Estuary Shelf Research Articles

Oxidation of petrogenic organic carbon in a large river-dominated estuary

Riverine export of petrogenic organic carbon (OCpetro) from continents to coastal oceans is a dynamic component of the global carbon budget and affects the long-term atmospheric carbon reservoir. In large fluvial systems, oxidation of OCpetro during transit releases a large flux of carbon dioxide to the atmosphere, influencing climate changes; however, the transport and fate of OCpetro and their controls along the fluvial–marine transition remain poorly constrained. Here, we combined Raman spectral, radiocarbon activity (F14C), mineralogical, and sedimentological techniques with multiple geochemical analyses to characterize the dynamics of OCpetro in the water column and sediment particles from the Yangtze River channel–estuary–shelf continuum systems.Our data show that much of the OCpetro present in suspended sediment (POCpetro) exported by the Yangtze River is “labile” fractions (mostly disordered materials) that can be degraded or lost during transport across the estuarine continuum, whereas the OCpetro deposited in seabed sediment are characterized by highly recalcitrant, unreactive, and graphitic carbon phases. As discrete “free” particles, coarse plant debris (>63 μm) with ages of several thousand years observed in the proximal delta of the Yangtze River exhibit nearly identical characteristics of disordered materials, and the presence of aged vascular plant detritus (carbon-rich and 14C-depleted materials) may lead to an overestimate of OCpetro in marine sediments. Using a Bayesian endmember mixing approach, a binary mixing model, and the ratio of fraction modern carbon (F14C) to Al/OC, we found a large decrease in POCpetro concentration and loading from the suspended sediments to seabed sediments, suggesting a loss of mineral-bound OCpetro fraction during sediment transport through the estuary and deposition on the shelf. We estimated that, on average, 46 ± 35 % of the POCpetro initially present in suspended sediments delivered to the Yangtze River Estuary during a flood event was primarily oxidized at the sediment–water interface, leaving the most graphitic carbon components transported laterally and efficiently reburied in shelf sediments. We found that during estuarine mixing, flocculation process-induced microaggregates may provide transient physical protection for POCpetro in the form of inclusions/aggregates with carbonate minerals; however, when POCpetro is physically and chemically separated from its mineral matrix via disaggregation and dissolution, it may be easily oxidized by microbial activity. In contrast, OC-phyllosilicates interactions exert a first-order control on the preservation of OCpetro in marine sediments. Our findings suggest that the importance of POCpetro oxidation and loss in carbon cycling and budget assessments of estuaries may be underestimated.

Hydrodynamical transport structure and lagrangian connectivity of circulations in the Pearl River Estuary

Using a three-dimensional (3D) hydrodynamic model, this study explored the seasonal hydrodynamic transport structure in the Pearl River Estuary and illustrated the intrinsic connectivity under multiscale motions from a Lagrangian perspective. Generally, the surface Lagrangian residual current (UL) is uniformly southwestward/southeastward in summer/winter, with a stronger intensity in the lower estuary. The bottom UL features in the southeastward direction in the upper estuary and northwest direction in the lower estuary. The fluvial–tide interaction line advances southeastward and northwestward in summer and winter, respectively. The UL captured the major transport processes and was in good agreement with the mean surface sediment transport patterns. In the transition region between Lantau Island and Neilingding Island, where it is largely affected by the interaction between the periodic tidal current and river discharge, the spatially averaged UL showed intensified intratidal variations and had a larger difference with the locally temporally averaged Eulerian residual current. The remarkable Lagrangian coherent structures that illustrated the transport paths and transport convergence regions were identified, which are generally consistent with the bottom sediment depocenters. Based on the Lagrangian connectivity analysis, it was revealed that a barrier between the western and eastern estuaries existed, which was weakened by the strong river discharge during summer. Two convergence regions near the Macau and Hong Kong waters were identified, where the sediment and pollutants easily settled. The study demonstrated the importance of the Lagrangian view in understanding the hydrodynamic process and transport structure in the estuary–shelf regions.

