02 May 2023
 | 02 May 2023
Status: this preprint is open for discussion.

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

Qian Leng, Xinyu Guo, Junying Zhu, and Akihiko Morimoto

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)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • 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.


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Short summary
Using a numerical model, we revealed that a large proportion of nutrients in a semi-enclosed sea (Seto Inland Sea, Japan) comes from the Pacific Ocean and supports the growth of more than half of phytoplankton in the sea. Such results imply that the human-made management of nutrient load from the land needs to consider the presence of oceanic nutrients that act as a background concentration and are not controlled by human activity.