DIN concentration in the SIS.
It is important to indicate the time required for the numerical model to become stationary; the DIN flow path during the set-up period is not the flow path when the model becomes stationary.
It should be noted that the seasonal variation of the nitrogen load from rivers is due to the seasonal variation of the river flow. Unlike Europe, the SIS receives a little precipitation in winter.