Multi-Year Winter Variations in Suspended Sediment Flux Through the Bohai Strait

remote sensing

Opinion Multi-Year Winter Variations in Suspended Sediment Flux through the Bohai Strait

Xingmin Liu 1,2, Lulu Qiao 1,2,*, Yi Zhong 1,2, Wenjing Xue 1,2 and Peng Liu 1,2

1 College of Marine Geosciences, Ocean University of China, 238 Songling Road, Qingdao 266100, China; [email protected] (X.L.); [email protected] (Y.Z.); [email protected] (W.X.); [email protected] (P.L.) 2 Key Lab of Submarine Sciences & Prospecting Techniques, MOE, Ocean University of China, 238 Songling Road, Qingdao 266100, China * Correspondence: [email protected]

Received: 4 November 2020; Accepted: 8 December 2020; Published: 11 December 2020

Abstract: The Bohai Strait is the only channel that allows material exchanges between the Bohai Sea and the Yellow Sea. It is also the only channel for the transportation of materials from the Yellow River to the Yellow Sea and the East China Sea. The supply of suspended sediment from the Bohai Sea plays a decisive role in the evolution of the mud area in the northern Yellow Sea and even the muddy area in the southern Yellow Sea. Previous studies have demonstrated that sediment exchange through the Bohai Strait occurs mainly in winter, but due to the lack of long-term observational data, changes in the sediment flux over multiple years have not been studied. In this paper, based on L1B data from the MODIS (Moderate Resolution Imaging Spectroradiometer) -Aqua satellite, an interannual time series of the suspended sediment concentration (SSC) at each depth layers in the Bohai Strait in winter was established through 16 cruises that benefited from the complete vertical mixing water in the strait in winter. The numerical model FVCOM, (Finite-Volume Community Ocean Model) which is forced by the hourly averaged wind field, reflected the effect of winter gales. With the model simulated winter current from 2002 to the present and the SSC at each layer, multi-year winter suspended sediment flux data were obtained for the Bohai Strait. This study found that in the winter, the suspended sediment output from the Bohai Sea to the Yellow Sea through the southern part of the Bohai Strait, while the suspended sediment input from the Yellow Sea to the Bohai Sea is through the northern part. In terms of long-term changes, the net flux ranged between 1.22 to 2.70 million tons in winter and showed a weak downward trend. The output flux and input flux both showed an upward trend, but the increase rate of the input flux was 51,100 tons/year, which was higher than the increase of the output flux rate (46,100 tons/year). These changes were mainly controlled by the increasing strength of east component of winter wind. And the weak decrease in net flux is controlled by the difference of output and input flux.

Keywords: Bohai Strait; suspended sediment ﬂux; winter wind; multi-year changes

1. Introduction Research on the suspended sediment fluxes of the Bohai Strait is relatively extensive, with a variety of technical methods. Seasonal variation in the flux and the differences in the suspended sediment concentration (SSC) between different water layers are mainly controlled by the advance and retreat of the cold water mass in the northern Yellow Sea and the temperature and salinity changes in the Bohai Strait [1–3]. Suspended sediment transport though the Bohai Strait is concentrated mainly on the south side of the Bohai Strait and occurs mainly in winter. This is driven by the eastward current in the southern Bohai Strait and the effects of winter wind [4–6], and the scope is mainly restricted

Remote Sens. 2020, 12, 4066; doi:10.3390/rs12244066 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 4066 2 of 14 by current fronts [7]. Concerning the factors that affect SSC and suspended sediment flux, previous studies have focused on the results related to wind magnitude, wind direction, water temperature, salinity, river-delivered sediments, shoreline evolution and even the sea ice [8–14]. Compared with the number of studies at the seasonal scale, rare works are conducted on the multi-year variations in flux in the Bohai Strait. Satellite remote sensing is widely used in suspended sediment distribution in the Bohai Strait and adjacent seas [1,11,15]. Obvious seasonal variation is found in the SSC near the Bohai Strait [1,9,15], with the highest value in winter and lower value in other seasons. This is caused by the wind velocity change, which can strengthen the hydrodynamic conditions such as waves and currents during a strong winter wind period. Thus, there is a strip of water with high SSC from the northern Shandong Peninsula to the east, bypassing the eastern Shandong Peninsula [2,5]. From the perspective of long-term changes, the SSC in different areas of the Bohai Sea are quite different. The study found that the areas around the Yellow River mouth, Bohai Bay, and Laizhou Bay experienced the largest changes. However, the interannual changes in the SSC in the central Bohai Sea and the northern Yellow Sea are very small [1,15]. Although satellite remote sensing is often used in suspended sediment study, it is typically applied to the sea surface water, and it has few applications in the study of middle-depth and deeper waters. In addition, in previous studies on the flux of suspended sediment, the surface flux is generally retrieved by remote sensing, while the mid-bottom flux generally relies on the short-term measured or model simulated SSC and current data [16–18]. However, due to the lack of accurate sediment movement parameters, the accuracy of the SSC simulation is not satisfactory enough. Moreover, due to the lack of long-term measured current data, there are very few studies on the interannual variation in suspended sediment flux through the Bohai Strait. Therefore, this paper uses remote sensing SSC, in situ measured SSC and the numerical simulate currents to study the multi-year winter variation of the sediment flux in the Bohai Strait. Sediment flux in the Bohai Strait in winter is of great significance to material exchanges between the Bohai Sea and the Yellow Sea, the water quality environment in the marine pastures of the northern Shandong Peninsula, and the formation of muddy areas in the northern Yellow Sea as well as the southern Yellow Sea [14,15]. This study is also valuable as a reference in how to retrieve mid-bottom SSC from sea surface satellite data.

