International Journal of Environmental Research and Public Health

Article Spatiotemporal Distribution Characteristics of Nutrients in the Drowned Tidal Inlet under the Influence of Tides: A Case Study of Zhanjiang Bay, China

Shuangling Wang 1,2, Fengxia Zhou 1,2, Fajin Chen 1,2,*, Yafei Meng 1,2 and Qingmei Zhu 1,2

1 College of Ocean and Meteorology, Guangdong Ocean University, Zhanjiang 524088, China; [email protected] (S.W.); [email protected] (F.Z.); [email protected] (Y.M.); [email protected] (Q.Z.) 2 Key Laboratory for Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China * Correspondence: [email protected]; Tel.: +86-759-239-6037

Abstract: The tidal dynamics and the characteristics of pollutant migration in the drowned-valley tidal inlet, a typical unit of coastal tidal inlets, are strongly influenced by geomorphological features. Along with the development of society and the economy, the hydrodynamic and water quality environment of the tidal inlet is also becoming more disturbed by human activities, such as recla- mation of the sea and the construction of large bridges. In this study, a typical drowned-valley tidal inlet, Zhanjiang Bay (ZJB), was selected for the establishment of a model via coupling of a



 tidal hydrodynamic model and water quality numerical model. This model can be used to simulate the migration and diffusion of pollutants in ZJB. The spatial and temporal variation processes of Citation: Wang, S.; Zhou, F.; Chen, F.; water quality factors of the bay under the influence of special geomorphic units was simulated at Meng, Y.; Zhu, Q. Spatiotemporal the tidal-inlet entrance, the flood/ebb tidal delta, and the tidal basin. The results show that ZJB Distribution Characteristics of Nutrients in the Drowned Tidal Inlet has strong tidal currents that are significantly affected by the terrain. Under the influence of the under the Influence of Tides: A Case terrain and tidal currents, the phosphorus and nitrogen concentration at the flood-tide and ebb-tide Study of Zhanjiang Bay, China. Int. J. moments showed obvious temporal and spatial differences in the ebb-tide delta, tidal-inlet entrance, Environ. Res. Public Health 2021, 18, flood-tide delta, and tidal basin. In this study, we analyzed the response mechanism of the water 2089. https://doi.org/10.3390/ quality environment to the drowned-valley tidal inlet, and this can provide theoretical guidance and ijerph18042089 a basis for decision-making toward protecting the ecology and water security of ZJB.

Academic Editors: Soon-Jin Hwang, Keywords: drowned-valley tidal inlet; phosphorus; nitrogen; numerical model; Zhanjiang Bay Young-Seuk Park and Ihn-Sil Kwak

Received: 7 January 2021 Accepted: 18 February 2021 1. Introduction Published: 21 February 2021 Tidal inlets are an important type of coastal dynamic geomorphology occurring on

Publisher’s Note: MDPI stays neutral tidal coasts. They are unified dynamic geomorphological systems that are interrelated with regard to jurisdictional claims in and influenced by the ebb-tide delta, tidal-inlet entrance, flood-tide delta, tidal basin, and published maps and institutional affil- other factors. The system is sensitive to the response and feedback of environmental iations. pollution [1,2]. With the continuous development of the social economy, the hydrodynamic water quality environment of tidal inlets is also increasingly being disturbed by human activities, including land construction around the sea, large-scale bridge construction, land source pollution emissions, and fishery farming [3–5]. How to slow down the problem of water pollution in coastal areas, especially tidal inlets, to ensure water ecological security Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. is an urgent scientific problem to be solved [6–9]. Inshore geomorphological units and This article is an open access article human activity interference are important factors affecting water quality [10–12]. Therefore, distributed under the terms and understanding the response mechanism of the sea area water quality environment to conditions of the Creative Commons special geomorphological units and human activities is the premise and basis for solving Attribution (CC BY) license (https:// the problem of inshore water pollution. creativecommons.org/licenses/by/ With regard to the tidal inlet, which is a special geomorphologic unit, the literature, 4.0/). both domestic and foreign, is mainly focused on the following aspects:

Int. J. Environ. Res. Public Health 2021, 18, 2089. https://doi.org/10.3390/ijerph18042089 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2021, 18, 2089 2 of 21

(1) The stability of a tidal inlet. Among the studies on the special geomorphologic units in the tidal areas near the shore, the stability of the tidal inlets has always been one of the hot spots and difficulties in research on the dynamic geomorphologic evolution of estuaries and coasts. By analyzing the tidal prism (P) and the cross-sectional area in a tidal-inlet (A) relationship of the tidal inlets, the sectional morphology of the tidal inlet was analyzed to judge the stability of the tidal inlets [2,13]. In addition, some scholars studied the stability of the tidal inlets from the hydrodynamic process of the tidal inlets by means of numerical models and, at the same time, pointed out the critical coefficients existing for the tidal inlets [14–16]. Numerous studies have also discussed the stability of the tidal inlets from the perspective of the coastal erosion and sediment movement of tidal inlets based on the differences among the tidal shapes in a tidal inlet [17–20]. (2) The tidal prism and water exchange. The amount of tidal prism directly affects the water exchange capacity as well as the rules of pollutant migration and diffusion in the bay. The duration of the water exchange in the tidal inlet is an important index for the vitality of a semi-closed bay. For the calculation of the static tidal prism capacity, a formula for calculating the linear tidal capacity is often used [2]. By virtue of the numerical model and the remote sensing data from the satellites, more scholars calculated the dynamic tidal prism capacity of a single-tide tidal inlet or a multi-tide tidal inlet [21]. With regard to water exchange, there are relatively more concepts available, such as the half-exchange time, persistence time, impact time, renewal time, and water age. At present, most of the calculations for the exchange capacity of water have been carried out through the numerical models for the tidal flow established based on studies using the two-dimensional convection–diffusion mathematical model [22,23]. (3) The tidal wave hydrodynamic characteristics of the tidal inlets. Study of the hydrody- namic processes of tidal inlets has provided a hydrodynamic field for assessing the stability, coastal erosion, material transport, and water quality environment of a tidal inlet [24–27]. The numerical simulation study of tidal hydrodynamics began in the 1950s. It gradually developed from a one-dimensional model to a two-dimensional model, and the numerical simulation and calculations were mainly carried out accord- ing to the law of water movement. In the 1970s, with the continuous exploration and extension of the research on the two-dimensional hydrodynamic models of the coastal waters, many scholars began to study the three-dimensional numerical simulation of inshore tides. With three-dimensional numerical simulation, it is possible to realize the dynamic simulation of the tides across the scales of space and time [15,28,29]. The research on the response of the water quality environment to near-shore tidal branch geomorphologic units is comparatively lacking. The western coast of Guangdong is located on the southwest side of the northern shelf of the South China Sea, which is connected with the Beibu Gulf by the Qiongzhou Strait. Zhanjiang Bay (ZJB) is a typical drowned-valley tidal inlet in this area, and the terrain is relatively flat. The special terrain makes the west Guangdong coastal current constitute the northern wing of the eastern cyclone vortex of the Qiongzhou Strait, and a part of the westward coastal current enters the Beibu Gulf westward through the Qiongzhou Strait (Figure1). Studies have shown that the west Guangdong coastal current has a significant impact on the transport of pollutants and nutrients in the coastal waters west of the Pearl River Estuary, Qiongzhou Strait, and Beibu Gulf [30,31]. Therefore, this study selected ZJB, a typical drowned-valley tidal inlet in the west of Guangdong Province, to study the feedback effect of ecological environment of the bay on dynamic process in the northwest of the South China Sea, which can expand the understanding of the dynamic forcing mechanism of the coastal ecosystem in the northwestern South China Sea. Int. J. Environ. Res. Public Health 2021, 18, 2089 3 of 21 Int. J. Environ. Res. Public Health 2021, 18, 2089 3 of 21

Figure 1. The location of Zhanjiang Bay (ZJB).

