Recent Spatio-Temporal Variations of Suspended Sediment Concentrations in the Yangtze Estuary

: Water and sediment are two of the most essential elements in estuaries. Their product, suspended sediment concentration (SSC), is involved in hydrology, geomorphology and ecology. This study was focused on the spatial and temporal variations of SSC in the Yangtze Estuary under new situations after the closure of ~50,000 dams in the Yangtze basin, including the Three Gorges Dam (TGD) in 2003. It was found that the SSC ﬁrst exhibited an increasing and then a decreasing trend longitudinally from Xuliujing Station to the outer estuary with the Turbidity Maximum Zone located in the mouth bar area. Vertically, the SSC in the bottom layers averaged 0.96 kg / m 3 , about 2.4 times larger than the surface layers (0.40 kg / m 3 ). During spring tides, the SSCs were always higher than those in neap tides, which was ﬁt for the cognition law. As for the seasonal variations in the North Branch and mouth bar area, the SSCs in the dry season were higher than those in the ﬂood season, while in the upper reach of the South Branch and outer estuary, the seasonal variation of SSCs reversed. This phenomenon primarily reﬂected the competition of riverine sediment ﬂux and local resuspended sediment ﬂux by wind-induced waves. As for the interannual changes, the SSCs demonstrated overall ﬂuctuant downward trends, determined by riverine sediment ﬂux and inﬂuenced by waves. This study revealed the new situation of SSC and can be a reference for other related researches in the Yangtze Estuary.


Introduction
Rivers are the lifeblood of human civilization and still play critical roles in human daily life. It attracts many studies in all kinds of aspects, such as hydrology, geomorphology, environments, ecology etc. [1][2][3][4][5][6]. Of all the riverine discharges, sediment and water are two of the most basic elements. They are also two necessary observation parameters in modern estuarine and coastal research. Their product, suspended sediment concentration (SSC), is a key factor influencing the nutrient concentration, the illumination intensity and the growth of phytoplankton in the water [5,[7][8][9][10]. In sedimentology, SSC determines the erosion/deposition processes at estuarine tidal flats and subaqueous deltas. In the tidal-cycle scale, net erosion/accretion relies on the competition between erosion flux and deposition flux, and deposition flux is positively correlated with SSC [11][12][13].
However, there are huge challenges for SSC under new circumstances. As reported by so many studies, sediment fluxes in many rivers, such as the Mississippi, Indus, Yellow and Volta Rivers, have Station ( Figure 1a) are 0.2 and 1.0 m, respectively [38]. Most of the North Branch is usually shallower than −5 m, while some local region of the North Branch can be deeper than −30 m. In the South Branch, the underwater shoals mainly developed along the north bank, while the deep waterway goes along the south bank primarily. The water depth becomes smaller outward in the mouth bar area (Figure 1b). Muddy deposits are found covering the channel bed in this region, mainly because of the settling process of fine suspended sediments during the slack water period. In the mouth bar area, the depth-averaged SSC at the mouth bar area can reach~0.3 kg/m 3 in a tidal cycle [39].
Water 2020, 12, x FOR PEER REVIEW 3 of 14 ( Figure 1a) are 0.2 and 1.0 m, respectively [38]. Most of the North Branch is usually shallower than −5 m, while some local region of the North Branch can be deeper than −30 m. In the South Branch, the underwater shoals mainly developed along the north bank, while the deep waterway goes along the south bank primarily. The water depth becomes smaller outward in the mouth bar area (Figure 1b). Muddy deposits are found covering the channel bed in this region, mainly because of the settling process of fine suspended sediments during the slack water period. In the mouth bar area, the depthaveraged SSC at the mouth bar area can reach ~0.3 kg/m 3 in a tidal cycle [39].

Materials and Methods
To study the spatial and temporal variations of suspended sediments, a total of 48 sampling sites were carefully selected and well distributed in the Yangtze Estuary (Figure 1). At each site, water samples were collected using 1000 ml water bottles at six layers in different depth (i.e., the surface layer, 0.2 H, 0.4 H, 0.6 H, 0.8 H, and the bottom layer, H: the height of the water column). During each field observation, sampling work was conducted hourly for at least one full semidiurnal tidal cycle. Suspended sediments were filtered through 0.45 mm filters from the water samples, dried at 45 °C for 48 h and weighed in a laboratory to measure the SSC. This fieldwork covers the years from 2009 to 2018 in both dry and flood seasons.