2. Study Area The Bohai Sea is a shallow semi-enclosed inner shelf sea in northeastern China (Figure1), with a total area of 77,000 km2 and an average water depth of 18 m [19]. The Bohai Sea is geographically composed by three shallow bays, Laizhou Bay, Bohai Bay, and Liaodong Bay, and the central Bohai Basin (Figure1). Four main rivers deliver fresh water and sediment to the Bohai Sea, namely the Yellow River, the Haihe River, the Luanhe River, and the Liaohe River, among which the Yellow River has the highest water discharge and sediment load to the sea [11]. Approximately 70~90% of the Yellow River’s discharged sediment deposited around the Yellow River mouth [15,20,21], and the rest was transported to the open seas [1,15]. The Bohai Sea is connected to the Yellow Sea through the Bohai Strait. The Bohai Strait gradually deepens from south to north with a maximum water depth of more than 60 m. The Bohai Strait is a necessary channel for water exchange between the Bohai Sea and the Yellow Sea, and it is also the only pathway for the long distance transportation of suspended sediments delivered by the Yellow River. The width of the Bohai Strait is 106 km, with a line of islands extending from the Shandong peninsula, starting with Nanchangshan to Beihuangcheng, covering half of this distance across the strait. Thus, there should be an insular baﬄe which must eﬀect on the circulation of sediment-laden waters. Remote Sens. 2020, 12, 4066 3 of 14 Remote Sens. 2020, 12, x FOR PEER REVIEW 3 of 16

Figure 1. Distribution of observation stations ((a) study area; (b) 5 cruises in spring; (c) 4 cruises in summer; (d) 4 cruises in autumn; (e) 4 cruises in winter). The regions in dark gray indicate the muddy Figure 1. Distribution of observation stations ((a) study area; (b) 5 cruises in spring; (c) 4 cruises in deposition areas. summer; (d) 4 cruises in autumn; (e) 4 cruises in winter). The regions in dark gray indicate the 3. Datamuddy and deposition Methodology areas.

3.3.1. Data Measured and Methodology Hydrological Parameters The measured data in this study mainly include SSC, water turbidity, temperature and salinity 3.1. Measured Hydrological Parameters by SeaBird 911 conductivity-temperature-depth system (CTD). These in situ data were mainly used forThe the retrievalmeasured of data remote in this sensing study data mainly and theinclude verification SSC, water of the turbidity, retrieved temperature results. The and resolution salinity of bythe SeaBird temperature, 911 conductivity-temperature-depth salinity, and turbidity data was system 1 m, and(CTD). the These resolution in situ of data the SSCwere data mainly was used based foron the the retrieval number of of remote layers insensing the di ffdataerent and water the veri depthsfication (generally of the 3–6retrieved layers). results. To ensure The resolution a sufficient ofdata the volume,temperature, this studysalinity, used and measured turbidity data data from was di 1ff erentm, and seasons the resolution for retrieval of the in orderSSC data to obtain was a basedretrieval on the model number that canof layers be applied in the to different all seasons. water The depths first dataset (generally is from 3–6 thelayers). joint To voyage ensure of thea sufficientNational data Natural volume, Science this Foundationstudy used ofmeasured China (11 data cruises from in different total, see season Figures 1for), undertaken retrieval in mainlyorder toby obtain the “Dongfanghong a retrieval model 2” scientificthat can researchbe applied ship to of all Ocean seasons. University The first of China.dataset Theis from second thedataset joint voyageis from of the the joint National voyage Natural of the QingdaoScience Foundation Marine Science of China and Technology (11 cruises Pilot in total, National see Figure Laboratory 1), undertaken“Transparent mainly Ocean” by the plan “Dongfanghong undertaken by 2” the scientif “Innovationic research 1” scientific ship of Ocean research University vessel of of the China. Yantai TheInstitute second of dataset Coastal is Zone from Research, the joint Chinese voyage Academy of the Qingdao of Sciences Marine in 2018 Science (3 cruises and inTechnology total, see Table Pilot1 ). National Laboratory “Transparent Ocean” plan undertaken by the “Innovation 1” scientific research vessel of the Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences in 2018 (3 cruises in total, see Table 1).

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Table 1. Information about the cruises from 2010 to 2019.