The MIKE3 Flow Model (FM) is a prominent three-dimensionalthree-dimensional water environment numerical simulationsimulation modeling,modeling, and and MIKE MIKE Ecological Ecological Modeling Modeling (ECO (ECO Lab) Lab) is ais piece a piece of nu- of numericalmerical simulation simulation software software for for ecological ecological modeling, modeling, both both of which of which were were developed developed by theby theDanish Danish Hydraulic Hydraulic Institute Institute (HDI), (HDI), and these and modelsthese mod haveels been have applied been applied worldwide worldwide[32–35]. [32MIKE3–35]. FM MIKE3 is based FM onis based a flexible on mesha flexible approach, mesh andapproach, it has been and developedit has been for developed applications for applicationswithin oceanographic, within oceanographic, coastal, and estuarine coastal, and environments. estuarine environments. The MIKE3 FM The hydrodynamic MIKE3 FM model and the MIKE ECO Lab model can be used to simulate the spatiotemporal distribu- hydrodynamic model and the MIKE ECO Lab model can be used to simulate the spatio- tion characteristics of nutrients in the drowned tidal channel under the influence of tides. temporal distribution characteristics of nutrients in the drowned tidal channel under the This study provides a scientific basis for controlling the water environment of ZJB, a typical influence of tides. This study provides a scientific basis for controlling the water environ- drowned-valley tidal inlet. ment of ZJB, a typical drowned-valley tidal inlet. 2. Study Area 2. Study Area Zhanjiang Bay (ZJB), located at the northwestern coastal region of the South China Sea (FigureZhanjiang2), is Bay the (ZJB), largest located drowned-valley at the northwestern tidal channel coastal on the region South of China the South Coast. China ZJB Seatakes (Figure the shape 2), is of the a treelargest branch, drowned extending-valley inland tidal channel for approximately on the South 50 kmChina from Coast. south ZJB to takesnorth. the It isshape surrounded of a tree by branch, Nansan extending Island, Donghai inland for Island, approximat and Leizhouely 50 Peninsula. km from south ZJB can to north.be roughly It is dividedsurrounded into fourby Nansan parts according Island, Donghai to its geomorphological Island, and Leizhou features: Peninsula. the ebb-tide ZJB candelta, be tidal-inlet roughly divided entrance, into flood-tide four parts delta, according and tidal to basin.its geomorphological ZJB has an expanse features: of water the ebbareas,-tide with delta, tides tidal of large-inlet capabilityentrance, flood and strong-tide delta, force and tidal stable basin. beaches ZJB and has wateran expanse troughs, of waterand represents areas, with the tides most of dynamic,large capability complex, and and strong vulnerable force and marine stable environment beaches and inwater the troughs,South China and represents Sea [36]. The the drowning-valley-type most dynamic, complex, tidal and channel vulnerable in ZJB marine has clear environment irregular

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in the South China Sea [36]. The drowning-valley-type tidal channel in ZJB has clear ir- regular semi-diurnal tidal features. The main tidal components M2, S2, O1, K1, M4, and MS4 are almost all introduced from the open sea. Due to the friction from the nearby islands and terrain, the characteristics of currents are more complicated. In recent years, with the demands of ZJB’s economic development, the development of ports and channels, energy bases, harbor industries, and tidal flats in the bay have been included in the economic development plan of ZJB. In addition to the above projects, pol- lution discharge from land sources, fishery, and aquaculture in addition to port and ship- Int. J. Environ. Res. Public Health 2021, 18, 2089 4 of 21 ping pollution have all changed the ZJB water environment to varying degrees, with the water quality affected to some extent [37]. Over recent decades, rapid economic develop- ment and urbanization have significantly impacted the ZJB environment. Eutrophication

andsemi-diurnal harmful algal tidal blooms features. have The occurred main tidal frequently components [38,39 M].2 A,S scientific2,O1,K1 ,Munderstanding4, and MS4 are of thealmost response all introduced mechanism from of thethe openwater sea. quality Due environment to the friction of from ZJB theto special nearby geomorpho- islands and logicalterrain, units the characteristics is the prerequisite of currents and basis are for more solving complicated. the water pollution problem.

Figure 2. The study area showing sampling sites (Z1–Z26) in ZJB. ZJB can be roughly divided into four parts: the ebb-tide Figuredelta, tidal-inlet 2. The study entrance, area showing flood-tide sampling delta, and sites tidal (Z1 basin.–Z26) in ZJB. ZJB can be roughly divided into four parts: the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin. In recent years, with the demands of ZJB’s economic development, the development 3.of Methodology ports and channels, and Model energy Setup bases, harbor industries, and tidal flats in the bay have been 3.1.included Model in Establishment the economic development plan of ZJB. In addition to the above projects, pollu- tion dischargeBased on the from MIKE3 land sources, hydrodynamic fishery, andmodule aquaculture (Flow Model in addition, FM) and to port water and ecological shipping mpollutionodule (MIKE have allECO changed Lab) (MIKE the ZJB Zero water release environment 2014), we to constructed varying degrees, a three with-dimensional the water hydrodynamicquality affected model to some and extent nutrient [37]. diffusion Over recent model decades, to simulate rapid economicthe ZJB hydrodynamic development processand urbanization and the migration have significantly and diffusion impacted process the of ZJB nutrients environment. at the ebb Eutrophication-tide delta, tidal and- inlharmfulet entrance, algal bloomsflood-tide have delta, occurred and tidal frequently basin. Comparison [38,39]. A scientific and analysis understanding of the distribu- of the response mechanism of the water quality environment of ZJB to special geomorphological tion of nutrients at the flood tide and ebb tide can provide a scientific basis for the optimi- units is the prerequisite and basis for solving the water pollution problem. zation of the pollution sources in ZJB. The research framework of this study is shown in Figure3. Methodology 3. and Model Setup 3.1. Model Establishment Based on the MIKE3 hydrodynamic module (Flow Model, FM) and water ecological module (MIKE ECO Lab) (MIKE Zero release 2014), we constructed a three-dimensional hydrodynamic model and nutrient diffusion model to simulate the ZJB hydrodynamic process and the migration and diffusion process of nutrients at the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin. Comparison and analysis of the distribution of nutrients at the flood tide and ebb tide can provide a scientific basis for the optimization of the pollution sources in ZJB. The research framework of this study is shown in Figure3. Int. J. Environ. Res. Public Health 2021, 18, 2089 5 of 21 Int. J. Environ. Res. Public Health 2021, 18, 2089 5 of 21

-Coastline data -Topographic data -Bias-correction -Hydrological,Tidal data Validation Hydrodynamic model application -Wind data (MIKE 3FM) -MIKE3 Application for Calibration -Water level Zhanjiang Bay -Salinity data -Temperature data

Observed temperature -Initial condition MIKE ECO Lab model and salinity data -Boundary condition

-Flow field distribution -Phosphorous distribution -Nitrogen distribution

FigureFigure 3.3. SchematicSchematic ofof thethe modelingmodeling frameworkframework forfor thisthis study.study.