Materials and Methods
To study the spatial and temporal variations of suspended sediments, a total of 48 sampling sites were carefully selected and well distributed in the Yangtze Estuary (Figure 1). At each site, water samples were collected using 1000 mL water bottles at six layers in different depth (i.e., the surface layer, 0.2 H, 0.4 H, 0.6 H, 0.8 H, and the bottom layer, H: the height of the water column). During each field observation, sampling work was conducted hourly for at least one full semidiurnal tidal cycle. Suspended sediments were filtered through 0.45 mm filters from the water samples, dried at 45 • C for Due to sampling capacity limitations, not all samples at every station during both spring and neap tides in both dry and flood seasons were collected every year. To eliminate the influence of wave-energy seasonal variation and get more data, the spatial analysis of SSC mainly adopted the data measured during spring tides in dry seasons of 2002-2004 and 2011. The seasonal variations of SSC in this study were mainly based on the data acquired in 2011. The interannual variations of SSC were based on the data collected in the flood season from 2001 to 2018. To reduce the accidental error from an individual sample, SSC in this study was primarily referred to as the depth-averaged data of 84 samples during a whole tidal cycle, except the analysis of vertical variations of SSC. The Yangtze's water discharge and sediment flux at Datong Station (tidal limit, Figure 1a) were acquired from the Yangtze Water Resources Commission (YWRC) [40]. Yangtze Sediment Bulletin is available at http://www.cjh.com.cn.

Longitudinal and Transverse Variations
Longitudinally, the depth-averaged SSCs for the whole tidal cycle in both the North and South Branches first increased and then decreased from the inner estuary to outer estuary ( Figure 2). The maximum of SSC in the Yangtze Estuary was found as 2.15 kg/m 3 in the middle reach of the North Branch, and the minimum of SSC was found as only 0.10 kg/m 3 in the outer estuary. Transversely, SSC averaged 1.45 kg/m 3 in the North Branch, which was 2.6 times larger than the South Branch (0.55 kg/m 3 , Table 1). Compared to the Yangtze Estuary, SSCs in the Hangzhou Bay were overall larger and averaged 2.15 kg/m 3 ( Figure 2, Table 1). Due to sampling capacity limitations, not all samples at every station during both spring and neap tides in both dry and flood seasons were collected every year. To eliminate the influence of wave-energy seasonal variation and get more data, the spatial analysis of SSC mainly adopted the data measured during spring tides in dry seasons of 2002-2004 and 2011. The seasonal variations of SSC in this study were mainly based on the data acquired in 2011. The interannual variations of SSC were based on the data collected in the flood season from 2001 to 2018. To reduce the accidental error from an individual sample, SSC in this study was primarily referred to as the depth-averaged data of 84 samples during a whole tidal cycle, except the analysis of vertical variations of SSC. The Yangtze's water discharge and sediment flux at Datong Station (tidal limit, Figure 1a) were acquired from the Yangtze Water Resources Commission (YWRC) [40]. Yangtze Sediment Bulletin is available at http://www.cjh.com.cn.

Longitudinal and Transverse Variations
Longitudinally, the depth-averaged SSCs for the whole tidal cycle in both the North and South Branches first increased and then decreased from the inner estuary to outer estuary ( Figure 2). The maximum of SSC in the Yangtze Estuary was found as 2.15 kg/m 3 in the middle reach of the North Branch, and the minimum of SSC was found as only 0.10 kg/m 3 in the outer estuary. Transversely, SSC averaged 1.45 kg/m 3 in the North Branch, which was 2.6 times larger than the South Branch (0.55 kg/m 3 , Table 1). Compared to the Yangtze Estuary, SSCs in the Hangzhou Bay were overall larger and averaged 2.15 kg/m 3 ( Figure 2, Table 1).   As for the South Branch, there were also great spatial differences. SSC was 0.74 kg/m 3 in the South Passage and was only 0.15 kg/m 3 in the upper reach of the South Branch. In the other regions, SSC ranged from 0.6 to 0.7 kg/m 3 ( Table 1).
The spatial pattern of SSC in the Yangtze Estuary confirmed previous studies [30,33]. It was highly related to the development of the Turbidity Maximum Zone, resulting from unique geomorphology, hydrology and mixing of salt and fresh water [30,36,41].

