Marine Pollution Bulletin 141 (2019) 91–103 Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Dilution characteristics of riverine input contaminants in the Seto Inland Sea T ⁎ Junying Zhua,b,c, Xinyu Guoa,b, , Jie Shia,c, Huiwang Gaoa,c a Key laboratory of Marine Environment and Ecology, Ocean University of China, Ministry of Education, 238 Songling Road, Qingdao 266100, China b Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama 790-8577, Japan c Laboratory for Marine Ecology and Environmental Sciences, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China ARTICLE INFO ABSTRACT Keywords: Riverine input is an important source of contaminants in the marine environments. Based on a hydrodynamic Dilution model, the dilution characteristics of riverine contaminants in the Seto Inland Sea and their controlling factors Riverine pollution were studied. Results showed that contaminant concentration was high in summer and low in winter. The Seto Inland Sea Contaminant concentration decreased with the reduction of its half-life period, and the relationship between Hydrodynamic model them followed power functions. Sensitivity experiments suggested that the horizontal current and vertical Residual currents stratification associated with air-sea heat flux controlled the seasonal cycle of contaminant concentration in the water column; however, surface wind velocity was the dominant factor affecting the surface contaminant concentration. In addition, contaminant concentration in a sub-region was likely controlled by the variations in river discharges close to the sub-region. These results are helpful for predicting contaminant concentrations in the sea and are expected to contribute to assessing the potential ecological risks to aquatic organisms. 1. Introduction populations, increasing the emergence of antibiotic resistant strains (Boon and Cattanach, 1999; Vivekanandhan et al., 2002; Baquero et al., With the rapid development of human life, more and more terres- 2008) and severely damaging fisheries production (Karunasagar et al., trial chemical substances, such as pharmaceuticals (Hirsch et al., 1999; 1994; Majtán et al., 2012). Previous studies have concluded that most Managaki et al., 2007), polychlorinated biphenyls (Burreau et al., antibiotics pose a relatively high ecological risk to relevant aquatic 2006), polycyclic aromatic hydrocarbons (Montuori and Triassi, 2012) organisms in marine environments (Lee et al., 2008; Zhao et al., 2010; and organochlorine pesticides (Hu et al., 2009), enter the natural water Zhang et al., 2012). Moreover, the degree of the effect of these con- environment and are detected in coastal seas around the world. The taminants on aquatic organisms mainly depends on their concentration adverse ecological risks of these contaminants to aquatic organisms are in the water. a major concern due to their potential toxic effects and biomagnifica- Riverine input is a major source of contaminants in marine en- tion in marine mammals (Halling-Sørensen et al., 1998; Burreau et al., vironment and a high concentration of contaminants in rivers has been 2006). observed in many countries (Xu et al., 2007; Minh et al., 2009; Zhao Among these contaminants, persistent organic pollutants (POPs) et al., 2010; da Silva et al., 2011; Zou et al., 2011; Zhang et al., 2012; have been of concern over the last decade (Iwata et al., 1993; Lohmann Grill et al., 2016). However, the contaminant concentrations in rivers et al., 2006) because they have a long half-life period and remain in and sea water are extremely different due to different hydrodynamic water environments for years. On the other hand, some contaminants, environments. When contaminants enter coastal seas from rivers, two such as antibiotics, have a short half-life period of a few days and are major questions are raised. The first is whether the ocean dilutes the relatively easy to remove from water environments. However, due to contaminant levels to a lower concentration (i.e., to a nontoxic degree) the potential threat to ecosystem balance and risk to health of humans without considerable consequences to aquatic organisms. The second is and animals, antibiotics in aquatic environments have generated what are the key factors controlling contaminant concentration in the growing interest among researchers in recent years (Marti et al., 2014; sea. Chow et al., 2015; Grill et al., 2016; Liu et al., 2017; Liu et al., 2018). To answer these questions, many fixed-point observations of the Antibiotics that enter water environments exert constant selection contaminant concentrations and risk assessments based on contaminant pressure on both bacterial fish pathogens and marine microbial concentrations have been conducted in some coastal seas (Gómez- ⁎ Corresponding author at: Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama 