Importance of large-scale coastal circulation on bay-shelf exchange and residence time in a subtropical embayment, the northern South China Sea
Introduction
In the past few decades, the amount of nutrients and other pollutants discharged into many coastal systems in the world has increased rapidly and has caused severe environmental problems, such as eutrophication, hypoxia (Rabouille et al., 2008; Lee and Lwiza, 2008; Zhang and Li, 2010; Scully, 2013; Caballero-Alfonso et al., 2015), degradation of aquatic ecosystems, and a decrease in fishery resources in these coastal systems (Rabalais et al., 2002). The ecological response of embayments to nutrient loads is regulated by the bay-shelf exchange process (Nixon et al., 1996; Josefson and Rasmussen, 2000). The bay-shelf exchange capability can be quantified by the time scale of transport in the water body, and the residence time is a commonly used parameter (Delhez et al., 2004). A coastal embayment with long residence time and weak water exchange facilitates the development of severe eutrophication and long-lasting hypoxia, and short residence time and strong water exchange has the opposite effect. Therefore, knowledge of the residence time and water exchange is essential to predict the possible pollutant effects in the water body and to plan better environmental management in coastal waters.
It is well known that the residence time and the bay-shelf exchange in a coastal embayment are controlled by rivers (Kenov et al., 2012; Wan et al., 2013; Ren et al., 2014; Du and Shen, 2016), tides (Banas and Hickey, 2005), local topography (Safak et al., 2015), and winds (Gong et al., 2009a; Pfeiffer-Herbert et al., 2015; Kang et al., 2017). The relative importance of these physical processes on the residence time and water exchange has been found to vary among embayment systems. However, the influences of the large-scale coastal circulation (e.g., coastal currents, coastal jets, upwellings, and downwellings) on the residence time in coastal embayments have been less well-documented. On the continental shelf, the large-scale coastal circulation dominates the transport pathways and residence times over other physical processes (Tang et al., 2003; Feng et al., 2016) and is closely coupled with the internal dynamics of many coastal embayments. Based on the observational and analytical model results, Wong and Moses-Hall (1998) suggested that the subtidal water exchange at the cross-section between the embayment and the coastal ocean is controlled by a combination of local and remote effects. For the local effects, winds in the embayment generate bidirectional transport (Wong and Valle-Levinson, 2002), and a down-estuary wind increases stratification and enhances gravitational circulation, whereas an up-estuary wind increases mixing (Scully et al., 2005). In contrast, by means of the Ekman effect, remote effects can increase the outflow (inflow) of the embayment from the shelf under upwelling-favorable (downwelling-favorable) remote winds on the shelf (Wong and Valle-Levinson, 2002). It should be noted that the remote effect is, in fact, the shelf processes. The relative importance of local and remote effects varies among embayment systems. Studies based on observations and analytical model results have shown that the currents generated by local wind forcing are stronger than those generated by remote effects in Delaware Bay (Janzen and Wong, 2002). In contrast, a field study in Chesapeake Bay suggested that local and remote effects have a similar influence under strong water column stratification (Wong and Valle-Levinson, 2002). In addition, the relative importance of remote effects in the bay-shelf exchange varies significantly by season, depending on the wind frequency and the degree of stratification in the water column (Wong and Valle-Levinson, 2002).
Daya Bay (DB) is a subtropical embayment on the southern coast of China, approximately 70 km east of the Pearl River Estuary (PRE) (Fig. 1). DB is typical of many coastal bay systems along the coast of the China Sea that are subject to significant coastal circulation, upwelling and downwelling (Hu and Wang, 2016). As the home of many marine species, including some endangered species (e.g., the sea turtle, Chen et al., 2015), DB has been designated as a marine-protected area since 1983. However, economic development and the local population have increased greatly since the mid-1980s. With the increase in sewage discharge associated with human activities and the discharge of cooling water from nuclear power plants, substantial degradation of water quality and serious marine pollution problems (e.g., eutrophication and harmful algal bloom) have been frequently reported in DB since the 2000s (Song et al., 2004; Wang et al., 2008; Wu et al., 2009; Song et al., 2009; Wu et al., 2017).
The environmental issues of DB are closely related to the bay-shelf exchange processes and residence time in this region. Previous studies showed that the residence time is sensitive to local winds (Wang et al., 2009; Li et al., 2009a). However, they did not address impacts of baroclinic effect (e.g. seasonal stratification) and the large-scale coastal circulation on the bay-shelf exchange and the residence time. Because DB is subject to strong seasonal variability in water column stratification and significant large-scale coastal circulation (Gan et al., 2009; Chen et al., 2017a, b), the abovementioned studies of residence time may substantially underestimate or overestimate the bay-shelf exchange and residence time in DB. Therefore, a better knowledge of the bay-shelf exchange and residence time in DB, especially the effects of the large-scale coastal circulation on the residence time, is needed for sustainable management of the coastal ecosystem.