Number of Season Symbol Survey Date Survey Elements Samples suspended sediment concentration Spring + 52 29 April 2010–4 May 2010 Remote Sens. 2020, 12, 4066 (SSC) 4 of 14 ● 66 11 May 2012–18 May 2012 SSC ◇ 47 18 April 2018–29 April 2018 SSC ▽ Table57 1. Information 28 April about 2019–6 the cruises May 2019 from 2010 to 2019. SSC △ 20 11 May 2019–14 May 2019 SSC SummerSeason Symbol● Number 25of Samples 21 June Survey 2011–27 Date June 2011 Survey Elements SSC Spring + 7152 298 July April 2016–14 2010–4 MayJuly 2016 2010 suspended sediment SSC concentration (SSC) ◇ 4566 17 August 11 May 2018–26 2012–18 MayAugust 2012 2018 SSC SSC • ▽ 5247 26 18 July April 2019–1 2018–29 August April 20182019 SSC SSC 3 57 28 April 2019–6 May 2019 SSC 5 14 September 2010–21 September Autumn ● 4920 11 May 2019–14 May 2019 SSCSSC 4 Summer 25 21 June 2011–272010 June 2011 SSC • 12 November 2012–19 November + 6171 8 July 2016–14 July 2016 SSCSSC 45 17 August 2018–262012 August 2018 SSC 3 52 26 July 2019–1 August 2019 SSC 5 9 September 2017–16 September Autumn ◇ 5949 14 September 2010–21 September 2010 SSCSSC • 2017 + 61 12 November 2012–19 November 2012 SSC Winter + 65 20 November 2011–1 December 2011 SSC 59 9 September 2017–16 September 2017 SSC Winter 3+● 6165 20 14 November January 2016–30 2011–1December January 2016 2011 Water temperature, SSC salinity, turbidity ▽ 6261 29 14 December January 2016–30 2017–8 January January 2016 2018 Water Water temperature, temperature, salinity, salinity, turbidity turbidity • 6222 29 November December 2017–8 2018–12 January December 2018 Water temperature, salinity, turbidity ◇5 34 Water temperature, salinity, turbidity 34 22 November 2018–122018 December 2018 Water temperature, salinity, turbidity 3 3.2. Fitting Fitting Methods Methods for for Each Each Water Water Layer Due to the influence inﬂuence of stronger winter winds in the Bohai Sea and the Bohai Strait, the water bodies becomebecome fullyfully mixed mixed (Figure (Figure2), with2), with the water the water turbidity turb inidity the upperin the and upper lower and layers lower relatively layers relativelyuniform. uniform. Therefore, Therefore, the winter the turbidity winter turbidity data over data many over years many can years be usedcan be to used establish to establish a ﬁtting a fittingrelationship relationship between between the surface the layer surface and layer deeper and layers deeper in order layers to translate in order the to surface translate remote the sensingsurface remoteretrieval sensing results retrieval into SSC results in the middleinto SSC and in the bottom middle layers. and bottom layers.

Figure 2. VerticalVertical distribution distribution of of measur measureded water water turbidity turbidity along along the the transect transect in inthe the Bohai Bohai Strait Strait in winter.in winter. According to the previous studies, the SSC in seawater had a significant correlation with the According to the previous studies, the SSC in seawater had a significant correlation with the distribution of seawater turbidity. Thus, turbidity values can be converted into SSC values [3,7]. distribution of seawater turbidity. Thus, turbidity values can be converted into SSC values [3,7]. Since surface SSC retrievals use measured SSC data from multiple cruises, it is necessary to convert Since surface SSC retrievals use measured SSC data from multiple cruises, it is necessary to convert the turbidity of each layer into the SSC before fitting the relationship between the surface turbidity the turbidity of each layer into the SSC before fitting the relationship between the surface turbidity and the turbidity of each layer. The conversion formula used in this article is the conversion method and the turbidity of each layer. The conversion formula used in this article is the conversion method established by Liu et al. The specific method is as follows (Figure3). established by Liu et al. The specific method is as follows (Figure 3). Figure4 shows that the surface SSC has obvious linear relationships with the SSCs of the other layers. The correlation coefficient (R2) gradually decreases from the surface layer to the bottom layer, but the lowest value is still above 0.84. The root mean square error (RMSE) gradually increases from the surface to the bottom and is nearly always below 3.3 mg/L (Table2), which indicates an acceptable fit for this article.

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Figure 3. RelationshipFigure 3. betweenRelationship measured between measured SSC (mg SSC/ L)(mg/L) and and measured measured turbidityturbidity (nephelometric (nephelometric turbidity turbidity units (NTU)). units (NTU)). Remote Sens. 2020Figure, 12 ,4 x shows FOR PEER that REVIEWthe surface SSC has obvious linear relationships with the SSCs of the other 6 of 16 layers. The correlation coefficient (R2) gradually decreases from the surface layer to the bottom layer, but the lowest value is still above 0.84. The root mean square error (RMSE) gradually increases from the surface to the bottom and is nearly always below 3.3 mg/L (Table 2), which indicates an acceptable fit for this article.

Table 2. Correlation of the fitting relationship between the SSCs of the surface layer and the other layers at the stations in the Bohai Strait.

