Research papersSpatiotemporal river flux and composition of nutrients affecting adjacent coastal water quality in Hainan Island, China
Graphical abstract
Introduction
Eutrophication is a global problem in coastal oceans that is caused by the excess nutrients (Boynton et al., 1995, Carpenter et al., 1998, Lancelot and Likens, 2009, Li et al., 2018, Markogianni et al., 2017). Globally, rivers play an important role in the terrestrial - marine linkage. Rapid increases in population and economic activity in the last decades have promoted eutrophication of coastal and estuarine waters though a large influx of nutrients and other biogenic elements (Bricker et al., 2008, Zhang et al., 2017, Zhang et al., 2019a, Zhang et al., 2019b). The increases in biologically available forms of nitrogen and phosphorus are linked to eutrophication in both estuaries and coastal marine ecosystems and consequent declines in ecological function, reduction in vital water supplies, and increasing frequency of harmful algal blooms, hypoxia and incidents of large fish kills worldwide (Conley et al., 2011, Lancelot and Likens, 2009, Tong et al., 2015, Tong et al., 2017, Turner and Rabalais, 1994, Wu et al., 2017). The stoichiometric ratio and the availability of N and P control both the phytoplankton primary productivity and composition of phytoplankton species (Redfield et al., 1963, Sundareshwar et al., 2003, Yuan et al., 2018). Total dissolved nitrogen (TDN) and total dissolved phosphorous (TDP) sources playe an important role in the cycling of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) in coastal waters (Ding et al., 2019, Liu et al., 2016, Lu et al., 2016, Zhang et al., 2019b). According to the surface water quality standard, TDN and TDP are the land surface water quality indicators (MEPC, 2002a, MEPC, 2002b). However, the DIN and DIP are the indicators of marine water quality in the Sea Water Quality Standard (GB3097-1997) in China (AQSIQ, 1997). The DIP is frequently the limiting nutrient for phytoplankton growth in many marine environments (Sundareshwar et al., 2003, Yuan et al., 2018, Zhang et al., 2019a). The excess TDN and TDP derived from extensive leaching from agricultural fertilizer, industrial and municipal sewage discharge have adverse effects on adjacent water quality.
Hainan Island, which is located in the northern part of the South China Sea, and separated by the Qiongzhou Strait from the Leizhou Peninsula of Guangdong, is the second largest island of China. Hainan Island is characterized by a radial river system, which mostly originates from the central mountainous area with high elevation (Zhao et al., 2015). With rapid industrial and economic development on Hainan Island, these rivers have been polluted to different degrees, but have led to some local adjacent coastal water pollution (Department of Ecological Environmental Protection in Hainan Province, 2017, Herbeck et al., 2011, Zhang et al., 2019a, Zhang et al., 2019b, Zhou, 2013). In addition, three red tides occurred in coastal waters adjacent to Haikou and Lingao city in 2016 (Department of Hainan Province Ocean and Fishery, 2017). In recent years, various studies on nutrients contamination in rivers, estuaries, and adjacent coastal areas have been conducted (Chang, 2006, Huang, 2012; Li et al., 2010, Qiu et al., 2019, Zhang et al., 2019a, Zhao et al., 2015, Zhou, 2013). Previous studies showed that the characteristics of land-based pollutants in Hainan Island and controlling land-based pollution were elaborated to divide the coastal zone pollution control of Hainan Island. However, the marine coastal water quality was not linked to the characteristics of land-based pollutants input, and the composition and riverine discharge of nutrients into the coastal river estuaries greatly changed under intense rainstorms and human activity, thereby resulting in a significant deterioration of the coastal environment and phytoplankton increases in coastal water (Chang, 2006, Chen et al., 2018; Zhang et al., 2019a, Zhou, 2013). However, few studies reported the whole-island seasonal nutrient flux transport to the coastal waters in Hainan Island, which is easily influenced by tropical typhoons. Land-ocean integration has been recognized as the key factor when dealing with sea use and marine pollution, which needs to coordinate the river watershed regions and coastal management (Powley et al., 2018, Schubel and Pritchard, 1986, Su et al., 2019, Wang et al., 2017, Zhang et al., 2017). Therefore, the spatiotemporal river fluxes and response to adjacent coastal water quality in Hainan Island is important for understanding the coastal estuarine systems and improving the management of nutrient pollution of estuaries.