3.2.3.2. Difference SchemeScheme andand DiscreteDiscrete EquationEquation ConsideringConsidering thethe conditionsconditions forfor thethe definitedefinite solution,solution, wewe cancan solvesolve thethe equationequation usingusing thethe discretediscrete form.form. TheThe MIKE3MIKE3 modelmodel appliesapplies C-typeC-type gridsgrids inin rectangularrectangular coordinatescoordinates andand adoptsadopts aa stablestable alternatingalternating directiondirection implicitimplicit (ADI)(ADI) schemescheme forfor thethe discretediscrete difference.difference. TheThe equationequation matrixmatrix appliesapplies doubledouble sweepsweep forfor thethe solution,solution, whichwhich hashas second-ordersecond-order accuracy.accuracy. OwingOwing toto aa largelarge areaarea ofof tidaltidal flatsflats inin thethe researchresearch area,area, whichwhich becomebecome drieddried outout oror flooded due to high or low tides, the existence of variable boundaries should be considered flooded due to high or low tides, the existence of variable boundaries should be consid- to accurately simulate the phenomenon. In our model, the dry–wet grid method was ered to accurately simulate the phenomenon. In our model, the dry–wet grid method was adopted to deal with flooding. When the water level drops to a certain point and the adopted to deal with flooding. When the water level drops to a certain point and the water water depth is less than the critical value of the dry point, the point is omitted from the depth is less than the critical value of the dry point, the point is omitted from the calcula- calculation. When the sea water rises and its depth is greater than the critical depth at the tion. When the sea water rises and its depth is greater than the critical depth at the wet wet point, the calculation is re-added. If the value of the water depth at the dry and wet point, the calculation is re-added. If the value of the water depth at the dry and wet points points is extremely large, the accuracy of the results is affected. If the value is extremely is extremely large, the accuracy of the results is affected. If the value is extremely small, small, an unstable calculation may occur. an unstable calculation may occur. 3.3. Boundary/Initial Conditions 3.3. Boundary/Initial Conditions The boundary conditions and initial conditions of the model are as follows (Figure4): ClosedThe boundary boundary: conditions Code1 in and the initial model conditions calculation of is the the model closed are boundary, as follows that (Figure is, the land4): boundary, U = 0 or V = 0. OpenClosed boundary: boundary: The Code1 south in boundary the model and calculation the east boundaryis the closed are boundary, the open boundaries that is, the inland the boundary, numerical U model. = 0 or V = 0. TheOpen model boundary: considers The south external boundary forces, suchand the as theeast tidal boundary current, are temperature, the open boundaries salinity, andin the wind. numerical At the model. opening boundary, the water level data come from the Oregon State UniversityThe model (OSU) considers Tidal Inversion external forces, Software such (OTIS). as the Thistidal studycurrent, selected temperature, six main salinity, tidal components,and wind. At M the2,S 2opening,O1,K1,M boundary,4, and MS the4, as water the external level data forces come of thefrom open the boundary. Oregon State The windUniversity data were (OSU) from Tidal the Inversion European Software Center for (OTIS). Medium-Range This study Weather selected Forecasts six main (ECMWF)tidal com- withponents, a high M2 resolution, S2, O1, K1, ofM 64, hand by MS 6 h.4, Theas the east external component forces U of and the northopen boundary. component The V of wind the winddata were speed from at 10 mthe height European from theCenter sea surface,for Medium with- aRange spatial Weather resolution Forecasts of 0.125 ◦(ECMWF)× 0.125◦, werewith interpolateda high resolution to each of 6 grid h by and 6 h generated. The east component as a wind field U and forcing north file. component V of the windThe speed temperature at 10 m height and saltfrom open the sea boundary surface, data with came a spatial from resolution National Centersof 0.125° for × 0.125°, Envi- ronmentalwere interpolated Prediction to each (NCEP) grid Global and generated Ocean Data as a Assimilation wind field forcing System file (GODAS). monthly averageThe data, temperature and the initialand salt temperature open boundary and salinitydata came field from came National from the Centers monthly for Envi- aver- ageronmental data of Prediction the World (NCEP) Ocean Atlas Global 2013 Ocean V2 (WOA13).Data Assimilation The above System data (GODAS) were taken monthly as the model-drivenaverage data, and forcing the initial conditions temperature and initial and conditions salinity field for came the interpolation from the monthly method. average The temperaturedata of the World and saltOcean boundary Atlas 2013 conditions V2 (WOA13). of the The water above quality data modelwere taken were as the the same model as- thosedriven of forcing the hydrodynamic conditions and model. initial Theconditions nutrient for boundary the interpolation conditions method. and backgroundThe temper- ature and salt boundary conditions of the water quality model were the same as those of

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the hydrodynamic model. The nutrient boundary conditions and background values were valuesinterpolated were interpolated with the measured with the measured values. The values. model The was model used was to useddetermine to determine the variation of thethe variation dry–wet of inundation the dry–wet grid inundation at the start grid of at the startopen of boundary, the open boundary, and the minimum and the water minimumdepth was water set at depth 0.05 was m. set at 0.05 m.

(a)

(b)

FigureFigure 4. 4Bathymetry. Bathymetry (a) ( anda) and computational computational flexible flexible grid mesh grid ( bmesh). (b).

3.4. Model Schematization The detailed bathymetry in numerical modeling plays a critical role in achieving ac- curate hydrodynamic simulations, especially in the drowned-valley tidal inlet. The mod- eling area of the horizontal plane is 110.1396°–110.6305° E and 20.9174°–21.5236° N. The

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3.4. Model Schematization The detailed bathymetry in numerical modeling plays a critical role in achieving accurate hydrodynamic simulations, especially in the drowned-valley tidal inlet. The modeling area of the horizontal plane is 110.1396◦–110.6305◦ E and 20.9174◦–21.5236◦ N. The model coastline adopted the high-precision data issued by National Oceanic and Atmospheric Administration’s National Geophysical Data Center (NOAA/NGDC) and was adjusted using the sea chart data of the coastal sea area to obtain accurate coastline data. The terrain data in the coastal area were obtained from the ETOPO1 topographic data with resolution of 10 × 100 and adjusted using the measured terrain data. The ETOPO1 is a 1 arc-minute global relief model of Earth’s surface that integrates land topography and ocean bathymetry. The coastline and topographic data obtained by using the above two kinds of data can accurately reflect the actual situation of the calculation area. The simulation grid cell selection for the study area is a flexible mesh (FM) or unstructured mesh, where the triangular cells of bathymetry are used to optimize the simulation, with small sizes near land domains and larger sizes in offshore settings. The triangular element sizes are about 800–1000 m offshore, with a total triangular area of 13,636 elements and 8820 nodes. The bathymetry and computational flexible grid mesh are as shown in Figure4. To obtain the tidal current dynamic data and water quality concentration data of ZJB, a total of 26 observation stations were set up for regular voyages (Figure2). The hydrometeorological parameters observed at each station mainly include the water depth, water temperature, salinity, flow velocity, flow direction, wind speed, and wind direction. Water samples were also collected at each site for the analysis of the chemical parameters. The samples were taken according to the standard level of oceanographic investigation and research. The analyzed water quality parameters mainly included dissolved oxygen and nu- trients. Water samples for nutrient analysis were immediately filtered through acid cleaned 0.45 µm acetate cellulose filters. The filtrates were collected in pre-cleaned polyethylene bottles and stored at −20 ◦C until laboratory analysis. Nutrients were determined using the Skalar San++ continuous flow analyzer. The water level of continuous observation stations was automatically collected by an SBE-26 wave tide meter, while the water level of the conventional observation station was measured by conductivity-temperature-depth (CTD) sensors. The dissolved inorganic nutrients included phosphate (PO4-P) and dissolved inorganic nitrogen (DIN). DIN is defined as the sum of the dissolved nitrate (NO3-N), nitrite (NO2-N), and ammonium (NH4-N). A total of four observation stations (Z4, Z8, Z11, and Z21) were set up for continuous voyages, and the continuous observation time was 75 h. Spring tides and neaps of different seasons were selected for observation. When conducting continuous observations, the measured hydrological parameters were the same as the above, and water samples were collected every three hours for chemical parameter analysis.