Surface-5 Surface-10 Surface-15 Surface-20 Surface-25 Surface-30 Surface-40 Surface-Bottom m m m m m m m R2 0.9989 0.9881 0.9871 0.9769 0.9421 0.8433 0.8618 0.9534 RMSE 0.193 0.639 0.952 2.303 1.471 1.54 1.63 3.304

Figure 4. FittingFigure 4. relationshipsFitting relationships between between the the SSCs SSCs of the the surface surface layer layer (horizontal (horizontal axis) and axis) the other and the other layers (vertical axis, (a) surface layer and 5 m layer, (b) surface layer and 10 m layer, (c) surface layer layers (vertical axis, (a) surface layer and 5 m layer, (b) surface layer and 10 m layer, (c) surface layer and 15 m layer, (d) surface layer and 20 m layer, (e) surface layer and 25 m layer, (f) surface layer and 15 m layer,and 30 (md )layer, surface (g) surface layer andlayer 20and m 40 layer, m layer, (e) ( surfaceh) surface layer layer andand bottom 25 m layer, layer; (blackf) surface points layer and 30 m layer,represent (g) surface all stations; layer red and points 40 m represent layer, (theh) surfaceBohai Strait layer stations). and bottom layer; black points represent all stations; red points represent the Bohai Strait stations). 3.3. Remote Sensing Data The remote sensing image data used were Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua satellite data downloaded from the NASA website (https://www.nasa.gov). The Level-1B (L1B) data for December, January, and February of each year from 2002 to 2018 (the winter of 16 years) were obtained. The images had a spatial resolution of 1 km. The remote sensing data in this study were processed and transformed mainly using the SeaWiFS Data Analysis System (SeaDAS) software officially provided by NASA for atmospheric correction, cloud removal and other preliminary processing. The L1B remote sensing data were batched into Level-2 remote sensing reflectance (Rrs). When establishing a remote sensing retrieval model, the Rrs that best matched the measured data in time and space was selected. A time control period within 3 h was effective for our data. A fitting relationship between the remote sensing data and the measured data was established. In addition, because there is visible sea ice in the Bohai Sea and the Yellow Sea in winter, sea ice has a great influence on SSC retrieval. Therefore, this study used methods described in previous studies to remove the influence of sea ice [14,15].

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Table 2. Correlation of the ﬁtting relationship between the SSCs of the surface layer and the other layers at the stations in the Bohai Strait.

Surface-5 m Surface-10 m Surface-15 m Surface-20 m Surface-25 m Surface-30 m Surface-40 m Surface-Bottom R2 0.9989 0.9881 0.9871 0.9769 0.9421 0.8433 0.8618 0.9534 RMSE 0.193 0.639 0.952 2.303 1.471 1.54 1.63 3.304

3.3. Remote Sensing Data Remote Sens. 2020, 12, x FOR PEER REVIEW 7 of 16 The remote sensing image data used were Moderate Resolution Imaging Spectroradiometer (MODIS)-AquaWith the above satellite method, data this downloaded study used fromthe meas theured NASA surface website SSC at (https: 28 stations//www.nasa.gov to fit the). remoteThe Level-1B sensing (L1B) data dataof the for Rrs555 December, band January, and establish and February a remote of sensing each year retrieval from 2002model. to 2018The retrieval(the winter fitting of 16 function years) were is as obtained.follows: The images had a spatial resolution of 1 km. The remote sensing data in this study were processed and transformed mainly using the SeaWiFS Data Analysis System (SeaDAS)SSC software = Exp o ffi(101.8cially × Rrs555) provided × 1.301 by NASA for atmospheric correction, cloudThe removal measured and surface other preliminary SSC had a high processing. degree of The fit L1B with remote Rrs555; sensing the R2 datawas 0.9522 were batched (Figure into5a), indicatingLevel-2 remote that the sensing established reflectance retrieval (Rrs). model When had establishing good accuracy a remote and sensing met the retrieval requirements model, theof this Rrs study.that best matched the measured data in time and space was selected. A time control period within 3 h was eFigureffective 2 forshows our data.that the A fitting area with relationship high SSC between values the in remotewinter sensingis mainly data concentrated and the measured in the southerndata was established.part of the Bohai In addition, Strait; becausethis may there indica iste visible that seasuspended ice in the sediment Bohai Sea transport and the Yellowin the Bohai Sea in Straitwinter, occurs sea ice mainly has a greatin this influence area. The on difference SSC retrieval. between Therefore, the acquisition this study times used methodsof the satellite described image in andprevious the in studies situ data to removewas less the than influence 3 h, the of spatial sea ice difference [14,15]. was less than 500 m, and the remaining data Withafter the abovesea ice method, was removed this study were used considered the measured as valid surface data SSCvalues. at 28 To stations control to the fit thequality remote of thesensing data, data when of thecalculating Rrs555 band the andaverage establish SSC ain remote winter, sensing if there retrieval were model.fewer valid The retrieval points in fitting the southernfunction isBohai as follows: Strait and nearby areas, the SSC value in that year was considered to be an SSC = Exp (101.8 Rrs555) 1.301 unreliable value. Figure 6 shows that the number of× valid points× was low in 2008 and 2009, and the numberThe of measured valid points surface in the SSC southern had a highBohai degree Strait ofwas fit extremely with Rrs555; lowthe in 2009 R2 was (Figure 0.9522 6). (Figure Therefore,5a), theindicating data from that 2009 the establishedwere consid retrievalered unreliable. model had In goodthe following accuracy analysis, and met when the requirements analyzing the of overallthis study. trends, unreliable results were not included.