In order to address the aforementioned gaps in our understanding of the riverine and coastal systems, the latest seasonal data, which cover land-based riverine inputs (From January to December) and coastal water quality (three water seasons), were also collected in 2017. We reported the concentration in estuaries of 19 major rivers and the coastal water quality response in Hainan Island in order to (1) assess the spatial nutrient contamination in the river estuaries; (2) quantify the spatial distribution of the nutrient fluxes of 19 rivers into coastal waters; (3) identify the spatiotemporal distribution of DIN and DIP concentrations in coastal water adjacent to Hainan Island; and (4) clarify the seasonal variation in N/P in the coastal rivers inputs form Hainan Island and the seawater response. This study provides a comprehensive data set on the river fluxes of major nutrients into the coastal water and the water quality response, which is beneficial for nutrient load reduction in Hainan Island within the Chinese total pollutant load control system, and will also provide fundamental information for marine environmental protection in Hainan Island.
Section snippets
Study area
Hainan Island is located at the southernmost end of China across from the Qiongzhou Strait to the north, and faces the Leizhou Peninsula (Fig. 1). Hainan Island is the second largest island in China after Taiwan Island, with a coastal water area of approximately 2 million km2, which accounts for 42.3% of the national sea area. The total coastline length of Hainan Province is 1928 km, of which the coastline length of Hainan Island makes up 1618 km. The average annual temperature (T) of the
Spatial patterns of TDN and TDP concentrations in the coastal rivers of Hainan Island
The spatial patterns of the TDN and TDP concentrations in the coastal rivers of Hainan Island are shown in Fig. 2. The results indicated that there was a wide variation in the spatial distribution. The annual average concentration of TDN was 77.23 ± 31.36 μmol/L, with a range of 14.99 μmol/L to 368.31 μmol/L. The maximum and minimum concentrations were found in B in July and October, respectively. According to the Environmental Quality Standards for Surface Water in China (MEPC, 2002a), the
Status of nutrient concentrations and composition
During the three periods, the DIN and DIP concentrations presented clear seasonal variations under the impacts of complex hydrological and biogeochemical conditions. According to the Seawater Quality Standard of China (GB3097-1997), the DIN and DIP concentrations at most monitoring stations met the national first-class seawater standards (AQSIQ, 1997). In comparison, the concentrations of DIN and DIP in this study were comparable to those found in most of the coastal waters listed in Table 2.
Conclusions
In summary, our study showed the spatiotemporal riverine nutrient fluxes in response to adjacent coastal water quality in Hainan Island. The spatial patterns of TDN and TDP concentrations in coastal rivers of Hainan Island indicated that the concentrations varied at different times and within different rivers. The annual average TDN and TDP concentrations were 77.23 ± 31.36 μmol/L and 3.25 ± 1.67 μmol/L, respectively. The river flux patterns of TDN and TDP were significantly different in
CRediT authorship contribution statement
Peng Zhang: Conceptualization, Writing - original draft, Writing - review & editing, Funding acquisition. Huaming Ruan: Software, Methodology, Validation, Visualization. Peidong Dai: Data curation, Software. Lirong Zhao: Methodology, Project administration. Jibiao Zhang: Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We deeply appreciate the journal editors and the anonymous reviewers for their constructive comments and suggestions. This study was funded by the Fund of Guangdong Ocean University. We gratefully acknowledge supported by Guangdong Ocean University Fund Project (R18021); The Science and Technology Special Project of Zhanjiang city (2019B01081), China; The First-class Special Fund (231419018) and the Innovation Strong School Project (230420021) of Guangdong Ocean University.
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