Title
Dilution characteristics of riverine input contaminants in the Seto Inland Sea

Members’ names and affiliations
NameInstitutionDepartment

Jie Shi

Ocean University Associate Professorof China, OUC
Ocean University Professor

Huiwang Gaoand Employmentposition

E-mailshijie@ouc.edu.cn
hwgao@ouc.edu.cn

of China, OUC
Xinyu Guo

Faculty member of Professor

guoxinyu@sci.ehime-

LaMeru.ac.jp

Aim
This project aims to carry out cooperative research with Prof. Guo Xinyu of Ehime
University on the dilution characteristics of riverine contaminants in the Seto Inland Sea(SIS) and their controlling factors. At the same time, I am trying to establish a physicalbiological coupled model in the SIS.

Procedure
I showed the recent studies about “Dilution characteristics of riverine input contaminantsin the Seto Inland Sea”, Then I tried to construct a three-dimensional low trophic-levelecosystem model in the Seto Inland Sea, and helped a student to study how to establish
ecological models and analysis model results. During the period, I discussed the problems
encountered in research and further research plan.

Results(1) Dilution characteristics of riverine input contaminants in the Seto Inland Sea
Based on a hydrodynamic model (Model domain was shown in Fig. 1) which can well
reproduce the seasonal variations in temperature and salinity of SIS. We first explored thespatial and temporal variations of concentrations of conservative contaminant and nonconservative contaminants (using antibiotics as an example), and then investigated themechanisms controlling contaminant concentration in the SIS.
For conservative contaminant, when riverine input contaminant concentration was 100,
the annual mean of C𝑣 in the entire SIS was 1.64, showing a high value of 2.03 in Julyand a low value of 1.39 in January (Fig. 2a). The annual mean of C𝑎 was 4.03, which
was more than twice the annual mean of C𝑣 . For C𝑣 , the high value occurred in Harima1 / 5

Nada, Osaka Bay, and the Bisan Strait with an annual mean of 4.58, 3.87, and 3.84,
respectively (Fig. 2b). The annual mean of C𝑣 were 2.18, 1.97, and 1.86 in Hiuchi-Nada,Suo-Nada, and Hiroshima Bay (Fig. 2b), respectively. Low value of C𝑣 was found in theKii Channel, Iyo-Nada, and the Bungo Channel with an annual mean value of 1.57, 1.11,
and 0.42, respectively (Fig. 2b).
Conservative contaminant exhibited a vertically mixed pattern with low concentrationin winter and a vertically varying pattern with high concentration in summer (Fig. 3).
Non-conservative contaminant showed a similar spatial distribution with the conservativecontaminant. As the half-life period of non-conservative contaminant decreased, thecontaminant concentration decreased following power functions. These were useful
information for environmental management to set standards for contaminants to protect
the health of the aquatic ecosystem of the SIS.
The roles played by surface wind velocity, air-sea heat flux and river discharge wereconfirmed with a series of numerical experiments (Fig. 4). The seasonal cycle of air-sea
heat flux was responsible for seasonal variations of contaminant concentrations in thewhole water column by influencing horizontal current and vertical stratification. The
surface wind velocity, which induced sea surface current, played a vital role affecting
surface contaminant concentration.

Fig. 1. Bathymetry (m) of the SIS and model domain. The solid black lines in (b) indicate
the Bungo Channel and Kii Channel, which are the southern boundaries of thehydrodynamic model. The red solid line running in a northwest direction in (b) is theKanmon Strait, which is closed in the model. The blue solid circles with lower case lettersa to u along the coast in (b) are the locations of 21 major rivers around SIS.

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Fig. 2 Time series of (a) 15-day running mean of C𝑣 and C𝑎 over the entire SIS of caseControl in four computational years. (b) is 7-day running mean of C𝑣 in selected subregions in the fourth year. 𝐶𝑣 is the volume-averaged concentration in the SIS, 𝐶𝑎 isthe area-averaged surface concentration in the SIS.

Fig. 3 Vertical distributions of contaminant concentrations in case Control along thetransection of SIS in (a) January, (b) April, (c) July and (d) October. The numbers ofstations correspond to numbers from 1 to 50 of the transection.

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Fig. 4 Time series of 7-day running mean of (a) C𝑣 in the numerical experiments and (b)their change ratios with respect to case Control. (c) and (d) are the same as (a) and (b),but for C𝑎 .
(2) construct a three-dimensional low trophic-level ecosystem model in the Seto
Inland Sea
based on a simple NPZD biogeochemical model and a 3-D hydrodynamical ocean
model in the Seto Inland Sea (SIS) (Chang et al, 2009), I tried to couple thehydrodynamic-biogeochemical module on line to address the ecosystem change in theSIS. I have written the biogeochemical codes coupled with hydrodynamic module, and it
also need to verify the feasibility of code through ideal experiment. At the same time, I
prepared input data for biogeochemical model from observations, literatures and othermaterials, including riverine input nutrients of 21 major rivers around the SIS, nutrients
input from the Bungo Channel and Kii Channel and benthic nutrients flux.

Publication/conference presentation
Publication:
Zhu J, Guo X, Shi J, et al. Dilution characteristics of riverine input contaminants in theSeto Inland Sea[J]. Marine pollution bulletin, 2019, 141: 91-103.
Oral presentation:

Title: Dilution characteristics of riverine input contaminants in the Seto Inland Sea and4 / 5

development of a NPDZ type biogeochemical model for the Seto Inland SeaLecturer: Junying Zhu
Time: February 15, 2019
Location: Ehime University

Perspectives in future
We will carry out further research with Prof. Guo Xinyu on the following subjects:
1. The interannual variations of the cold water mass in Iyo-Nada.
2. The influence factors of seasonal warming range of cold water mass.
3. The physical-biological coupled model.

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