Estimation of Bed Shear Stresses in the Pearl River Estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler （PC-ADP） and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary. The bed shear stresses were estimated by four different methods： log profile （LP）, eddy correlation （EC）, turbulent kinetic energy （TKE）, and inertial dissipation （ID）. The results show that （a） all four methods for estimating bed stresses have advantages and disadvantages, and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors; （b） the LP method was found to be the most suitable to estimate the bed stresses in non-stratified, quasi-steady, and homogeneous flows; and （c） in the estuary where the semi-diurnal tidal current is dominant, bed shear stresses exhibit a strong quarter-diurnal variation.

Statistical Modeling and Trend Detection of Extreme Sea Level Records in the Pearl River Estuary

Sea level rise has become an important issue in global climate change studies. This study investigates trends in sea level records, particularly extreme records, in the Pearl River Estuary, using measurements from two tide gauge stations in Macao and Hong Kong. Extremes in the original sea level records （daily higher high water heights） and in tidal residuals with and without the 18.6-year nodal modulation are investigated separately. Thresholds for defining extreme sea levels are calibrated based on extreme value theory. Extreme events are then modeled by peaks-over-threshold models. The model applied to extremes in original sea level records does not include modeling of their durations, while a geometric distribution is added to model the duration of extremes in tidal residuals. Realistic modeling results are recommended in all stationary models. Parametric trends of extreme sea level records are then introduced to nonstationary models through a generalized linear model framework. The result shows that, in recent decades, since the 1960s, no significant trends can be found in any type of extreme at any station, which may be related to a reduction in the influence of tropical cyclones in the region. For the longer-term record since the 1920s at Macao, a regime shift of tidal amplitudes around the 1970s may partially explain the diverse trend of extremes in original sea level records and tidal residuals.

Sampling Surface Particle Size Distributions and Stability Analysis of Deep Channel in the Pearl River Estuary

Particle size distributions （PSDs） of bottom sediments in a coastal zone are generally multimodal due to the complexity of the dynamic environment. In this paper, bottom sediments along the deep channel of the Pearl River Estuary （PRE） are used to understand the multimodal PSDs＇ characteristics and the corresponding depositional environment. The results of curve-fitting analysis indicate that the near-bottom sediments in the deep channel generally have a bimodal distribution with a fine component and a relatively coarse component. The particle size distribution of bimodal sediment samples can be expressed as the sum of two lognormal functions and the parameters for each component can be determined. At each station of the PRE, the fine component makes up less volume of the sediments and is relatively poorly sorted. The relatively coarse component, which is the major component of the sediments, is even more poorly sorted. The interrelations between the dynamics and particle size of the bottom sediment in the deep channel of the PRE have also been investigated by the field measurement and simulated data. The critical shear velocity and the shear velocity are calculated to study the stability of the deep channel. The results indicate that the critical shear velocity has a similar distribution over large part of the deep channel due to the similar particle size distribution of sediments. Based on a comparison between the critical shear velocities derived from sedimentary parameters and the shear velocities obtained by tidal currents, it is likely that the depositional area is mainly distributed in the northern part of the channel, while the southern part of the deep channel has to face higher erosion risk.

Numerical Simulation and Risk Analysis of Coastal Inundation in Land Reclamation Areas:A Case Study of the Pearl River Estuary

Coastal inundation results in many human casualties and significant economic losses.In this study,an inundation model with an unstructured mesh was developed using the ADvanced CIRCulation(ADCIRC)and Simulating WAves Nearshore(SWAN)models to simulate 43 inundation events from 1998 to 2018.A combined wind-pressure field derived from the assimilation windpressure model was used to drive the coupled ADCIRC-SWAN model.On the basis of the model results,a sensitivity study of the influence of land reclamation on coastal inundation was conducted.The results of the study showed that coastline changes caused by reclamation significantly influence the distribution of coastal inundation,particularly in areas where narrow waterways,bays,and peninsulas are newly formed.Combining the extreme inundation events calculated using the Gumbel and Weibull distributions,the 10-year,50-year,100-year,and 200-year return coastal inundations were obtained for the Pearl River Estuary in China.The risk of coastal inundation was analyzed according to the probability of the inundation depth exceeding 1 m.A hazard grade zoning map was drawn to guide disaster relief and mitigation in the reclamation area.