790-8577, Japan. E-mail address: [email protected] (X. Guo). https://doi.org/10.1016/j.marpolbul.2019.02.029 Received 14 November 2018; Received in revised form 13 February 2019; Accepted 13 February 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved. J. Zhu, et al. Marine Pollution Bulletin 141 (2019) 91–103 Fig. 1. (a) Location of the Seto Inland Sea (SIS). (b) 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 the hydrodynamic model. The red solid line running in a northwest direction in (b) is the Kanmon Strait, which is closed in the model. The blue solid circles with lower case letters a to u along the coast in (b) are the locations of 21 major rivers (Table 1), and the green solid circles are the locations of the five secondary rivers. (c) Monthly observation stations in the SIS. The hollow blue circles and solid red circles in (c) are the monthly sampling stations. The solid red circles and lines with numbers from 1 to 50 denote selected stations to validate the model results in the vertical direction usedinFig. 4. The dashed black rectangles in (c) represent selected sub-regions for the purpose to show the seasonal variation of mean contaminant concentration in the sub-region. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Gutiérrez et al., 2007; Zhao et al., 2010). Two related factors were of riverine contaminants in the entire SIS and their influencing factors proposed. Dilution factors, defined as the concentration ratio of con- by using a hydrodynamic model. We first explored the spatial and taminants in a river (or effluent) and sea water near the outfalls, have temporal variations of concentrations of conservative contaminant and been used to represent the dilution levels of sea water (Minh et al., non-conservative contaminants (using antibiotics as an example), and 2009; Zhang et al., 2012). The other is the contaminant flux from rivers, then investigated the mechanisms controlling contaminant concentra- which is the product of contaminant concentrations and river dis- tion in the SIS. charge, and it is often used to estimate the contribution of rivers to contaminant concentration in sea water (Wolschke et al., 2011; Zou et al., 2011; Montuori and Triassi, 2012). However, the key factors 2. Model configuration and validation responsible for contaminant concentration in the ocean were still un- clear, and it should be explored in a specific estuary or coastal sea. 2.1. Simulation and validation of temperature and salinity Due to limitation in observation, there is insufficient data to show continuous spatio-temporal variation of contaminant concentration in The SIS is a shallow semi-enclosed shelf sea located in western 2 sea water. In addition, previous dilution studies about contaminants Japan, with a surface area of 23,000 km and an average depth of 38 m have mostly focused on the areas around the mouths of rivers; there- (Fig. 1a). It opens to the Pacific Ocean via the Bungo Channel and Kii fore, there is a lack of information on dilution characteristics in offshore Channel, through which tidal waves enter the SIS. It is surrounded by and shelf areas. Modeling is a powerful tool for coupling processes the islands of Honshu, Shikoku, and Kyushu, which are rapidly devel- under different disciplines and spatio-temporal scales. To address these oping, urbanized regions in Japan. There are 21 first-order rivers and issues, a hydrodynamic model is required. 643 secondary order rivers emptying into the SIS. It has a complicated The Seto Inland Sea (SIS) region (Fig. 1) is one of the most in- geographical character, including a series of broad basins (called dustrialized areas in Japan. Murata et al. (2011) reported the results of “nada” in Japanese), such as Suo-Nada, Iyo-Nada, Hiuchi-Nada, and a nationwide survey of commonly used human and veterinary anti- Harima-Nada, which are connected to each other through narrow biotics in 37 Japanese rivers. They found that concentrations of the sum straits. The predominant tidal constituent in the SIS is the lunar semi- of 12 target antibiotics in 8 rivers, which directly emptied into the SIS, diurnal M2 (Yanagi, 1981). Tidal currents are strong in the narrow ranged from low ng/L to 60 ng/L. The SIS is a major fishing ground straits (> 1 m/s) and weak in the broad basins (approximately 0.1 m/s) (Nagai, 2003) that includes aquacultures sites for fish, bivalves, and (Takeoka, 2002). seaweed; therefore, it is necessary to evaluate the dilution levels of Appendix A describes the details of hydrodynamic model. Model antibiotics in the whole SIS. domain ranges from 130.98°E to 135.5°E and from 32.8°N to 34.8°N The aim of this study was to investigate the dilution characteristics (Fig. 1b). The model includes 21 major rivers and 5 secondary rivers whose locations along the coastline are shown in Fig.