Vertical Variation
Based on the calculations, SSCs varied from 0.05 to 1.97 kg/m 3 in the surficial layers and varied from 0.11 to 2.84 kg/m 3 in the bottom layers. SSCs in the bottom layers averaged 0.96 kg/m 3 , about 2.4 times larger than the surface layers (0.40 kg/m 3 ). The SSC of the bottom layer can even be 9.1 times larger than the surface layer in the North Channel ( Figure 3). All these samples were subject to the same patterns as the surficial SSCs were much lower than the bottom layers. This phenomenon is generally known, mainly due to a decreased upward diffusion of suspended sediments [34]. Meanwhile, compared to the inner estuary, the differences of SSCs between the surface and bottom layers were much larger in the North Branch and outer estuary (Figure 3), where either water was shallow or wave energy was strong.

Differences of Suspended Sediment Concentrations between Spring and Neap Tides
In the Yangtze Estuary and Hangzhou Bay, the depth-averaged SSC was 0.84 kg/m 3 during spring tides, more than twice the SSC during neap tides (0.41 kg/m 3 ). Generally, most sites revealed this consistent phenomenon (Figure 4), reflecting the effects of hydrodynamic forces.
The differences of SSCs between spring and neap tides in the North Branch were much larger than the other regions (Figure 4), considering that the North Branch was controlled by tidal forces and the Yangtze's water discharge into the coastal zone via the North Branch was less than 2%. This difference of SSCs reached its maximum as 1.51 kg/m 3 at the bend in the North Branch, mainly due to the combined effects of powerful tidal force during spring tides and large topographic changes.  In the South Branch, SSC varied from 0.08 to 1.17 kg/m 3 during spring tides and from 0.04 to 0.61 kg/m 3 during neap tides. The differences of SSCs between spring and neap tides in the mouth bar area were much larger than those in the upper reach of South Branch (Figure 4). The variation range of SSC was 0.08-0.26 kg/m 3 during spring tides and 0.04-0.10 kg/m 3 during neap tides in the upper reach of the South Branch. In the mouth bar area, depth-averaged SSC during spring tides was 0.68 kg/m 3 , about 2.5 times larger than that during neap tides (0.27 kg/m 3 ). The differences of SSCs between spring and neap tides were small in the region controlled by riverine forces and were large in the region controlled by oceanic forces.
In the outer estuary and Hangzhou Bay region, the difference of SSCs between spring and neap tides were relatively small because of weak re-suspension under deep waters. Hangzhou Bay is a macro-tidal estuary, where SSC kept at a high level during both spring and neap tides [42,43].

Differences of Suspended Sediment Concentrations between Flood and Dry Seasons
Based on measured data, the mean SSC in the Yangtze Estuary was 0.74 kg/m 3 during dry seasons, 17% higher than that in flood seasons (0.63 kg/m 3 ) ( Figure 5, Table 2). About half of the In the South Branch, SSC varied from 0.08 to 1.17 kg/m 3 during spring tides and from 0.04 to 0.61 kg/m 3 during neap tides. The differences of SSCs between spring and neap tides in the mouth bar area were much larger than those in the upper reach of South Branch (Figure 4). The variation range of SSC was 0.08-0.26 kg/m 3 during spring tides and 0.04-0.10 kg/m 3 during neap tides in the upper reach of the South Branch. In the mouth bar area, depth-averaged SSC during spring tides was 0.68 kg/m 3 , about 2.5 times larger than that during neap tides (0.27 kg/m 3 ). The differences of SSCs between spring and neap tides were small in the region controlled by riverine forces and were large in the region controlled by oceanic forces.
In the outer estuary and Hangzhou Bay region, the difference of SSCs between spring and neap tides were relatively small because of weak re-suspension under deep waters. Hangzhou Bay is a macro-tidal estuary, where SSC kept at a high level during both spring and neap tides [42,43].