Estimation of Turbulent Kinetic Energy Dissipation Rate in the Bottom Boundary Layer of the Pearl River Estuary

A structure function approach is applied to estimate the turbulent kinetic energy （TKE） dissipation rate in the bottom boundary layer of the Pearl River Estuary （PRE）. Simultaneous measurements with an acoustic Doppler velocimeter （ADV） supplied independent data for the verification of the structure function method. The results show that, 1） the structure function approach is reliable and successfully applied method to estimate the TKE dissipation rate. The observed dissipation rates range between 8.3 ×10^-4 W/kg and 4.9× 10^-6 W/kg in YM01 and between 3.4×10^-4 W/kg and 4.8×10^-7 W/kg in YM03, respectively, while exhibiting a strong quarter-diurnal variation. 2） The balance between the shear production and viscous dissipation is better achieved in the straight river. This first-order balance is significantly broken in the estuary by non-shear production/dissipation due to wave-induced fluctuations.

Estimation of Peak Water Level in Pearl River Estuary under the Background of Sea Level Rise

[Objective] The study aimed to predict the peak water level in Pearl River Estuary under the background of sea level rise. [Method] The changing trends of peak water level at Denglongshan station and Hengmen station were analyzed firstly on the basis of regression models, and then sea level rise in Pearl River Estuary in 2050 was predicted to estimate the 1-in-50-year peak water level in the same year. [Result] Regression analyses showed that the increasing rate of peak water level over past years was 6.3 mm/a at Denglongshan station and 5.8 mm/a at Hengmen station. In addition, if sea level will rise by 20, 30 and 60 cm respectively in 2050, it was predicted that the 1-in-50-year peak water level will reach 3.04, 3.14 and 3.44 m at Denglongshan station, and 3.19, 3.29 and 3.59 m at Hengmen station separately. [Conclusion] The estimation of peak water level in Pearl River Estuary could provide theoretical references for water resources planning.

Multilocus DNA metabarcoding diet analyses of small cetaceans:a case study on highly vulnerable humpback dolphins and finless porpoises from the Pearl River Estuary,China

Accurate diet identification of top predators is crucial to fully understand their ecological roles.Compared to terrestrial animals,gathering dietary information from cetaceans is notoriously difficult.Here,we applied a multilocus metabarcoding approach to investigate the diet of vulnerable Indo-Pacific humpback dolphins and Indo-Pacificfinless porpoises from the Pearl River Estuary(PRE),China.Our analyses identified 21 preyfish species from the 42 humpback dolphin stomachs,as well as 10 species offish and 1 species of cephalopod from the 13finless porpoise stomachs.All of the taxa were assigned to the species level,highlighting that the multimarker approach could facilitate species identification.Most of the prey species were small-and medium-sizedfishes that primarily fed on zooplankton.The calculated similarity index revealed a moderated dietary overlap between the 2 cetaceans,presumably due to the feeding of the 2 predators in association withfishing vessels in the PRE.A more diverse diet was observed in humpback dolphins in the closedfishing season compared to thefishing season,implying the influence on the dolphin diet due to the availability of commercialfishery resources.However,according to the results of species rarefaction curves,ourfindings on the feeding habits of the 2 cetaceans are still limited by insufficient sample size and therefore should be interpreted with caution.This study represents afirst attempt to apply the multilocus DNA metabarcoding technique in the diet analysis of small cetaceans,although more efforts are needed to improve this type of analysis.

Distribution of heavy metals in sediments of the Pearl River Estuary,Southern China:Implications for sources and historical changes

The distribution of heavy metals （Pb, Zn, Cd and As） in sediments of the Pearl River Estuary was investigated. The spatial distribution of heavy metals displayed a decreasing pattern from the turbidity maxima to both upstream and downstream of the estuary, which suggested that suspended sediments played an important role in the trace metal distribution in the Pearl River Estuary. In addition, metal concentrations were higher in the west part of the estuary which received most of the pollutants from the Pearl River. In the sediment cores, fluxes of heavy metals were consistent with a predominant anthropogenic input in the period 1970-1990. From the mid-1990s to the 2000s, there was a significant decline in heavy metal pollution. The observed decline has shown the result of pollution control in the Pearl River Delta. However, it is noteworthy that the metal concentrations in the most recent sediment still remained considerably high. Taken together, the enrichment of heavy metals in sediments was largely controlled by anthropogenic pollution.