3.5. Model Calibration and Validation The parameters of the module were set as follows: calculation of the time-step interval, 30 min; initial water level, 0 m; roughness height data, 0.1 m; and nitrate first-order decay rate at 20◦ C, 0.1/day. The computing time was from 14 August 2017, 0 points, to 24 points on the 16th. The model was validated based on the tide level and nutrition concentration. Measured data obtained from 14 to 16 August were used to validate the model. There were four continuous observation stations (Z4, Z8, Z11, and Z21) (Figure2), and the water levels and nutrient concentrations monitored at these continuous observation stations can be used for model validation. In this study, the numerical simulation is divided into two parts: hydrodynamic simulation and water quality simulation. The main calibration parameters for hydrody- namic simulation are Manning number and Courant–Friedrich–Levy (CFL), and the main calibration parameters for water quality simulation are pollutant diffusion coefficient and degradation coefficient. Int. J. Environ. Res. Public Health 2021, 18, 2089 8 of 21 Int. J. Environ. Res. Public Health 2021, 18, 2089 8 of 21

match simulated results and observed data, including the water level and salinity concen- The Manning number and Courant–Friedrich–Levy (CFL) were specified considering tration in different locations, by changing the Manning number and CFL in MIKE 3. The the variable depth for calibrating the MIKE 3 FM model. The CFL number is accounted in Manningthe MIKE number 3 FM for is thedefined numerical as a function stability criterion. of water The depth calibration and can process be calculated aims to match based on depthsimulated and drag results coefficient and observed [40]. data,The CFL including number the wateris accounted level and in salinity the MIKE concentration 3 FM module for inthe different numerical locations, stability by changing criterion. the In Manning order to number ensure and the CFL accuracy in MIKE of 3. the The model, Manning accord- ingnumber to the is results defined of as model a function verification, of water depth the Manning and can be number calculated and based CFL on number depth and in the modeldrag are coefficient 0.018 s/m [401/3]. and The 0.8, CFL respectively. number is accounted in the MIKE 3 FM module for the numericalDue to the stability high criterion.requirement In order of global to ensure sensitivity the accuracy analysis of the for model, computer according and the to long runningthe results time of of model the model, verification, this study the Manning carried on number the local and CFLsens numberitivity, used in the the model disturbance are 0.018 s/m1/3 and 0.8, respectively. method, only changed one input parameter at a time, with the other parameters remaining Due to the high requirement of global sensitivity analysis for computer and the long unchangedrunning timeto obtain of the parameter model, this sensitivity study carried of the on law, the and local the sensitivity, water quality used the model distur- param- etersbance were method, continuously only changed adjusted one and input calibrated parameter [41 at a,42 time,]. Sensitivity with the other analysis parameters parameters includeremaining the diffusion unchanged coefficient to obtain and parameter degradation sensitivity coefficient of the law,of PO and4-P theand water DIN. quality The results showmodel that parameters diffusion werecoefficient continuously can be adjustedregarded and ascalibrated an insensitive [41,42 ].parameter, Sensitivity while analysis degra- dationparameters coefficient include has the high diffusion sensitivit coefficienty, and andits sensitivity degradation is coefficientthat the degradation of PO4-P and coefficient DIN. of POThe4- resultsP is greater show than that diffusionthe degradation coefficient coefficient can be regarded of DIN. as On an insensitivethe basis of parameter, the hydrody- namicwhile model, degradation according coefficient to the has results high sensitivity,of local sensitivity and its sensitivity analysis, is thatthe thedegradation degradation coeffi- coefficient of PO -P is greater than the degradation coefficient of DIN. On the basis of the cients of PO4-P an4d DIN are constantly adjusted according to the simulation conditions hydrodynamic model, according to the results of local sensitivity analysis, the degrada- untiltion the coefficients best calibration of PO4-P result and DINis achieved. are constantly The diffusion adjusted coefficient according to of the PO simulation4-P and DIN is 1.6 m2/s, the degradation coefficient of PO4-P and DIN are 1.68 × 10−7/s and 5.1 × 10−8/s, conditions until the best calibration result is achieved. The diffusion coefficient of PO4-P 2 −7 respectively.and DIN is 1.6 m /s, the degradation coefficient of PO4-P and DIN are 1.68 × 10 /s and 5.1The× 10 simulation−8/s, respectively. results were compared with the measurements. Figures 5 and 6 show the validationThe simulation results resultsof the water were compared level (Z4, with Z8, theZ11, measurements. and Z21), phosphorus Figures5 and concentrations6 show (Z4the and validation Z21), and results nitrogen of the waterconcentrations level (Z4, Z8, (Z8 Z11, and and Z11 Z21),). The phosphorus validatio concentrationsn results demon- strate(Z4 that and Z21),the errors and nitrogen between concentrations the calculated (Z8 and and Z11). measured The validation water results levels demonstrate were predomi- that the errors between the calculated and measured water levels were predominantly nantly within 10 cm; in addition, the variation pattern of the calculated tidal current ve- within 10 cm; in addition, the variation pattern of the calculated tidal current velocity locitywas was consistent consistent with with that ofthat the of measured the measured tidal current tidal current velocity, veloc and theity, errors and the between errors be- tweenthe calculatedthe calculated and measured and measured nitrogen nitrogen concentrations concentrations were within were 20%. within The calibration20%. The cali- brationresults results demonstrate demonstrate that the modelthat the produces model relativelyproduces good relatively simulation good results, simulation the model results, theparameter model parameter selection wasselection reasonable, was reasonable, and the calculation and the results calculati canon characterize results can the characterize ocean thecurrents ocean currents in the study in the area. study area.

Figure 5. Cont.

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FigureFigureFigure 5.5. VerificationVerification 5. Verification ofof of thethe the tidaltidal tidal levelslevels levels at at thethe Z4, Z4,Z4, Z8, Z8,Z8, Z11, Z11Z11 and,, andand Z21 Z21Z21 stations. stations.stations.

FigureFigureFigure 6.6. ValidationValidation 6. Validation ofof thethe of the phosphorusphosphorus phosphorus concentrationsconcentrations concentrations at atat the thethe Z4 Z4Z4 and andand Z21 Z21Z21 stations stationstation andss nitrogen andand nitrogennitrogen concentrations concentrationsconcentrations at the Z8 atat and thethe Z8Z8 andand Z11Z11Z11 stationstation stations.ss..

4.4. ResultsResults andand DiscussionDiscussion 4.1.4.1. ResultsResults ofof thethe CalculationCalculation ofof thethe FlowFlow FieldField inin ZJBZJB ZJBZJB takestakes thethe shapeshape ofof aa treetree branchbranch andand isis thethe largestlargest drowneddrowned--valleyvalley tidaltidal channelchannel onon thethe SouthSouth ChinaChina Coast.Coast. UnderUnder thethe influenceinfluence ofof thethe terrain,terrain, thethe flowflow fieldsfields ofof thethe ebbebb-- tidetide delta,delta, tidaltidal--inletinlet entrance,entrance, floodflood--tidetide deltadelta,, andand tidaltidal basinbasin areare clearlyclearly different.different. TheThe widthwidth ofof eacheach segmentsegment narrowsnarrows goinggoing upstreamupstream suchsuch thatthat thethe energyenergy isis concentratedconcentrated afterafter