Figure 5. A fitting relationship between in situ SSC and Rrs555. The remote sensing retrieval formula Figure(a) and the5. A verification fitting relationship results (b). between in situ SSC and Rrs555. The remote sensing retrieval formula (a) and the verification results (b). Figure2 shows that the area with high SSC values in winter is mainly concentrated in the southern part of the Bohai Strait; this may indicate that suspended sediment transport in the Bohai Strait occurs mainly in this area. The difference between the acquisition times of the satellite image and the in situ data was less than 3 h, the spatial difference was less than 500 m, and the remaining data after the sea ice

Remote Sens. 2020, 12, 4066 7 of 14 was removed were considered as valid data values. To control the quality of the data, when calculating the average SSC in winter, if there were fewer valid points in the southern Bohai Strait and nearby areas, the SSC value in that year was considered to be an unreliable value. Figure6 shows that the number of valid points was low in 2008 and 2009, and the number of valid points in the southern Bohai Strait was extremely low in 2009 (Figure6). Therefore, the data from 2009 were considered unreliable. Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 16 In the following analysis, when analyzing the overall trends, unreliable results were not included.

Figure 6. Effective data volume of satellite images at different locations in the Bohai Strait. Figure 6. Effective data volume of satellite images at different locations in the Bohai Strait. 3.4. Other Data 3.4. OtherIn this Data paper, the runoff and sediment transport data for the Yellow River were derived from the statisticsIn this paper, from the Lijin runoff Station and in sediment the “Chinese transport River data Sediment for the Bulletin”.Yellow River In addition,were derived this studyfrom theused statistics cross-calibrated from Lijin multi-platform Station in the (CCMP)“Chinese wind River field Sediment data to Bulletin”. analyze the In eaddition,ffect of wind this onstudy the usedtransport cross-calibrated of suspended multi-platform sediment. The (CCMP) CCMP wind wind fiel fieldd data data to are analyze vector datathe effect derived of wind from remoteon the transportsensing systems of suspended (RSS); theirsediment. temporal The CCMP resolution wind is fie 6 h,ld anddata their are vector spatial data resolution derived is from 0.25◦ remote0.25◦ × sensing(http://www.remss.com systems (RSS); their/measurements temporal /resolutionccmp). is 6 h, and their spatial resolution is 0.25° × 0.25° (http://www.remss.com/measurements/ccmp). 3.5. Suspended Sediment Flux Calculation Method 3.5. SuspendedUsing the Sediment current Flux data Calculation simulated Method by the Finite-Volume Community Ocean Model (FVCOM) and theUsing predicted the current data fordata the simulated suspended by sediments the Finite-Volume in various layersCommunity of the BohaiOcean Strait Model in winter(FVCOM) over andmany the years, predicted the output data for and the input suspended fluxes of sediments the suspended in various sediments layers were of the calculated Bohai Strait quantitatively. in winter overTo study many the years, trend ofthe the output suspended and sedimentinput fluxe transports of the in thesuspended Bohai Strait sediments in winter were over manycalculated years, quantitatively.we first used the To equation study the [22 trend,23] (1):of the suspended sediment transport in the Bohai Strait in winter Z H over many years, we first used the equation [22,23] (1): FS = CUdz (1) 0 The single-width fluxes at various points across the Bohai Strait were calculated, where F 𝐹 𝐶𝑈𝑑𝑧 (1) S represents the single-width flux at a given point, H represents the water depth (unit: m), C represents predicted SSC (unit: mg/L) fitted at each layer, and U represents the seasonal average velocity (unit: m/s). Then,The Equation single-width (2) was fluxes used toat calculatevarious thepoints suspended across the sediment Bohai fluxStrait across were the calculated, entire Bohai where Strait: Fs represents the single-width flux at a given point, H represents the water depth (unit: m), C Z represents predicted SSC (unit: mg/L) fitted at eachS layer, and U represents the seasonal average Fr = FSds (2) velocity (unit: m/s). Then, Equation (2) was used 0to calculate the suspended sediment flux across the entire Bohai Strait: where S represents the distance between the first and last points in the section. Considering that the satellite image and the measured data were fitted in the form of a seasonal average, and considering the quality of the𝐹 satellite 𝐹 image,𝑑𝑠 the seasonal average result was used for(2) the where S represents the distance between the first and last points in the section. Considering that the satellite image and the measured data were fitted in the form of a seasonal average, and considering the quality of the satellite image, the seasonal average result was used for the flux calculation. The established FVCOM applied hourly wind data from 2012 to 2018. The model also considered the effects of strong winter winds and different wind speed and direction conditions. The model has been verified in detail to prove its reliability [24].