Interplay between Asian Monsoon and Tides Affects the Plume Dispersal of the New Hu-Wei River off the Coast of Midwest Taiwan

In the coupled estuary–shelf system, plumes originating from the New Hu-Wei and Choshui rivers, consisting of many terrestrial materials, could contaminate the water of the Mailiao industrial harbor. To determine the contribution of the two rivers to pollution, the interaction between river-forced, tide-generating, and monsoon-driven water motions in and around the Mailiao industrial zone harbor was examined by performing a series of numerical model experiments. We used a three-dimensional general circulation model to examine the interplay between Asian monsoon-driven, river-forced, and tide-induced water motions, one of which could primarily affect the plume. The model-derived results for different river discharges revealed that almost all of the ammonium entering the harbor had a slope-positive trend, with oscillations in response to flood–ebb tidal cycles. The ammonium increased with time and flux, except for the 10 m3/s flux. Although the river discharge flux exceeded 200 m3/s, the ammonium entering the harbor was the same as that of the 200 m3/s flux; the ammonium concentration did not increase significantly with time after the flux exceeded 200 m3/s. In addition, irrespective of flood or ebb tidal currents being suppressed by strong Asian monsoons, this mechanism avoided contaminating the water quality of the harbor while northeasterly winds prevailed. By contrast, the southwesterly monsoon drove the geostrophic current northward along the coast; concurrently, the coastal sea level increased to form the surface isobar slope up toward the coast, producing a secondary flow to accelerate geostrophic alongshore currents. The northward geostrophic currents compressed the plumes shoreward, forming a relatively narrow-band plume; the coupling model demonstrated that the southwesterly monsoon-driven current pushed plumes favorably along the west pier into the harbor.

Plume spreading test case for coastal ocean models

Abstract. We present a test case of river plume spreading to evaluate numerical methods used in coastal ocean modeling. It includes an estuary–shelf system whose dynamics combine nonlinear flow regimes with sharp frontal boundaries and linear regimes with cross-shore geostrophic balance. This system is highly sensitive to physical or numerical dissipation and mixing. The main characteristics of the plume dynamics are predicted analytically but are difficult to reproduce numerically because of numerical mixing present in the models. Our test case reveals the level of numerical mixing as well as the ability of models to reproduce nonlinear processes and frontal zone dynamics. We document numerical solutions for the Thetis and FESOM-C models on an unstructured triangular mesh, as well as ones for the GETM and FESOM-C models on a quadrilateral mesh. We propose an analysis of simulated plume spreading which may be useful in more general studies of plume dynamics. The major result of our comparative study is that accuracy in reproducing the analytical solution depends less on the type of model discretization or computational grid than it does on the type of advection scheme.

The Assimilation of Temperature and Salinity Profile Observations for Forecasting the River–Estuary–Shelf Waters

AbstractCurrent down‐scaling numerical modeling system around Hong Kong achieved a considerable prediction skill for the estuarine–shelf circulation off the Pearl River Estuary without data assimilation (DA). In order to further improve the reliability of this modeling system, the cost‐effective Ensemble optimal interpolation approach is implemented to test the potential benefits from assimilating the cruise temperature and salinity (T/S) profiles to reproduce the variable coast waters. Regarding assimilation parameters (e.g., assimilation window, observation spatiotemporal scales, and ensemble composition), four parallel experiments are conducted in summer 2015. Against the assimilated T/S profiles, the vertical structures of the analyzed T/S are improved by the DA, although the waters experience strong mixing on the shelf. Compared with the run without DA, the root mean square errors of the predicted T/S are generally reduced by 9.8%–23.5% and 4.2%–14.0% in the assimilation runs. The results also show the salinity stratification is improved in the shelf by the assimilation of T/S profiles, although the improvement is sensitive to the selected ensemble and the assimilation window. Further, we investigate the impact of the temporal scales of the river‐estuary‐shelf (RES) waters on the assimilation results by the sampling of the model‐state ensemble. The water exchanges between the estuary and the shelf are also better captured through this assimilation. The assimilation impact analysis shows that DA has advantages in reproducing the distribution of water masses of the RES waters, although the quality of the reproduced water mass distribution is related to the adopted sample ensemble in DA.

Impacts of fluvial flood on physical and biogeochemical environments in estuary–shelf continuum in the East China Sea

Land-ocean interaction plays an essential role in the transport fate of terrestrial matters in the coastal and shelf regions. Flood discharge from a mega river, containing massive water, sediment, and nutrient loads, could result in substantial and complex impacts on the physical and biogeochemical dynamics of coastal systems. In this study, field campaigns were conducted in a region from the Changjiang River Estuary to the East China Sea (ECS) before and after a significant flood. The impacts of the flood on physical and biogeochemical environments were assessed. The results revealed that the fluvial flood enhanced the offshore expansion of the low-salinity river plume and associated sediment/nutrient fronts. However, the area of elevated chlorophyll-a at the river mouth did not expand noticeably. A numerical model was applied to quantify the contribution of the Three Gorges Dam (TGD) to the spatial intensity and temporal duration of fluvial flood effects on estuary–shelf continuum. The results predicted a maximum of 2° latitudinal offshore displacement of the shelf water. Salinity and nitrate exhibited conservative expansions, with a longer relaxation time (~2 months) than chlorophyll-a and phosphate. After the TGD-regulated flow event ceased, salinity and nitrate effects persisted, but phosphate and chlorophyll-a recovered rapidly. The flood decreased the dissolved oxygen (DO) concentration around the river mouth and the offshore region, but not in the nearshore transient area. In contrast, the non-TGD regulation increased the regional DO concentration, which reduced the hypoxia risk. The TGD has become a crucial anthropogenic driver of environmental changes in the Changjiang Estuary-ECS continuum.