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4. Results and Discussion 4.1. Results of the Calculation of the Flow Field in ZJB ZJB takes the shape of a tree branch and is the largest drowned-valley tidal channel on the South China Coast. Under the influence of the terrain, the flow fields of the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin are clearly different. The width of each segment narrows going upstream such that the energy is concentrated after the tidal wave enters, and the upstream tidal range increases, thereby increasing the tidal capacity. In the narrowed section, the scouring force of the tide is strengthened, and the depth of the deep groove is stabilized. Due to the influence of the terrain, the tide in ZJB becomes more complicated when it rises and falls. In the throat section of the mouth, due to the narrow tube effect, the flow velocity is particularly high at the ebb and flood tide. The drowning-valley-type tidal channel in ZJB has obvious irregular semi-diurnal tidal features. The main tidal components M2,S2,O1,K1,M4, and MS4 are almost all introduced from the open sea. There is no river influence in and out of ZJB, and the flow power is mainly the tidal current [43]. Due to the friction from nearby islands and terrain, the current characteristics are more complicated [44]. Based on the calculated results, the ZJB area has strong tidal currents that are signif- icantly affected by the terrain. ZJB has an irregular semi-diurnal tidal pattern, with two high tides and two low tides in one day, with diurnal inequality. Influenced by Donghai Island, Nanshan Island, and the continental shoreline, a stable tidal flood and ebb-tide channel and deep trough have been formed [43]. In the tidal-inlet entrance and bay, affected by the terrain, the tidal current is reciprocating [43]. The flood tide is mainly westward WSW–WNW, the ebb tide is mainly in the ESE direction, and the ebb-tide speed is generally greater than the flood-tide speed. Affected by the terrain, the tidal level gradually increases from the outside of the bay to the inside of the bay, and the tidal range increases from the outside of the bay to the inside of the bay. The average high water level of the tidal-inlet entrance is 0.97 m, and the low water level is 0.80 m. The reciprocating flow is in the direction of the channel. The flow rate of the ebb tide is greater than that of the flood tide. The velocity is strongest near the tidal inlet of the bay. At the moment when the velocity of the flood tidal current of a tide reaches its maximum, the tidal current direction in the deep-water area runs from east to west and is mainly perpendicular to the sea contour line. The maximum velocity at the tidal outlet was about 0.98 m/s (Figure7). At the moment when the velocity of the ebb tidal current of a tide reaches its maximum, the tidal current direction in the deep-water area is from west to east, and it is mainly perpendicular to the sea contour. The maximum velocity at the tidal outlet was about 1.05 m/s (Figure8). The maximum surface velocity is distributed in the tidal-inlet entrance, and the maximum flow velocity on the surface of the flood and ebb tide was 0.98 and 1.05 m/s, respectively. In the vertical distribution of velocity, the maximum velocity appears in the surface layer and in the middle layer; however, the difference of velocity between the surface layer, the middle layer, and the bottom layer was not large (Figures9 and 10). It can be seen from the results that flood-/ebb-tide delta and tidal-inlet entrance water bodies had a stronger exchange with the outer sea, while the tidal basin water bodies had a weaker exchange with the outer sea bodies, which is similar to other research results [45]. Int. J. Environ. Res. Public Health 2021, 18, 2089 10 of 21

the tidal wave enters, and the upstream tidal range increases, thereby increasing the tidal capacity. In the narrowed section, the scouring force of the tide is strengthened, and the depth of the deep groove is stabilized. Due to the influence of the terrain, the tide in ZJB becomes more complicated when it rises and falls. In the throat section of the mouth, due to the narrow tube effect, the flow velocity is particularly high at the ebb and flood tide. The drowning-valley-type tidal channel in ZJB has obvious irregular semi-diurnal tidal features. The main tidal compo- nents M2, S2, O1, K1, M4, and MS4 are almost all introduced from the open sea. There is no river influence in and out of ZJB, and the flow power is mainly the tidal current [43]. Due to the friction from nearby islands and terrain, the current characteristics are more com- plicated [44]. Based on the calculated results, the ZJB area has strong tidal currents that are signif- icantly affected by the terrain. ZJB has an irregular semi-diurnal tidal pattern, with two high tides and two low tides in one day, with diurnal inequality. Influenced by Donghai Island, Nanshan Island, and the continental shoreline, a stable tidal flood and ebb-tide channel and deep trough have been formed [43]. In the tidal-inlet entrance and bay, af- fected by the terrain, the tidal current is reciprocating [43]. The flood tide is mainly west- ward WSW–WNW, the ebb tide is mainly in the ESE direction, and the ebb-tide speed is generally greater than the flood-tide speed. Affected by the terrain, the tidal level gradu- ally increases from the outside of the bay to the inside of the bay, and the tidal range increases from the outside of the bay to the inside of the bay. The average high water level of the tidal-inlet entrance is 0.97 m, and the low water level is 0.80 m. The reciprocating flow is in the direction of the channel. The flow rate of the ebb tide is greater than that of the flood tide. The velocity is strongest near the tidal inlet of the bay. At the moment when the velocity of the flood tidal current of a tide reaches its maximum, the tidal current direction in the deep-water area runs from east to west and is mainly perpendicular to the sea contour line. The maximum velocity at the tidal outlet was about 0.98 m/s (Figure 7). At the moment when the velocity of the ebb tidal current

Int. J. Environ. Res. Public Healthof2021 a ,tide18, 2089 reaches its maximum, the tidal current direction in the deep-water area11 is of 21from west to east, and it is mainly perpendicular to the sea contour. The maximum velocity at the tidal outlet was about 1.05 m/s (Figure 8).

Int.Figure J. Environ. 7Figure. Flow Res. 7. PublicfieldFlow Healthplane field 2021distribution plane, 18 distribution, 2089 near nearthe tidal the tidalinlet inlet at the at themoment moment when when the the velocity velocity of of the the flood flood tide tide reaches reaches 11its of 21

maximum.its maximum.

FigureFigure 8. Flow 8. Flow field field plane plane distribution distribution near near the the tidal tidal inlet inlet at atthe the moment moment when when the the velocity velocity of of the the ebb ebb tide tide reaches reaches its maximum.its maximum.

The maximum surface velocity is distributed in the tidal-inlet entrance, and the max- imum flow velocity on the surface of the flood and ebb tide was 0.98 and 1.05 m/s, respec- tively. In the vertical distribution of velocity, the maximum velocity appears in the surface layer and in the middle layer; however, the difference of velocity between the surface layer, the middle layer, and the bottom layer was not large (Figures 9 and 10). It can be seen from the results that flood-/ebb-tide delta and tidal-inlet entrance water bodies had a stronger exchange with the outer sea, while the tidal basin water bodies had a weaker exchange with the outer sea bodies, which is similar to other research results [45].

Figure 9. Flow field vertical distribution near the tidal inlet at the moment when the velocity of the flood tide reaches its maximum.

Int. J. Environ. Res. Public Health 2021, 18, 2089 11 of 21

Figure 8. Flow field plane distribution near the tidal inlet at the moment when the velocity of the ebb tide reaches its maximum.

The maximum surface velocity is distributed in the tidal-inlet entrance, and the max- imum flow velocity on the surface of the flood and ebb tide was 0.98 and 1.05 m/s, respec- tively. In the vertical distribution of velocity, the maximum velocity appears in the surface layer and in the middle layer; however, the difference of velocity between the surface layer, the middle layer, and the bottom layer was not large (Figures 9 and 10). It can be

Int. J. Environ. Res. Public Health 2021seen, 18 ,from 2089 the results that flood-/ebb-tide delta and tidal-inlet entrance water 12bodies of 21 had a stronger exchange with the outer sea, while the tidal basin water bodies had a weaker exchange with the outer sea bodies, which is similar to other research results [45].

Int. J. Environ.FigureFigure 9Res.. Flow Public 9. Flow field Health field vertical 2021 vertical, 18distribution, 2089 distribution near near the the tidal tidal inlet inlet at at the the moment moment when thethe velocityvelocity of of the the flood flood tide tide reaches reaches its12 of 21

maximum.its maximum.

FigureFigure 10. Flow 10. Flow field field vertical vertical distribution distribution near near th thee tidal tidal inlet inlet at at the the moment moment when thethe velocityvelocity of of the the ebb ebb tide tide reaches reaches its maximum.its maximum.

TheThe three three-day-day time time series ofof seasea level level variations variations at differentat different stations stations in ZJB in during ZJB during the the tidetide gauge gauge-mooring-mooring period fromfrom 8 8 to to 10 10 July July 2017 2017 is shown is shown in Figure in Figure 11. The 11. water The level water at level at the tidaltidal inletinlet changes changes the the most, most, followed followed by theby floodthe flood tidal tidal delta delta and the and ebb the tidal ebb delta. tidal delta.

Figure 11. The sea level time series during the three days at different stations in ZJB.

4.2. Results of the Spatiotemporal Distribution of the Phosphorus Concentration Based on the MIKE3 hydrodynamic module (Flow Model, FM) and considering the influence of temperature and salt in ZJB, the two-dimensional and vertical three-dimen- sional hydrodynamic processes of ZJB were calculated. Combined with the measured nu- trient concentration in ZJB, the MIKE ECO Lab model was used to simulate the distribu- tion of nutrients in ZJB during the tidal cycle. The spatiotemporal distribution of the phos- phorus concentrations at the flood tidal delta, tidal-inlet entrance, the ebb tidal delta, and

Int. J. Environ. Res. Public Health 2021, 18, 2089 12 of 21

Figure 10. Flow field vertical distribution near the tidal inlet at the moment when the velocity of the ebb tide reaches its maximum.