Spatial and Seasonal Variations of Nutrients in Sediment Profiles and Their Sediment-Water Fluxes in the Pearl River Estuary,Southern China

Three cruises were launched in the Pearl River Estuary （PRE） in 2005 to investigate the biogeochemical cycling of nutrients associated with early diagenesis related to degradation of organic matter. Seasonal and spatial variations of pore water nutrient concentrations and profile patterns in sediments were studied. Nutrient fluxes at the sediment-water interface （SWI） were measured by incu- bation experiments, and we here discussed the accumulation and transformation processes of nutrients at the SWI. The nutrients generally decreased from the Pearl River outlets downstream, indicating an- thropogenic influences on the nutrient inputs in the estuary. NO3-N concentration was the highest of the three forms of DIN （dissolved inorganic nitrogen, the sum of NH4-N, NO3-N and NO2-N） in the overly- ing water, and NH4-N was the main component of DIN in pore water. The gradual increase of NH4-N and the rapid decrease of NO3-N with sediment depth provided the evidence for anaerobic conditions below the SWI. Negative fluxes of NO3-N and positive fluxes of NH4-N were commonly observed, sug- gesting the denitrification of NO3-N at the SWL The DIN flux direction suggested that the sediment was the sink of DIN in spring, however, the sediment was generally the source of DIN in summer and winter. PO4-P distribution patterns were distinct while SiO4-Si inconspicuously varied in sediment profiles in different seasons. The flux results indicated that PO4-P mainly diffused from the water column to the sediment while SiO4-Si mainly diffused from the sediment to the water column. Generally, the incu- bated fluxes were the coupling of diffusion, bioturbation and biochemical reactions, and were relatively accurate in this study.

Distribution Characteristics and Sources of Sedimentary Organic Matter in the Pearl River Estuary and Adjacent Coastal Waters,Southern China

Geochemical descriptors （carbon, nitrogen contents, and their isotopes） of sedimentary organic matter （OM） were investigated in the Pearl River estuary （PRE） and adjacent coastal waters.The comprehensive geochemical data of surface and deep sediments were provided, and the sources of sedimentary OM were discussed in this area mainly impacted by human activities. Surface （1-10 cm）and deep （190-200 cm） sedimentary total organic carbon （TOC） were 1.2% and 0.6%, and total nitrogen （TN） contents were 0.09% and 0.05%, with δ^13C of-24.3%0 and -24.5%0, on the average, respectively.Higher TOC and TN contents combining with relatively negative 613C values in surface sediments suggested the increasing OM inputs from rivers in recent years. Terrestrial organic carbon （OC） contents were 0.70% and 0.36%, and marine OC contents were 0.50% and 0.28% on the average, respectively, in surface and deep sediment. Surface terrestrial and marine OC were lower in river outfalls due to the high current energy, and deep terrestrial and marine OC showed the increasing trend away from the coast. Lower δ^15 was mainly attributed to the influence of river sewage discharge in this study.

Numerical Experiment Research of a Marine Fog Event in the Pearl River Estuary Region

A marine fog event that occurred near the Pearl River Estuary region on 26March 2002 was investigated with the fifth-generation Pennsylvania State University/National Centerfor Atmospheric Research Mesoscale Model (MM5). The results of numerical experiment are veryconsistent with the surface observations, especially in the processes of marine fog formation andevolution. Besides, a series of sensitivity numerical experiments were performed by varying thedistribution of landuse type and the turbulence exchange process. It is shown that the marine fogforms along the coastal line. Tests have indicated that when the distribution of landuse type ismodified, the regions where the marine fog can intrude into are obviously different. The turbulenceprocess in the model is important for fog formation and evolution. If the influence of turbulenceprocess is ignored in numerical simulation experiment, the simulated maximum of the cloud liquidwater content and the height of fog top will be decreased greatly.