Many numerical studies of the bay-shelf exchange and the residence time do not account for the effects of the large-scale coastal circulation. In this study, we investigate the residence time and the bay-shelf exchange with a nested numerical ocean model, with special focus on the effects and relative contributions of the large-scale coastal circulation and other drivers on the residence time and the bay-shelf exchange of DB. The paper is organized as follows. First, the results of a 2-year numerical simulation of the circulation from 2006 to 2007 in DB are given in Sections 2 Nested ocean model and observations, 3 Model validation. We then present the monthly mean circulations in summer and winter in the surface and at selected transects in DB. The residence time and the bay-shelf exchange in DB were calculated based on the model results in Section 4. The main factors that control the spatial and temporal variation of the residence time and the bay-shelf exchange are discussed in Section 5. Finally, the paper ends with a summary and conclusions in Section 6.
Section snippets
Nested ocean model
To better resolve the ocean dynamics in DB and its adjacent waters from the regional scale to the local scale, this study includes a nested ocean model with two submodels: parent and child models (Fig. 1a and b). The nested ocean circulation model is based on the Estuarine, Coastal, and Ocean Model (ECOM, Blumberg, 2002), which has been successfully applied to studies of coastal circulations and water exchange, including the residence time, upwelling and river plume (Li et al., 2009a; Jing et
Model validation
We first assess the model performance in simulation of the tidal and non-tidal sea levels in the coastal waters adjacent to DB (child model; Fig. 1b). Four principle tidal constituents (M2, S2, K1 and O1) in DB (Song et al., 2016) were calculated from the child model results and from observations from four tide gauges in Hong Kong waters during 2007 (Table 2). The results show that the child model reasonably reproduces the tidal sea level in this region. The biases in amplitudes for all four
Monthly mean circulation and hydrography
Fig. 7 presents the spatial distribution of the monthly mean circulation, salinity, and temperature near the surface in July 2006 and January 2007 as produced by the parent and child models. The mean circulation on the shelf of the NSCS and in DB exhibits significant seasonal variability. Dominated by the southwesterly monsoon during July (Fig. 2a), the simulated mean currents feature a northeastward coastal current with a speed of approximately 0.4 m/s at the surface along the shelf. After
Impact of the mean circulation on the residence time
The model results demonstrate that the residence time in DB features significant spatial and temporal variability. The seasonal variability is closely related to the water exchange and circulation structure at the bay-shelf interface. The current along the bay shows a marked two-layer circulation structure in summer and a clear transverse structure in winter (Fig. 9c, g). As a result, the net water exchanges in the upper layer (0–6 m) of DB are offshore in summer and onshore in winter,
Summary and conclusions
In this study, the importance of the large-scale coastal circulation on the bay-shelf water exchange and the residence time of DB in summer 2006 and winter 2007 were investigated using a nested 3D ocean circulation model system with a particle-tracking module. The model performance was assessed by sea levels at tidal gauges and by observed current, salinity, and temperature at mooring and cruised stations. The simulated sea levels and 3D currents in DB and its adjacent waters show reasonable
Acknowledgements
This research was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFC0402603), and the opening foundation from State Key Laboratory of Tropical Oceanography (LTO), South China Sea Institute of Oceanology (SCSIO), Chinese Academy of Science through grant #LTO1508. This research was also supported by the National Natural Science Foundation of China (NSFC) through grant #41506102 and #41576089, Joint Research Projects NSFC-NOW (Netherlands
References (62)
- et al.
Biogeochemical and environmental drivers of coastal hypoxia
J. Mar. Syst.
(2015) - et al.
Dispersal of the Pearl River plume over continental shelf in summer
Estuar. Coast Shelf Sci.
(2017) - et al.
Far-reaching transport of Pearl River plume water by upwelling jet in the northeastern South China Sea
J. Mar. Syst.
(2017) - et al.
Using a food-web model to assess the trophic structure and energy flows in Daya Bay, China
Continent. Shelf Res.
(2015) - et al.
A modeling study on the response of Chesapeake Bay to hurricane events of Floyd and Isabel
Ocean Model.
(2012) - et al.
Residence time in a semi-enclosed domain from the solution of an adjoint problem
Estuar. Coast Shelf Sci.
(2004) - et al.
Water residence time in Chesapeake bay for 1980-2012
J. Mar. Syst.
(2016) - et al.
Ocean circulation drives heterogeneous recruitments and connectivity among coral populations on the North West Shelf of Australia
J. Mar. Syst.
(2016) - et al.
Interaction of a river plume with coastal upwelling in the northeastern South China Sea
Estuar. Coast Shelf Sci.