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flux calculation. The established FVCOM applied hourly wind data from 2012 to 2018. The model also considered the effects of strong winter winds and different wind speed and direction conditions. The model has been verified in detail to prove its reliability [24]. Remote Sens. 2020, 12, x FOR PEER REVIEW 9 of 16 4. Results 4. Results 4.1. Multi-Year Variations in the Surface SSC in the Bohai Strait 4.1. Multi-Year Variations in the Surface SSC in the Bohai Strait The surface SSC of the Bohai Strait exhibits obvious temporal and special variations. The interannualThe surface variation SSC of inthe the Bohai surface Strait SSC exhibits in the northern obvious Bohaitemporal Strait and is small,special with variations. the diff erenceThe interannual variation in the surface SSC in the northern Bohai Strait is small, with the difference between the highest and the lowest value is only approximately 5 mg/L (Figure7). The surface SSC between the highest and the lowest value is only approximately 5 mg/L (Figure 7). The surface SSC in the southern Strait varies greatly, and the difference between the highest and lowest values is in the southern Strait varies greatly, and the difference between the highest and lowest values is approximately 28 mg/L. The southern part of the Bohai Strait is bounded by the northern islands of the approximately 28 mg/L. The southern part of the Bohai Strait is bounded by the northern islands of Shandong Peninsula. The magnitude of change in the western strait is greater than that in the eastern. the Shandong Peninsula. The magnitude of change in the western strait is greater than that in the Theeastern. difference The betweendifference the between highest the and highest lowest and value lowe overst value these over years these in the years eastern in the part eastern of the part strait of is lessthe than strait 10 is mg less/L. than This 10 maybe mg/L. causedThis maybe by the caused barrier bye theffect barrier of the effect island of [the17]. island [17].

Figure 7. Figure 7.Multi-year Multi-year variationsvariations in the su surfacerface SSC SSC in in the the Bohai Bohai Strait. Strait. 4.2. Multi-Year Variations in SSC Flux in the Bohai Strait in Winter

Due to the north-side inflow and the south-side outflow current, the suspended sediment will be transported in different directions between the southern and northern Bohai Strait (Figure8) which directed from the Bohai Sea to the Yellow Sea in the south, and from the Yellow Sea to the

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Bohai Sea in the north. However, as along with the water depth deepen, the location of the boundary between waters with opposite flux directions changes significantly. The surface boundary is relatively stable throughout the year, mainly located near 38.56◦N. There is a clear interannual change in the mid-level shear front, and the boundary generally exists between 38.47◦N and 38.53◦N. The position of the bottom boundary is also relatively stable near 38.4◦N. The location of the boundary gradually moves southwardly with increasing depth. This is mainly because the surface currents of the Bohai Strait is mainly controlled by the east component of the winter wind, which clearly exhibited a stronger output flux corresponds to the heavy northwesterly wind (Figure8a). The bottom of the Bohai Strait is invaded mainly by a warm current from the Yellow Sea. The intensity of the Yellow Sea Warm Current gradually weakens from the bottom to the top, so the location of the boundary gradually moves south from the surface to the bottom [1,24,25]. Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 16

Figure 8. Single-widthFigure 8. Single-width fluxes fluxes in the in Bohaithe Bohai Strait Strait at at thethe surface surface layer layer (a), middle (a), middle layer (b layer), and (bottomb), and bottom layer (c); Positivelayer (c); values Positive indicate values indicate output ou fluxtput fromflux from the the Bohai Bohai Sea Sea to to the the Yellow Yellow Sea, and and negative negative values values indicate input flux from the Yellow Sea to the Bohai Sea; The red line represents the indicate input flux from the Yellow Sea to the Bohai Sea; The red line represents the boundary between boundary between the input flux and the output flux. the input flux and the output flux.

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To better study the vertical differences in the multi-year variations of the suspended sediment flux in the Bohai Strait in winter, fluxes at two stations located in the southern and northern Bohai Strait respectively were compared (Figure9). Most of the output flux occurred at the surface layer in the southern Bohai Strait, which gradually decrease downward. This is mainly because the southern part of the Bohai Strait is affected by strong northwesterly winds in winter, and the SSC mixes well vertically, while the surface current velocity is significantly higher than the bottom current. The multi-year variations of suspended sediment flux in the northern Bohai Strait were more complicated than those in the southern part. The middle and lower layers in the northern part of the strait were mainly represented by the westward input flux. This process is close related to the Yellow Sea Warm Current, which is a upwind current carrying a large amount of suspended sediment through the Yellow Sea into the Bohai Strait, and finally, into the Bohai Sea. The higher output flux in the upper layers in 2004, 2005, 2007 to 2009, is roughly corresponding to the larger north component of Remotewinter Sens. wind. 2020, 12, x FOR PEER REVIEW 12 of 16