A REVIEW OF HEAVY METAL POLLUTION IN THE PEARL RIVER ESTUARY

Since 70'' s, the economy and society in the Pearl River Delta region havebeen undergoing a great development. Pollution control and environment protection have received muchpublic attention. As heavy metals are an important toxic pollutant source in this area and are notbiodegradable in general, a lot of scientists in China, especially in Southern China, devote theirfocus on studying the pollution caused by heavy metals in the Pearl River Estuary (PRE). This paperpresents a comprehensive review on the heavy metal pollution research and its related research inthis area. It is suggested that the study of heavy metal pollution in PRE should be coupled withwater movement, sediment motion and tidal characteristics in this region. Studying core sediments invarious locations is recommended to reveal the history of heavy metal pollution in PRE and improvethe understanding of the fate and transport of metals.

Relationship between nutrients and plankton biomass in the turbidity maximum zone of the Pearl River Estuary

Nutrients,dissolved and particulate organic carbon and plankton（bacterio-,phyto-and zoo-）were compared in the turbidity maximum zone（TMZ） and adjacent areas（non-TMZ） in the Pearl River estuary.Our results showed that high levels of suspended substances had marked effect on dynamics of nutrients and plankton in the TMZ.Based on the cluster analysis of total suspended solids（TSS） concentrations,all stations were divided into two groups,TMZ with average TSS of 171 mg/L and non-TMZ of 45 mg/L.Suspended substances adsorbed PO^（3-）_4 and dissolved organic carbon,resulting in higher particulate phosphorus and organic carbon（POC）and lower PO^（3-）_4 and DOC in the TMZ,compared to the non-TMZ.However,suspended substances had limited effect on nitrogenous nutrients.Phytoplankton growth was light-limited due to high concentrations of suspended substances in the TMZ and a peak of phytoplankton abundance appeared in the non-TMZ.In contrast,the highest bacterial abundance occurred in the TMZ,which was likely partly responsible for low DOC levels.Two peaks of zooplankton abundance observed in the TMZ and non-TMZ in the Pearl River estuary were primarily supported by bacteria and phytoplankton,respectively.Our finding implied that high levels of suspended solids in the TMZ affect the trophic balance.

Spatial assessment of water quality using chemometrics in the Pearl River Estuary, China

A cruise was commissioned in the summer of 2009 to evaluate water quality in the Pearl River Estuary （PRE）. Chemometrics such as Principal Component Analysis （PCA）, Cluster analysis （CA） and Self-Organiz- ing Map （SOM） were employed to identify anthropogenic and natural influences on estuary water quality. The scores of stations in the surface layer in the first principal component （PC1） were related to NH4-N, PO4-P, NO2-N, NO3-N, TP, and Chlorophyll a while salinity, turbidity, and SiO3-Si in the second principal component （PC2）. Similarly, the scores of stations in the bottom layers in PC1 were related to PO4-P, NO2-N, NO3-N, and TP, while salinity, Chlorophyll a, NH4-N, and SiO3-Si in PC2. Results of the PCA identified the spatial distribution of the surface and bottom water quality, namely the Guangzhou urban reach, Middle reach, and Lower reach of the estuary. Both cluster analysis and PCA produced the similar results. Self-organizing map delineated the Guangzhou urban reach of the Pearl River that was mainly influenced by human activities. The middle and lower reaches of the PRE were mainly influenced by the waters in the South China Sea. The information extracted by PCA, CA, and SOM would be very useful to regional agencies in developing a strategy to carry out scientific plans for resource use based on marine system functions.