(2009) - et al.
The hydrodynamic response of the York River Estuary to tropical cyclone Isabel, 2003
Estuar. Coast Shelf Sci.
(2007)
The influence of wind on the water age in the tidal Rappahannock River
Mar. Environ. Res.
A numerical model study of barotropic subtidal water exchange between estuary and subestuaries (tributaries) in the Chesapeake Bay during northeaster events
Ocean Model.
Wind effects on the lateral structure of density-driven circulation in Chesapeake Bay
Estuar. Coast Shelf Sci.
West Florida shelf circulation and temperature budget for the 1999 spring transition
Continent. Shelf Res.
Nutrient retention by benthic macrofaunal biomass of Danish Estuaries: importance of nutrient load and residence time
Estuar. Coast Shelf Sci.
Dynamics of water and salt exchange at Maryland Coastal Bays
Estuar. Coast Shelf Sci.
Characteristics of bottom dissolved oxygen in long Island sound, New York
Estuar. Coast Shelf Sci.
Storm surge induced flux through multiple tidal passes of Lake Pontchartrain estuary during Hurricanes Gustav and Ike
Estuar. Coast Shelf Sci.
A simple boundary condition for unbounded hyperbolic flows
J. Comput. Phys.
Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE
Comput. Geosci.
Dynamics of wind-driven estuarine-shelf exchange in the Narragansett Bay estuary
Continent. Shelf Res.
Comparison of hypoxia among four river-dominated ocean margins: the Changjiang (Yangtze), Mississippi, Pearl, and Rhone Rivers
Continent. Shelf Res.
Predicting water age distribution in the Pearl River Estuary using a three-dimensional model
J. Mar. Syst.
Controls on residence time and exchange in a system of shallow coastal bays
Continent. Shelf Res.
Variation of phytoplankton biomass and primary production in Daya Bay during spring and summer
Mar. Pollut. Bull.
Harmful algal blooms (HABs) in Daya Bay, China: an in situ study of primary production and environmental impacts
Mar. Pollut. Bull.
AVHRR satellite remote sensing and shipboard measurements of the thermal plume from the Daya Bay, nuclear power station, China
Rem. Sens. Environ.
Ecological environment changes in Daya bay, China, from 1982 to 2004
Mar. Pollut. Bull.
Monitoring cooling water discharge using Lagrangian coherent structures: a case study in Daya Bay, China
Mar. Pollut. Bull.
Identification of anthropogenic effects and seasonality on water quality in Daya Bay, South China Sea
J. Environ. Manag.
Scenarios of nutrient alterations and responses of phytoplankton in a changing Daya Bay, South China
J. Mar. Syst.
Cited by (12)
Modelled coastal circulation and Lagrangian statistics from a large coastal embayment: The case of Bay of Plenty, Aotearoa New Zealand
2023, Estuarine, Coastal and Shelf ScienceGame analysis of nuclear wastewater discharge under different attitudes: Seeking a potential equilibrium solution
2021, Science of the Total EnvironmentResponse of freshwater transport during typhoons with wave-induced mixing effects in the Pearl River Estuary, China
2021, Estuarine, Coastal and Shelf ScienceDistribution and potential provenance of trace elements in a 120-year dated sediment core from west Daya Bay, northeastern South China Sea
2021, Marine Pollution BulletinCitation Excerpt :The possible sources of elevated Zn levels in the corals were identified as domestic and industrial sewage discharge and inputs from other point sources such as oil terminals and ports (Chen et al., 2010). Contaminants are transported and distributed currents (north-south currents during rising and ebb tides) throughout the entire region (Zhang et al., 2019). Thus, the high burial fluxes of Zn, Pb, Cd, and Hg can likely be attributed to the increased natural weathering of soil materials or inputs from rapid industrial development and urbanization in the past few decades.
Evaluation of the total maximum allocated load of dissolved inorganic nitrogen using a watershed–coastal ocean coupled model
2019, Science of the Total EnvironmentCitation Excerpt :The nested model has 16 sigma levels in the vertical direction, which is sufficiently fine to represent the tracer movements in DB. The details of the model configuration and the validation results are the same as those introduced in a previous study by Zhang et al. (2018). Following Jiang (2009), 49 outlets around DB were finally selected for both PS and NPS input for this study.
Estuarine salinity recovery from an extreme precipitation event: Hurricane Harvey in Galveston Bay
2019, Science of the Total EnvironmentCitation Excerpt :The shelf current also plays an important role for the strong tidal pumping at the bay entrance. Shelf transport is known to affect greatly the water exchange between ocean and estuary (Du and Shen, 2017; Zhang et al., 2019). Salinity snapshots on September 13–14, about two weeks after the stormwater release, show clearly how the shelf current facilitated the salt exchange (Fig. 11).