FigureFigure 9. TheThe multi-yearmulti-year variations variations in single-widthin single-width sediment sediment fluxes atfluxes different at different locations inlocations the Bohai in Strait. the BohaiLocation Strait. details Location are provided details in are Figure provided1a. The in flux Figure at the 1a. station The flux in the at southernthe station Bohai in the Strait southern (a) and Bohaiin the Straitnorthern (a) and Strait in ( bthe). Positive northern values Strait indicate (b). Positive output values flux from indicate the Bohai output Sea flux to the from Yellow the Sea,Bohai and Sea negative to the Yellowvalues indicateSea, and input negative flux fromvalues the indicate Yellow Seainput to flux the Bohai from Sea.the Yellow Sea to the Bohai Sea. 5. Discussion 5. Discussion 5.1. Multi-Year Variations in the Output Flux of Suspended Sediment in the Bohai Strait in Winter 5.1. Multi-Year Variations in the Output Flux of Suspended Sediment in the Bohai Strait in Winter The output and input flux represents the sediment transported from the Bohai Sea to the Yellow Sea and fromThe output the Yellow and Sea input to the flux Bohai represents Sea, respectively. the sediment The sumtransported vector of from the output the Bohai and inputSea to flux the is Yellowthe net Sea flux. and To furtherfrom the analyze Yellow the Sea relationship to the Bohai between Sea, respectively. the fluxes ofThe the sum Bohai vector Strait of and the variousoutput andinfluencing input flux factors, is the this net section flux. To calculates further theanalyze sediment the relationship volume of the between Yellow River,the fluxes the averageof the Bohai wind Straitspeed and in winter various each influencing year, the east factors, component this section of wind calculates (east is representedthe sediment by volume positive of values, the Yellow west is River,represented the average by negative wind speed values), in andwinter the each south year component, the east (southcomponent is represented of wind (east by positive is represented values, bynorth positive is represented values, west by negativeis represented values); by the negative trend linesvalues), of each and influencingthe south component factor are calculated(south is representedseparately (Figure by positive 10). values, north is represented by negative values); the trend lines of each influencing factor are calculated separately (Figure 10). The output flux of suspended sediment in the Bohai Strait in winter ranged between 3.86 and 6.41 million tons during the study period and exhibited a slow upward trend overall, with an increase of approximately 46,100 tons/year (Figure 10c). The output flux has a strong correlation with the east component of winter wind. When the east component is larger, the output flux in the Bohai Strait is also larger resulted from the increased currents. In 2003 and 2014, when the east component was relatively high, the output flux in the Bohai Strait was the largest (above 6.321 million tons in both years). The lowest values of the east component, in 2012 and 2013, corresponded to the lowest output flux values from 2002 to 2017 (3.884 million tons and 3.856 million tons, respectively). This trend is the opposite of the decreasing trend of the average wind speed in the Bohai Sea and the Yellow River in winter. Therefore, the output flux of suspended sediment in the Bohai Strait is mainly controlled by the east component of wind in winter. The intense east component of wind caused the water piling up to southeast while the sea surface height in Bohai Sea fell significantly, effectively forming a northward pressure gradient [26]. A large quantity of the water escape from Bohai Sea to Yellow Sea through Bohai Strait. Meanwhile, the relatively high sea surface height is distributed along the northern coast of the Shandong Peninsula, and the Northern Shandong Coastal Current is markedly enhanced because of geostrophic balance. Furthermore, intensified waves facilitate the rapid increase of SSC in the shallow water of southern Bohai Sea, which is then carried out of Bohai Sea by the Northern Shandong Coastal Current. This period is the main transport stage of the sediments from Bohai Sea [16,17].

Remote Sens. 2020, 12, 4066 11 of 14 Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 16

Figure 10. Suspended sediment fluxes and influencing factors in the Bohai Strait in winter ((a) net flux, Figure 10. Suspended sediment fluxes and influencing factors in the Bohai Strait in winter ((a) net (b) input flux, (c) output flux, (d) wind speed, (e) south component of winter wind, (f) east component flux, (b) input flux, (c) output flux, (d) wind speed, (e) south component of winter wind, (f) east of winter wind, (g) sediment runoff). component of winter wind, (g) sediment runoff). The output flux of suspended sediment in the Bohai Strait in winter ranged between 3.86 and 6.415.2. Multi-Year million tons Variations during the in studythe Input period Flux andof Suspended exhibited Sediment a slow upward in the Bohai trend Strait overall, in Winter with an increase of approximatelyFrom 2002 to 46,100 2017, tonsthe /inputyear (Figureflux of 10suspendedc). The output sediment flux hasin the a strong Bohai correlation Strait in winter with theranged east componentbetween 2.233 of winterand 4.047 wind. million When tons. the eastThe componentmagnitude isof larger,the change the output in the flux input in theflux Bohai was Straitmuch issmaller also larger than resultedthat in the from output the increased flux. The currents.overall input In 2003 flux and also 2014, showed when an the upward east component trend, and was the relativelyincrease rate high, was the higher output than flux that in the of Bohaithe output Strait flux, was thereaching largest 51,100 (above tons/year 6.321 million (Figure tons 10b). in both The years).change Thein the lowest input values flux depended of the east mainly component, on the in Yellow 2012 and Sea 2013, Warm corresponded Current flowing to the westward lowest output into fluxthe Bohai values Sea, from and 2002 its to main 2017 (3.884channel million varied tons within and 3.856the strait. million The tons, Yellow respectively). Sea Warm This Current trend isis thean oppositeupwind positive-pressure of the decreasing trendflow originating of the average from wind the southern speed in theYellow Bohai Sea Sea [27], and which the Yellow is also River related in winter.to the strength Therefore, of east the outputcomponent flux ofwinter suspended wind (Fig sedimenture 10f). in theIts velocity Bohai Strait is small, is mainly and the controlled SSC in the by theYellow east componentSea is lower of windthan inthat winter. in the The Bohai intense Sea, east thus component the input of windflux of caused suspended the water sediment piling up is torelatively southeast lower while than the seaoutput surface in winter. height inThus Bohai both Sea of fell the significantly, input and output effectively flux forming are induced a northward by the east component of winter wind.