Differences of Suspended Sediment Concentrations between Flood and Dry Seasons
Based on measured data, the mean SSC in the Yangtze Estuary was 0.74 kg/m 3 during dry seasons, 17% higher than that in flood seasons (0.63 kg/m 3 ) ( Figure 5, Table 2). About half of the measured data exhibited the opposite phenomenon. There were spatial differences in the seasonal variations of SSC ( Figure 5).
Based on the five samples in the North Branch, SSCs averaged 1.58 kg/m 3 in the dry season, all higher than those in the flood season (1.07 kg/m 3 ). The same seasonal variation also occurred in most of the mouth bar area ( Figure 5, Table 2). This phenomenon mainly reflected ocean-dominated sediment dynamics [39]. The Yangtze's input of suspended sediment flux contributed less to the SSC in these regions than the contribution from re-suspended local sediment flux by ocean-controlled waves and currents. The mouth bar area was open to the sea, where the wind was strong and the water was shallow. Wind-induced waves during dry seasons were usually much stronger than those during flood seasons, while riverine water and sediment discharge during dry seasons were much smaller than those during flood seasons [34,35,44]. water was shallow. Wind-induced waves during dry seasons were usually much stronger than those during flood seasons, while riverine water and sediment discharge during dry seasons were much smaller than those during flood seasons [34,35,44]. According to the data measured during this observation in 2011, the wind speed varied from 2.9 to 4.5 m/s and averaged 4.1 m/s in the flood season, while it varied from 4.6 to 6.0 m/s and averaged 5.3 m/s in the dry season. The river water discharges during the observations in the flood and dry seasons were ~28,200 and 12,800 m 3 /s, respectively. The water discharge during the flood season was 1.5 times larger than that in the dry season, while the wind speed during the dry season was ~30% larger than that in the flood season. This proved the above mechanism.  seasons were~28,200 and 12,800 m 3 /s, respectively. The water discharge during the flood season was 1.5 times larger than that in the dry season, while the wind speed during the dry season was~30% larger than that in the flood season. This proved the above mechanism. This phenomenon reversed in the upper reach of the South Branch and the outer estuary ( Figure 5). Only at one site of the upper reach of the South Branch, the SSC in the dry season is around the same level as that in the flood season. At the remaining sites, the SSCs in the flood season were always higher than those in the dry season. The SSCs in this region averaged 0.21 kg/m 3 during the flood season, 22% higher than that in the dry season ( Figure 5, Table 2). The ocean played a minor role at the upper reach of the South Branch, where water was deep and wind-induced waves from the outer sea were inaccessible. Thus, SSCs in this region were mainly determined by the Yangtze's riverine suspended sediment flux. The riverine SSC in the flood season was still much higher than that in the dry season, though it decreased by >80% after the closure of all these dams [45]. In the outer estuary, where water was deep, the wave's energy was too weak to bring sediments into water columns.  [23,45]. In the first years after the construction of the TGD, the filling of the reservoir had a significant influence on the discharge regime in the lower Yangtze River. The TGD let the finest sediment (fine silt and clay) pass but retained the morphologically important coarse silt fraction. This might explain the declines in the sediment flux and SSC in the years 2003, 2004 and 2006 ( Figure 6). As the input from riverbed erosion downstream of the TGD, the Yangtze's sediment flux recovered but remained in a downward tendency [25,45].
SSC at the Xuliujing Station exhibited an overall decreasing trend (Figure 7). The SSCs in the North and South Channels and the North and South Passages all showed decreasing trends as a whole, including some distinct annual fluctuations before 2016, after which the SSCs rose again (Figure 7). The high values of SSC in the mouth bar area after 2016 might be attributed to the recent increase of August wind velocities ( Figure 6b). As mentioned above, the SSC in the reach of the inner estuary was mainly controlled by riverine inputs, while the SSC in the mouth bar area not only reflected the riverine inputs but was also influenced by local wind energies.