Int. J. Environ. Res. Public Health 2021, 18, 2089The three-day time series of sea level variations at different stations in ZJB13 of 21during the tide gauge-mooring period from 8 to 10 July 2017 is shown in Figure 11. The water level at the tidal inlet changes the most, followed by the flood tidal delta and the ebb tidal delta.

FigureFigure 11. 11The. The sea sea level level time time series series during during the three the three daysat days different at different stations station in ZJB. s in ZJB. 4.2. Results of the Spatiotemporal Distribution of the Phosphorus Concentration 4.2. Results of the Spatiotemporal Distribution of the Phosphorus Concentration Based on the MIKE3 hydrodynamic module (Flow Model, FM) and considering the in- fluenceBased of temperature on the MIKE3 and salt hydrodynamic in ZJB, the two-dimensional module (Flow and Model vertical, three-dimensionalFM) and considering the hydrodynamicinfluence of temperature processes of ZJBand were salt calculated.in ZJB, the Combined two-dimensional with the measuredand vertical nutrient three-dimen- concentrationsional hydrodynamic in ZJB, the processes MIKE ECO of Lab ZJB model were wascalculated. used to simulateCombined the with distribution the measured of nu- nutrientstrient concentration in ZJB during in the ZJB, tidal the cycle. MIKE The ECO spatiotemporal Lab model distribution was used of to the simulate phosphorus the distribu- concentrationstion of nutrients at the in floodZJB during tidal delta, the tidal-inlettidal cycle. entrance, The spatiotemporal the ebb tidal delta, distribution and the tidal of the phos- basinphorus during concentrations flood tide and at ebbthe tideflood are tidal shown delta, are showntidal-inlet in Figure entra 12nce,a,b, the respectively. ebb tidal delta, and To quantify the spatial variations of the nutrient concentration across the modeling domain, the maximum and minimum phosphorus concentrations during the flood tidal and ebb tidal were calculated. The results showed that the spatiotemporal distribution of phosphorus in ZJB was significantly different over the flood-tide duration and ebb-tide duration. The phosphorus concentration field in the offshore areas changed periodically with the movement of the tidal current. Areas with high concentrations of pollutants were primarily concentrated near the discharge outlet (urban areas and aquaculture areas), and the contour lines of the phos- phorus concentration distribution were relatively dense. The concentration of phosphorus in the tidal basin of ZJB was the highest, and the concentration of phosphorus in the tidal-inlet entrance was the lowest at both the flood-tide and ebb-tide moments. The variation characteristics of the phosphorus concentration were roughly consistent with the depth contour. Station 4 in Figure 12 is within the tidal basin of ZJB, where the velocity is relatively slow, and fish farming (oysters, fish, shrimp, etc.) and urban sewage are concentrated. In estuaries and near-shore areas, due to the influence of the tidal cycle and land-source pollution, the distribution of phosphorus concentration has a negative correlation with salinity [36]. Under the influence of tidal cycles and land-source pollution, the concentration phosphorus in station 4 shows a fluctuation in levels (Figure 13). Int. J. Environ. Res. Public Health 2021, 18, 2089 13 of 21

Int. J. Environ. Res. Public Health 2021, 18the, 2089 tidal basin during flood tide and ebb tide are shown are shown14 in of 21Figure 12a,b, re- spectively.

(a)

(b)

FigureFigure 12. 1The2. The spatiotemporal spatiotemporal distribution distribution of phosphorus of phosphorus concentrations concentrations across the modeling across domain the modeling do- atmain the moment at the whenmoment the velocity when ofthe the velocity flood tide of ( athe) and flood ebb tide tide (b ()a reaches) and ebb its maximum tide (b) (mg/L).reaches its maximum (mg/L).

To quantify the spatial variations of the nutrient concentration across the modeling domain, the maximum and minimum phosphorus concentrations during the flood tidal and ebb tidal were calculated. The results showed that the spatiotemporal distribution of phosphorus in ZJB was significantly different over the flood-tide duration and ebb-tide

Int. J. Environ. Res. Public Health 2021, 18, 2089 14 of 21

duration. The phosphorus concentration field in the offshore areas changed periodically with the movement of the tidal current. Areas with high concentrations of pollutants were primarily concentrated near the discharge outlet (urban areas and aquaculture areas), and the contour lines of the phos- phorus concentration distribution were relatively dense. The concentration of phosphorus in the tidal basin of ZJB was the highest, and the concentration of phosphorus in the tidal- inlet entrance was the lowest at both the flood-tide and ebb-tide moments. The variation characteristics of the phosphorus concentration were roughly consistent with the depth contour. Station 4 in Figure 12 is within the tidal basin of ZJB, where the velocity is rela- tively slow, and fish farming (oysters, fish, shrimp, etc.) and urban sewage are concen- trated. In estuaries and near-shore areas, due to the influence of the tidal cycle and land- Int. J. Environ. Res. Public Health 2021, source18, 2089 pollution, the distribution of phosphorus concentration has a negative correlation15 of 21 with salinity [36]. Under the influence of tidal cycles and land-source pollution, the con- centration phosphorus in station 4 shows a fluctuation in levels (Figure 13).

FigureFigure 13.13. TemporalTemporal distributiondistribution ofof phosphorusphosphorus concentrationsconcentrations andand salinitysalinity atat stationstation 4 during the three days. three days.

DuringDuring thethe slack slack water water period, period, the the range range of phosphorusof phosphorus concentrations concentrations at the at ebb-tidethe ebb- delta,tide delta, tidal-inlet tidal- entrance,inlet entrance, flood-tide flood delta,-tide delta, and tidal and basintidal basin were, were, respectively, respectively,0.025–0.071 0.025–, 0.071–0.083,0.071, 0.071– 0.083–0.142,0.083, 0.083– and0.142, 0.142–0.280 and 0.142– mg/L.0.280 mg/L. During During the ebb the slack ebb period,slack period, the phos- the phorusphosphorus concentration concentration ranges ranges at ebb-tide at ebb delta,-tide delta, tidal-inlet tidal- entrance,inlet entrance, flood-tide flood delta,-tide delta, and tidaland basintidal basin were, were, respectively, respectively, 0.045–0.083, 0.045– 0.083–0.129,0.083, 0.083– 0.129–0.155,0.129, 0.129– and0.155, 0.155–0.280 and 0.155 mg/L.–0.280 Throughmg/L. Through the effect the ofeffect the ebbof the tide, ebb some tide, pollutantssome pollutants can also can be also brought be brought to the to outer the outer bay areabay toarea be to purified be puri infied the in outer the outer sea, reducing sea, reducing the possibility the possibility of water of water pollution pollution in the in bay. the bay. However, the water environment is poor due to the small area, narrow topography, smallHowever, flow velocity the water of part environment of the tidal is basin, poor due and to because the small the area, coastal narrow area topography, of this part issmall the locationflow velocity of a densely of part populatedof the tidal area basin, that and includes because the the industrial coastal area zone of and this port part of is Zhanjiangthe location City, of wherebya densely the populated main terrestrial area that pollutants includes are the also industrial discharged zone into and the port sea of areaZhanjiang here [ 38City,39,]. whereby The tidal the inlets main connecting terrestrial pollutants oceans to estuariesare also discharged profoundly into influence the sea thearea ecology here [38,39 of estuarine]. The tidal water inlets bodies connecting [4]. Studies oceans have to estuaries analyzed profoundly the dissolved influence oxygen the dynamicsecology of [46 estuarine], nutrient water dynamics, bodies phytoplankton [4]. Studies have communities analyzed [ 47the,48 dissolved], and composition oxygen dy- of fishnamics assemblages [46], nutrient [4] at dynamics, coastal tidal phytoplankton inlets. Chinese communities white dolphin, [47,48 mangrove], and composition crabs, and sea of starsfish assemblages are common [4] in ZJB,at coastal some tidal studies inlets have. Chinese shown thatwhite the dolphin, spatiotemporal mangrove distribution crabs, and characteristicssea stars are common of nutrients in ZJB, and some tides studies in ZJB and have coastal shown urbanization that the spatiotemporal can affect the distribu- ecology oftion these characteristics species [49– 51of ].nutrients and tides in ZJB and coastal urbanization can affect the ecologyAt the of these moment species when [49 a– spring51]. tide reaches high tide, the flood tidal current is about to turn into an ebb tidal current, and the phosphorus-affected area reaches its maximum during the rise of the flood tide. Due to the effect of the westward flood tidal current, the phosphorus concentration distribution migrates westward along the tidal-inlet entrance, and the seawater with low phosphorus content in the open sea enters ZJB through the tidal-inlet entrance. The concentration line of phosphorus curves from the ebb-tide delta to the flood-tide delta, and the 0.06–0.08 mg/L concentration envelope line penetrates into the flood-tide delta from the tidal-inlet entrance. The sea areas where the phosphorus concentration is 0.06–0.08 mg/L have an area of 63.11 km2, and the sea areas where the phosphorus concentration is in the range of 0.08–0.14 mg/L have an area of 65.67 km2 (Figure 14a). Int. J. Environ. Res. Public Health 2021, 18, 2089 15 of 21

