Elsevier

Journal of Hydrology

Volume 591, December 2020, 125293
Journal of Hydrology

Research papers
Spatiotemporal river flux and composition of nutrients affecting adjacent coastal water quality in Hainan Island, China

https://doi.org/10.1016/j.jhydrol.2020.125293Get rights and content

Highlights

  • The total TDN and TDP fluxes in 19 coastal rivers were 1.8 × 104 t and 1.1 × 103 t.

  • DIN and DIP concentration have significantly seasonal variations in Hainan seawater.

  • Riverine nutrients source was the predominate factor of impacting seawater quality.

  • The hotspots DIN and DIP concentrations of Hainan Island seawater were identified.

  • The tropical storms can increase riverine nutrients flux and response in seawater.

Abstract

Human activities have changed global nutrients cycling and appreciably altered marine ecosystems in riverine-estuarine systems. This is evident by the significant shifts in the supply of nitrogen and phosphorus. However, very little is known about the concentrations and fluxes and of these nutrients and their seasonal changes in Hainan Island, China. We examined Hainan Island, which is the second largest island in China, to assess the spatiotemporal concentrations and fluxes of nutrients using seasonal data on a whole-of-system scale covering river estuaries and adjacent coastal water (2017). The annual average total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations were 77.23 ± 31.36 μmol/L and 3.25 ± 1.67 μmol/L, respectively. The annual fluxes of pollutants from the discharge of coastal rivers on Hainan Island into coastal waters were 1.8 × 104 t for TDN and 1.1 × 103 t for TDP, respectively. The month with the largest contribution to the annual pollutants load was October, which contributed 13.60% of TDN and 17.79% of TDP. In addition, the largest river fluxes were Nandu River, which contributed 7.5 × 103 t of TDN and 364 t of TDP. The annual average concentrations of pollutants in the surface coastal water were 7.60 ± 10.71 μmol/L for dissolved inorganic nitrogen (DIN) and 0.24 ± 0.31 μmol/L for dissolved inorganic phosphorus (DIP). The annual average N/P ratios in coastal rivers and seawater were 30 and 65, which were significantly higher than the Redfield ratio (N/P = 16). The DIN and DIP concentrations of most monitoring stations met the national first-class seawater standards, but the problem of nutrient pollution in some local coastal water areas was still outstanding. The coastal water quality monitoring should consider the influence of tropical storms influence on Hainan Island. The effective improvement of coastal water quality should be based on land–ocean coordination monitoring of coastal water quality and the riverine nutrient flux of Hainan Island.

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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