Spectral characteristics of dissolved organic matter in sediment pore water from Pearl River Estuary

As key parts of land-sea transition zones,estuary ecosystems play a very important role in the ocean carbon cycle processes.The sources,degradation,and preservation of dissolved organic matter(DOM)in estuaries have long been the subject of intense study.To examine the aforementioned issues,this study examined three-dimensional fluorescence spectroscopy and ultraviolet-visible absorption spectroscopy to determine the spatial distribution and sources of DOM in the pore water of three sedimentary cores from the Pearl River Estuary(S1,S2 and S3,with increasing salinity).Using the parallel factor analysis(PARAFAC)method to analyze the three-dimensional fluorescence spectrum data,five fluorescent components were obtained—three humic-like components(C1,C3,and C4),and two protein-like components(C2 and C5).C2 exhibited a significant positive correlation with the sediment microbial deoxyribose nucleic acid(DNA)concentration(R~2=0.69,P<0.01),indicating that the protein-like component C2 might be derived from the catabolism of in situ microbes.C5 displayed a relatively weak correlation with DNA concentration(R~2=0.40,P<0.05),presumably due to the incorporation of phenolic compounds,which have a fluorescence peak very similar to that of protein-like components.The source of humic-like fluorescent components is extremely complex.The content at station S1 was relatively high(1.45–8.83 R.U.),which implies that terrestrial inputs had a significant influence.The three humic-like components showed similar distributions at S2 and S3,and the fluorescence intensity was rather low;this result indicates that the DOM at these two stations was more likely affected by the metabolism of algae and microorganisms.The humification index(HIX)and the fluorescence intensity of protein-like components increased and decreased,respectively,with depth.There was a significant positive correlation between the relative content of protein-like components and the spectral slope ratio(SR),which indicates that DOM transitioned from low-molecular-weight protein-like components in the surface sediment to high-molecular-weight humic-like components in the subsurface.This study provides valuable information for understanding the pore water size/reactivity(PWSR)model of DOM and its biochemical processes occurring in estuary sediments.

Chemometric study of spatial variations of environmental and ecological characteristics in the Zhujiang River (Pearl River) Estuary and adjacent waters

Chemometric approach based on principal component analysis（PCA） was utilized to examine the spatial variances of environmental and ecological characteristics in the Zhujiang River（Pearl River） Estuary and adjacent waters（ZREAW） in the South China Sea. The PCA result shows that the ZREAW can be divided into different zones according to the principal components and geographical locations of the study stations,and indicates that there are distinct regional variances on environmental features and the corresponding phytoplankton biomass and community structures among different areas. The spatial distribution of ecological features was implied to be influenced by various degrees of the different water resources,such as the Pearl River discharges,the coastal current and the oceanic water from the South China Sea. The variation of the biomass maximum zone and the complex impacts on the spatial distributions of phytoplankton biomass and production were also evaluated.

Turbidity maximum formation and its seasonal variations in the Zhujiang(Pearl River)Estuary,southern China

Real-time observations in the field and numerical simulations（with Delft3D） were combined to study the formation, distribution and the relevant influencing factors of turbidity maximum（TM） in the Zhujiang（Pearl River） Estuary（ZE）. The spatial distribution pattern of the TM varies with the longitudinal distributions of salinity and suspended sediment concentration（SSC）. The SSC is enhanced and the TM is intensified during dry seasons,whereas the center of the TM moves upstream by a distance of 10 km during wet seasons. The formation of the TM is influenced by a complex combination of numerous factors, including tides, river discharges and topography, wherein sediment resuspension and vertical circulation dominate the formations and variability of the TM.

Variations of suspended sediment transport caused by changes in shoreline and bathymetry in the Zhujiang(Pearl)River Estuary in the wet season