Remote Sens. 2020, 12, 4066 12 of 14 pressure gradient [26]. A large quantity of the water escape from Bohai Sea to Yellow Sea through Bohai Strait. Meanwhile, the relatively high sea surface height is distributed along the northern coast of the Shandong Peninsula, and the Northern Shandong Coastal Current is markedly enhanced because of geostrophic balance. Furthermore, intensiﬁed waves facilitate the rapid increase of SSC in the shallow water of southern Bohai Sea, which is then carried out of Bohai Sea by the Northern Shandong Coastal Current. This period is the main transport stage of the sediments from Bohai Sea [16,17].

5.2. Multi-Year Variations in the Input Flux of Suspended Sediment in the Bohai Strait in Winter From 2002 to 2017, the input flux of suspended sediment in the Bohai Strait in winter ranged between 2.233 and 4.047 million tons. The magnitude of the change in the input flux was much smaller than that in the output flux. The overall input flux also showed an upward trend, and the increase rate was higher than that of the output flux, reaching 51,100 tons/year (Figure 10b). The change in the input flux depended mainly on the Yellow Sea Warm Current flowing westward into the Bohai Sea, and its main channel varied within the strait. The Yellow Sea Warm Current is an upwind positive-pressure flow originating from the southern Yellow Sea [27], which is also related to the strength of east component winter wind (Figure 10f). Its velocity is small, and the SSC in the Yellow Sea is lower than that in the Bohai Sea, thus the input flux of suspended sediment is relatively lower than output in winter. Thus both of the input and output flux are induced by the east component of winter wind.

5.3. Multi-Year Variations in the Net Flux of Suspended Sediment in the Bohai Strait in Winter Overall, the net flux in the Bohai Strait ranged from 1.22 to 2.70 million tons. The net flux changed steadily over the years, showing a very slow downward trend, with a decline of only 4960 tons/year (Figure 10a). The net flux is controlled by the difference between the input and output. In the years when the east component was larger, the net flux in the Bohai Strait was also larger. In the years when the east component was above 1.6 m/s, the net flux in the Bohai Strait exceeded 2.5 million tons. The net flux in 2013, which corresponded to the smallest east component of winter wind, was the lowest, approximately 1.22 million tons. The net flux in the Bohai Strait has little to do with the sediment transport volume of the Yellow River, which indicates that the sediment in the Bohai Strait is mainly derived from the resuspension of sediments in the Bohai Sea.

6. Conclusions Considering the vertical well mixed water in the Bohai Strait in winter, remote sensing data from MODIS-Aqua in combination with in situ measured water turbidity and SSC were used to establish a remote sensing model in order to translate the surface remote sensing retrieval results into SSC in each water layers in winter. Then, by incorporating current data simulated by FVCOM, the suspended sediment flux in the Bohai Strait in winter of 2002 to 2018 was obtained. The output suspended sediment flux occurs in the southern part of the Bohai Strait in winter from the Bohai Sea to the northern Yellow Sea, and the sediment flux gradually decreases from the surface downward. This is mainly because of the stronger wind-driven current at the surface layer than that at the bottom, while the input sediment flux directing from the northern Yellow Sea to the Bohai Sea occurs in the middle and lower layers of the northern part of the strait. This process is mainly affected by the Yellow Sea Warm Current which carries suspended sediment flowing into the Bohai Strait. In terms of long-term changes, the net sediment flux to the Yellow Sea ranged between 1.22 and 2.70 million tons in winter, showing a slight downward trend. That is because of a relatively smaller increase rate of output flux by 46,100 tons/year comparing to the increase rate of input flux by 51,100 tons/yr. The multi-year winter variations in the suspended sediment flux in the strait are mainly controlled by the east component of the winter wind. This is necessary for the accurate study of sediment transport in different types of straits in the world. Remote Sens. 2020, 12, 4066 13 of 14

Author Contributions: Conceptualization, L.Q.; methodology, L.Q., X.L. and Y.Z.; formal analysis and validation, L.Q. and X.L.; writing—original draft preparation, X.L. and Y.Z.; writing—review & editing, L.Q. and W.X.; data curation, X.L. and P.L.; visualization, X.L. and P.L.; supervision, L.Q. and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This study was supported by the National Key Research and Development Program of China (grant number 2017YFE0133500), National Natural Science Foundation of China (NSFC grant number 42076179), the Shandong Key Research and Development Program (grant number 2019GHY112057), the National Natural Science Foundation of China (NSFC grant numbers 41476030, 41806072), the Ministry of Science and Technology of China (grant number 2016YFA0600903), and the Project of Taishan Scholar. Acknowledgments: The field cruises were supported by the NSFC ship time sharing project (Transparent Marine). Part of the mooring data are archived in Ocean and Atmosphere Data Center of Ocean University of China. Thanks for all the efforts of Dongfanghong 2 and Chuangxin 1. Conflicts of Interest: The authors declare no conflict of interest.

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