At the moment when a spring tide reaches high tide, the flood tidal current is about to turn into an ebb tidal current, and the phosphorus-affected area reaches its maximum during the rise of the flood tide. Due to the effect of the westward flood tidal current, the phosphorus concentration distribution migrates westward along the tidal-inlet entrance, and the seawater with low phosphorus content in the open sea enters ZJB through the tidal-inlet entrance. The concentration line of phosphorus curves from the ebb-tide delta to the flood-tide delta, and the 0.06–0.08 mg/L concentration envelope line penetrates into the flood-tide delta from the tidal-inlet entrance. The sea areas where the phosphorus con- 2 Int. J. Environ. Res. Public Health 2021, 18, 2089 centration is 0.06–0.08 mg/L have an area of 63.11 km , and the sea areas16 ofwhere 21 the phos- phorus concentration is in the range of 0.08–0.14 mg/L have an area of 65.67 km2 (Figure 14a).

(a)

(b)

Figure 14. TheFigure spatiotemporal 14. The spatiotemporal distribution of distribution the phosphorus of the phosphorusconcentration concentration near the tidal near inlet the at tidal the inletmoment at the when the velocity of themoment flood tide when (a) and the velocityebb tide of(b the) reaches flood its tide maximum (a) and ebb (mg/L) tide (. b) reaches its maximum (mg/L).

At the momentAt the when moment a spring when tide a spring reaches tide low reaches tide, the low ebb tide, tidal the current ebb tidal is aboutcurrent is about to to turn into aturn flood-tide into a flood current,-tide and current, the phosphorus-affected and the phosphorus- areaaffected reaches area itsreaches maximum its maximum dur- during the riseing ofthe the rise ebb of tide.the ebb Due tide. to theDue effect to the of effect the eastward of the eastward flood tidal flood current, tidal current, the the phos- phosphorusphorus concentration concentration distribution distribution migrates migrates eastward eastward along the along tidal-inlet the tidal entrance,-inlet entrance, and and the seawater with higher phosphorus concentration in ZJB enters the South China Sea through tidal-inlet entrance. The concentration line of phosphorus curves from the flood-tide delta to the ebb-tide delta, and the 0.06–0.08 mg/L concentration envelope line penetrates into the ebb-tide delta from tidal-inlet entrance. The sea areas where the phosphorus concentration is greater than 0.06–0.08 mg/L have an area of 63.98 km2, and the sea areas where the phosphorus concentration is in the range of 0.08–0.14 mg/L have an area of 88.55 km2 (Figure 14b). Un- der the influence of the terrain and tidal current, the phosphorus concentration throughout the duration of the flood tide and ebb tide shows clear temporal and spatial differences in the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin [52]. Int. J. Environ. Res. Public Health 2021, 18, 2089 16 of 21

the seawater with higher phosphorus concentration in ZJB enters the South China Sea through tidal-inlet entrance. The concentration line of phosphorus curves from the flood-tide delta to the ebb-tide delta, and the 0.06–0.08 mg/L concentration envelope line penetrates into the ebb-tide delta from tidal-inlet entrance. The sea areas where the phosphorus concentration is greater than 0.06–0.08 mg/L have an area of 63.98 km2, and the sea areas where the phos- phorus concentration is in the range of 0.08–0.14 mg/L have an area of 88.55 km2 (Figure Int. J. Environ.14b). Res. Under Public Health the influence2021, 18, 2089 of the terrain and tidal current, the phosphorus concentration 17 of 21 throughout the duration of the flood tide and ebb tide shows clear temporal and spatial differences in the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin [52].

4.3. Results of Spatiotemporal4.3. Results Distribution of Spatiotemporal of Nitrate Distribution Concentration of Nitrate Concentration The spatiotemporal The distribution spatiotemporal of the distribution nitrogen concentrations of the nitrogen at concentrations the flood tidal at the flood tidal delta, delta, tidal-inlet entrance,tidal-inlet ebb tidal entrance, delta, ebb and tidal tidal delta, basin andduring tidal the basin flood during and ebb the tides flood and ebb tides are are shown in Figureshown 15a,b, respectively. in Figure 15a,b, The respectively. spatial distribution The spatial of distributionthe nitrogen ofconcen- the nitrogen concentration tration is similar to thatis similar of phosphorus to that of concentration phosphorus concentration due to the effects due toof thethe effectsland-source of the land-source sewage sewage discharge anddischarge aquaculture, and aquaculture,and the concentration and the concentration of the tidal basin of the is tidalthe highest basin is the highest during during the period ofthe the period flood ofand the ebb flood tides. and In ebb the tides. throat In section the throat of the section tidal of-inlet the tidal-inleten- entrance, due trance, due to the narrowto the tube narrow effect, tube the effect, velocity the velocity of the flow of the is flowparticularly is particularly large at large the at the time of flood andInt. J. ebbEnviron. tides, Res. thePublic turbulent Health 2021 diffusion, 18, 2089 is strong, and the concentration of nitrogen at the 17 of 21 time of flood and ebb tides, the turbulent diffusion is strong, and the concentration of nitrogen at the tidal-tidal-inletinlet entrance entrance is the is lowest. the lowest.

(b) (a)