A wave-current-sediment coupled numerical model is employed to study the responses of suspended sediment transport in the wet season to changes in shoreline and bathymetry in the Zhujiang(Pearl)River Estuary(ZRE)from 1971 to 2012.It is shown that,during the wavy period,the large wave-induced bottom stress enhances sediment resuspension,resulting in an increase in the area of suspended sediment concentration(SSC)greater than 100 mg/L by 183.4%.On one hand,in spring tide,the change in shoreline reduces the area of SSC greater than 100 mg/L by 17.8%in the west shoal(WS)but increases the SSC,owing to the closer sediment source to the offshore and the stronger residual current at the Hengmeng(HEM)and Hongqili(HQL)outlets.The eastward Eulerian transport is enhanced in the WS and west channel(WC),resulting in a higher SSC there.The reclamation of Longxue Island(LXI)increases SSC on its east side and east shoal(ES)but decreases the SSC on its west and south sides.Moreover,in the WC,the estuarine turbidity maximum(ETM)is located near the saltwater wedge and moves southward,which is caused by the southward movement of the maximum longitudinal Eulerian transport.In neap tide,the changes are similar but relatively weaker.On the other hand,in spring tide,the change in bathymetry makes the SSC in the WS increase,and the area of SSC greater than 100 mg/L increases by 11.4%and expands eastward and southward,which is caused by the increases in wave-induced bottom stress and eastward Eulerian transport.On the east side of the WC,the eastward Eulerian transport decreases significantly,resulting in a smaller SSC in the middle shoal(MS).In addition,in the WC,the maximum SSC is reduced,which is caused by the smaller wave-induced bottom stress and a significant increase of 109.88%in southward Eulerian transport.The results in neap tide are similar to those in spring tide but with smaller changes,and the sediment transports northward in the WC owing to the northward Eulerian transport and vertical shear transport.This study may provide some references for marine ecological environment security and coastal management in the ZRE and other estuaries worldwide affected by strong human interventions.

The dynamic observation of dissolved organic matter in the Zhujiang(Pearl River)Estuary in China from space

The distributions of estuarine colored dissolved organic matter（CDOM）are the combined results of physicalbiogeochemical processes.Remote sensing is needed to monitor highly dynamically estuarine CDOM.Using in situ data from four seasonal cruises,an algorithm is developed to estimate CDOM absorption coefficient at 400nm（aCDOM（400））in the Zhujiang（Pearl River）Estuary（ZJE）.The algorithm uses band ratios of Rrs（667）/Rrs（443）and Rrs（748）/Rrs（412）.By applying it to moderate resolution imaging spectroradiometer onboard Aqua satellite（MODIS/Aqua）data from 2002 to 2014,seasonal climatology aCDOM（400）in the ZJE is calculated.CDOM distributions are majorly influenced by water discharge from the Zhujiang River and underwater topography.Along the section vertical to a water depth gradient,the seasonal aCDOM（400）exponentially decreased（y=aebx,b〈0）,but with great differences among seasons.Riverine fresh water is the primary source of CDOM in the ZJE.Fulvic acid fraction decreases with increasing salinity.Using developed algorithms,conservative CDOM mixing equation,and river discharge,effective riverine end-member concentration and flux of dissolved organic carbon（DOC）in summer and winter from 2002 to 2014 are first estimated from the MODIS/Aqua data.Both effective riverine end-member DOC concentration and flux are positively related to the river discharge,significantly in summer with R-2 of 0.698 for concentration and 0.965 7 for flux.

Effect of river plume on phytoplankton community structure in Zhujiang River estuary

To examine the phytoplankton assemblages and the effect of diluted waters on them,a research cruise was conducted from July 19 to August 7,2015 in the Zhujiang(Pearl)River estuary in the northern South China Sea(21°N–23.5°N,111°E–117°E).Samples were collected from 65 stations including one for time-series sampling.A total of 212 phytoplankton taxa were identified from 61 genera belonging to 4 phyla.Among them,122 species identified from 42 genera of Bacillariophyta and 83 species from 15 genera of Pyrrophyta.Chain-forming diatoms dominated the phytoplankton community where Pseudonitzschia delicatissima,Guinardia striata,Thalassionema nitzschioides,and P.pungens comprised about 52%of the total abundance.However,higher cell abundances concentrated on both sides of the estuary,because of low salinity and high nutrients brought by diluted water.In addition,Canonical Correspondence Analysis revealed that salinity and dissolved inorganic nitrogen shaped the species composition in the study area.Furthermore,the Jaccard similarity index showed prevailing high similarity in the distribution of species in low-salinity diluted waters,and the Bray-Curtis similarity depicted distinguished grouping for phytoplankton assemblages along the salinity gradient.However,phytoplankton diel vertical cycles showed maximum abundance occurred at 2:00 am,which was mainly contributed by benthic phytoplankton species Pseudo-nitzschia spp.and T.nitzschioides.