The Yangtze's water discharge in August has exhibited a periodic oscillation at a relatively stable mean level of around 42,100 m 3 /s since 2002 (Figure 6a). The Yangtze's sediment flux in August experienced a similar periodic oscillation with the water discharge (linear-regression analysis: R = 0.7) but displayed an overall downward trend (Figure 6a). Compared to 70 Mt in August 2002, it decreased to only 18 Mt in August 2003, mainly owing to the impoundment of the TGD in June 2003 [23,45]. In the first years after the construction of the TGD, the filling of the reservoir had a significant influence on the discharge regime in the lower Yangtze River. The TGD let the finest sediment (fine silt and clay) pass but retained the morphologically important coarse silt fraction. This might explain the declines in the sediment flux and SSC in the years 2003, 2004 and 2006 ( Figure 6). As the input from riverbed erosion downstream of the TGD, the Yangtze's sediment flux recovered but remained in a downward tendency [25,45].  Sediment coarsening of the bed in the mouth bar area was reported by previous studies during the process of the delta's transition from accumulation to erosion [25,46]. This is primarily ascribed to the re-suspension of the finer portion in bed sediments. The supplementation to local SSC from re-suspended bed sediments especially in the mouth bar area also contributed to this fluctuation of SSC.
Considering there will be more dams and water-soil conservation projects in the Yangtze basin [23,45], while the large lakes connected to the Yangtze and its riverbed may supplement much more sediment, the future of the sediment transport to the estuary will probably be complex.
Thus, the evolution process of SSC will be complicated, accompanied by fluctuations resulted from wind-induced waves, especially under strong winds' impacts during dry seasons. Apart from these factors, human interventions in the Yangtze Estuary including reclamations, reservoirs, water engineering and dredging works also have significant effects on local SSC, which requires further analyses in future studies.
pre-2003 level (0.22 kg/m 3 ), the SSC in 2017 (0.08 kg/m 3 ) decreased by 64%. The Yangtze's sediment flux declined sharply in 2006 (Figure 6a) and the SSC was only 0.07 kg/m 3 at an extremely low level (Figure 6b). After 2006, the SSC recovered with the increase of annual riverine sediment flux. The SSCs at the Xuliujing Station have trended down in general since 2010, closely related to the Yangtze's sediment flux. Due to lack of data in most of the years, the SSC at the Bzk Station (Figure 7) in the North Branch also decreased from 1.42 kg/m 3 in the pre-2003 period to 0.79 kg/m 3 in 2008, and then to 0.35 kg/m 3 in 2010 (Figure 7).
The SSCs in the North and South Channels and the North and South Passages all showed decreasing trends as a whole, including some distinct annual fluctuations before 2016, after which the SSCs rose again (Figure 7). The high values of SSC in the mouth bar area after 2016 might be attributed to the recent increase of August wind velocities ( Figure 6b). As mentioned above, the SSC in the reach of the inner estuary was mainly controlled by riverine inputs, while the SSC in the mouth bar area not only reflected the riverine inputs but was also influenced by local wind energies.  Sediment coarsening of the bed in the mouth bar area was reported by previous studies during the process of the delta's transition from accumulation to erosion [25,46]. This is primarily ascribed to the re-suspension of the finer portion in bed sediments. The supplementation to local SSC from resuspended bed sediments especially in the mouth bar area also contributed to this fluctuation of SSC.

Conclusions
Going from the Xuliujing Station to the outer estuary longitudinally, the SSC first tended to increase and then to decrease, mainly due to the existence of the turbidity maximum zone in the mouth bar area. Vertically in the water column, the differences between the surface and the bottom layers were quite large. Though the hydrodynamic force was strong in the Yangtze Estuary, the upward diffusion of SSC in the water column was still weak. In the Yangtze Estuary, the SSC during spring tides were always higher than that during neap tides. As for the seasonal variations, the SSC in the dry seasons were higher than that in the flood season in the North Branch and mouth bar area, mainly reflecting the dominated impactor of wind-induced waves. While in the inner estuary of the South Branch and outer estuary, the seasonal variation of SSC reversed. It mainly reflected the domination of the Yangtze River and weak influence from waves due to land's cut-off or deep-water environment. The SSCs at most of the stations showed overall decreasing trends on interannual scales, accompanied by some distinct annual fluctuations resulting primarily from increased wind-wave energies. Climatic change is also a vital factor affecting the estuarine SSC, which makes the SSC's evolution unpredictable and highlights the need for more studies.