Figure 15. The spatiotemporal distribution of the nitrate concentrationFigure 15. acrossThe spatiotemporal the modeling distribution domain at the of momentthe nitrate when concentration across the modeling do- the velocity of the flood tide (a) and ebb tide (b) reaches its maximummain at the (mg/L). moment when the velocity of the flood tide (a) and ebb tide (b) reaches its maximum (mg/L). With the exception of the tidal basin, the nitrogen concentration in other waters of ZJB was within the second-classWith range the of exception water quality of the standards, tidal basin, and the thenitrogen nitrogen concentration in other waters of concentration was relatively low.ZJB The was spatial within and the temporal second-class distribution range of of water phosphorus quality was standards, and the nitrogen con- significant at the time of floodcentration and ebb tides,was relatively while the low. spatial The and spatial temporal and temporal difference distribution of phosphorus was of nitrogen was relatively small,significant especially at inthe the time flood/ebb of flood tidaland ebb delta. tides, During while the the flood- spatial and temporal difference of and ebb-tide periods, the nitrogennitrogen concentration was relatively in the small, flood/ebb especially tidal in delta the flood/ebb was mostly tidal delta. During the flood- and within the range of 0.1–0.2 mg/L.ebb During-tide periods, the slack the water nitrogen period, concentration the nitrogen in concentration the flood/ebb tidal delta was mostly within ranges at the ebb-tide delta, tidal-inletthe range entrance, of 0.1–0.2 flood-tide mg/L. During delta, the and slack tidal water basin period,were, the nitrogen concentration respectively, 0.098–0.127, 0.097–0.106,ranges at 0.072–0.174, the ebb-tide and delta, 0.174–1.405 tidal-inlet mg/L. entr Duringance, flood the ebb-tide delta, and tidal basin were, slack period, the nitrogen concentrationrespectively, range 0.098 at–0.127, the ebb-tide 0.097–0.106, delta, 0.072 tidal-inlet–0.174, entrance, and 0.174–1.405 mg/L. During the ebb flood-tide delta, and tidal basinslack were, period, respectively, the nitrogen 0.135–0.058, concentration 0.058–0.098, range 0.098–0.193,at the ebb-tide delta, tidal-inlet entrance, and 0.193–1.412 mg/L. flood-tide delta, and tidal basin were, respectively, 0.135–0.058, 0.058–0.098, 0.098–0.193, At the moment when a springand 0.193 tide– reaches1.412 mg/L. high tide, the flood tidal current is about to turn into an ebb tidal current,At and the the moment nitrogen-affected when a spring area tide reaches reaches its high maximum tide, the flood tidal current is about during the rise of the flood tide.to turn Due into to the an effect ebb tidal of the current, westward and floodthe nitrogen tidal current,-affected the area reaches its maximum dur- nitrogen concentration distributioning the migrates rise of westwardthe flood tide. along Due the tidal-inletto the effect entrance, of the westward and flood tidal current, the the seawater with a low nitrogennitrogen content concentration in the open seadistribution enters ZJB migrates through westward the tidal-inlet along the tidal-inlet entrance, and entrance. The concentration linethe ofseawater nitrogen with curves a low from nitrogen the ebb-tide content deltain the to open flood-tide sea enters ZJB through the tidal-inlet delta, and the 0–0.1 mg/L concentrationentrance. The envelope concentration line penetrates line of intonitrogen the flood-tide curves from delta the ebb-tide delta to flood-tide delta, and the 0–0.1 mg/L concentration envelope line penetrates into the flood-tide delta from the tidal-inlet entrance. The sea areas where the nitrogen concentration is in the range of 0–0.1 mg/L have an area of 27.74 km2 (Figure 16a). At the moment when a spring tide reaches low tide, the ebb tidal current is about to turn into a flood-tide current, and the nitrogen-affected area reaches its maximum during the rise of the ebb tide. Due to the effect of the east-westward flood tidal current, the ni- trogen concentration distribution migrates eastward along the tidal-inlet entrance, and the seawater with a higher nitrogen concentration in ZJB enters the South China Sea through the tidal-inlet entrance. The concentration line of nitrogen curves from the flood- tide delta to the ebb-tide delta, and the 0–0.1 mg/L concentration envelope line penetrates into the ebb-tide delta from the tidal-inlet entrance. The sea areas where the nitrogen con- centration is in the range of 0–0.1 mg/L have an area of 57.16 km2 (Figure 16b) [53].

Int. J. Environ. Res. Public Health 2021, 18, 2089 18 of 21

The nitrogen concentration field in the offshore areas changes periodically with the Int. J. Environ. Res. Public Health 2021, 18movement, 2089 of the tidal current. Under the influence of the terrain and tidal current, the18 of 21 nitrogen concentrations at the flood-tide and ebb-tide moments show obvious temporal and spatial differences at the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin. Decreased water velocities in the tidal basin generally lead to the accumulation of fromnutrients. the tidal-inlet Nutrient entrance. concentrations The sea are areas also whereexpected the to nitrogen be higher concentration due to the reduced is in the ex- range ofchange 0–0.1 mg/Lwith the have more an oligotrophic area of 27.74 ocean km2 water(Figure [4]. 16 a).

(a)

(b)

FigureFigure 16. 16.The The spatiotemporal spatiotemporal distribution distribution ofof thethe nitratenitrate concentration near near the the tidal tidal inlet inlet at atthe the moment moment when when thethe velocity velocity of theof the flood flood tide tide (a )(a and) and ebb ebb tide tide (b ()b) reaches reaches its its maximum maximum (mg/L).(mg/L).

At the moment when a spring tide reaches low tide, the ebb tidal current is about to turn into a flood-tide current, and the nitrogen-affected area reaches its maximum during the rise of the ebb tide. Due to the effect of the east-westward flood tidal current, the nitrogen concentration distribution migrates eastward along the tidal-inlet entrance, and the seawater with a higher nitrogen concentration in ZJB enters the South China Sea through the tidal-inlet entrance. The concentration line of nitrogen curves from the flood- tide delta to the ebb-tide delta, and the 0–0.1 mg/L concentration envelope line penetrates into the ebb-tide delta from the tidal-inlet entrance. The sea areas where the nitrogen concentration is in the range of 0–0.1 mg/L have an area of 57.16 km2 (Figure 16b) [53]. Int. J. Environ. Res. Public Health 2021, 18, 2089 19 of 21

The nitrogen concentration field in the offshore areas changes periodically with the movement of the tidal current. Under the influence of the terrain and tidal current, the nitrogen concentrations at the flood-tide and ebb-tide moments show obvious temporal and spatial differences at the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin. Decreased water velocities in the tidal basin generally lead to the accumulation of nutrients. Nutrient concentrations are also expected to be higher due to the reduced exchange with the more oligotrophic ocean water [4].

5. Conclusions Based on the analysis of natural conditions of the ZJB areas, the spatial and temporal variation processes of water quality factors of the bay under the influence of special geomorphic units were simulated at the tidal-inlet entrance, the flood/ebb tidal delta, and the tidal basin using the MIKE3 FM hydrodynamic model and the MIKE ECO Lab model. The results of the present study can serve as a reference for the numerical simulation of similar projects. The main conclusions of the present study are as follows. (1) ZJB has strong tidal currents that are significantly affected by the terrain. In the narrowed section, the scouring force of the tide is strengthened, and the depth of the deep groove is stabilized. Due to the influence of the terrain, the tide in ZJB becomes more complicated when it rises and falls. In the throat section of the mouth, due to the narrow tube effect, the flow velocity is particularly high during ebb and flood tides. There are significant differences in the tidal current velocity between the deep-water areas and the shoals. (2) Under the influence of the terrain, the nutrient concentration changes greatly at the tidal-inlet entrance, flood/ebb tidal delta, and tidal basin with the change of the tide. The nutrients migrate southwestward with the flood tidal current and northeastward with the ebb tidal current. The dilution and dispersion of the nutrients are affected by the ocean currents in different tidal periods. At the time of flood tide and ebb tide, affected by the velocity and water level, the concentration of phosphorus in the tidal basin demonstrated a slight change while changing greatly at the ebb- tide delta, tidal-inlet entrance, and flood-tide delta. Except for the tidal basin, the nitrogen concentration in other waters of ZJB was within the second-class range of the water quality standard, and the nitrogen concentration was relatively low. Under the influence of the terrain and tidal current, the phosphorus concentration at the flood-tide and ebb-tide moments showed clear temporal and spatial differences at the ebb-tide delta, tidal-inlet entrance, flood-tide delta, and tidal basin.

Author Contributions: All authors contributed to the data assessment and analysis strategy. S.W. and F.C. conceived and designed the study. Y.M. and Q.Z. performed sample collection and con- tributed to the experiment and measurement. F.Z. and F.C. collaborated in discussing the manuscript and modifying the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was co-funded by the Foundation for Young Talents in General Colleges and Universities of Guangdong Province (2019KQNCX044); the Program for Scientific Research Start-up Funds of Guangdong Ocean University (R20018); the Foundation for Young Talents in General Colleges and Universities of Guangdong, China (2019KQNCX044); the Guangdong Natural Science Foundation of China (2016A030312004), the Innovation Team Plan in Universities of Guangdong Province(2019KCXTF021), and the National Natural Science Foundation of China (U1901213). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. Int. J. Environ. Res. Public Health 2021, 18, 2089 20 of 21

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