Assimilating In Situ and Remote Sensing Observations in a Highly Variable Estuary–Shelf Model

AbstractTo improve the forecasting performance in dynamically active coastal waters forced by winds, tides, and river discharges in a coupled estuary–shelf model off Hong Kong, a multivariable data assimilation (DA) system using the ensemble optimal interpolation method has been developed and implemented. The system assimilates the conductivity–temperature–depth (CTD) profilers, time series buoy measurement, and remote sensing sea surface temperature (SST) data into a high-resolution estuary–shelf ocean model around Hong Kong. We found that the time window selection associated with the local dynamics and the number of observation samples are two key factors in improving assimilation in the unique estuary–shelf system. DA with a varied assimilation time window that is based on the intratidal variation in the local dynamics can reduce the errors in the estimation of the innovation vector caused by the model–observation mismatch at the analysis time and improve simulation greatly in both the estuary and coastal regions. Statistically, the overall root-mean-square error (RMSE) between the DA forecasts and not-yet-assimilated observations for temperature and salinity has been reduced by 33.0% and 31.9% in the experiment period, respectively. By assimilating higher-resolution remote sensing SST data instead of lower-resolution satellite SST, the RMSE of SST is improved by ~18%. Besides, by assimilating real-time buoy mooring data, the model bias can be continuously corrected both around the buoy location and beyond. The assimilation of the combined buoy, CTD, and SST data can provide an overall improvement of the simulated three-dimensional solution. A dynamics-oriented assimilation scheme is essential for the improvement of model forecasting in the estuary–shelf system under multiple forcings.

A numerical study of coupled estuary–shelf circulation around the Pearl River Estuary during summer: Responses to variable winds, tides and river discharge

The mean (sub-tidal) circulations in the Pearl River Estuary (PRE) and over the adjacent shelf are interactive. They are driven by multi-forcing of winds, tides, and the buoyancy of river discharge. Utilizing a validated three-dimensional, high resolution numerical model, we find that the circulation in the PRE during summer is dominated by an advective gravitational two-layer circulation in the upper estuary and the landward part of the lower PRE, where the surface flow pattern also varies with the upwelling winds. The circulation in the seaward part of the lower PRE is governed by both gravitational circulation and geostrophic intrusive current from the shelf. The pattern and intensity of these circulations are largely modulated by variable wind forcing. The cross-shore upwelling shelf circulation off the PRE enhances the water exchange rate between the shelf and the PRE, but the net intrusive transport into the PRE is negatively correlated with the intensity of upwelling-favorable wind stress. Relatively strong water exchange rate between the shelf and estuary occurs during upwelling, which reduces the flushing time of the estuary. Although the ebbing/flooding tide strengthens/weakens the eastward alongshore upwelling current, tidal effect on the upwelling circulation and on the net transport between the shelf and estuary is not significant over the sub-tidal period. The shelf influences the estuary mainly through the intrusions of the shelf waters at the western bank and along the two navigation channels of the PRE; and they are governed by geostrophic cross-shelf upwelling circulation and by gravitational intrusive currents due to pressure gradients, yielded by the alongshore variation of the upwelling shelf circulation and by the buoyancy forcing of the river plume, respectively.

Distribution of hopanoids along a land to sea transect: Implications for microbial ecology and the use of hopanoids in environmental studies

Bacteriohopanepolyols (BHPs) are lipid constituents of diverse bacteria and have great potential as taxonomically and environmentally diagnostic biomarkers. In order to examine their environmental behavior and potential for tracing biogeochemical processes, we analyzed BHPs and geohopanoids (the diagenetic products of BHPs) in soils and surface sediments from the middle Yangtze River catchment to the East China Sea (ECS) shelf. These data are compared to an up‐to‐date survey of BHP distributions in soils, including regions collectively covering the Arctic, temperate, subtropics, and tropics. Regional climatic differences, particularly temperature, likely exert an important control on BHP distributions in soils. In the aquatic (river–estuary–shelf) setting, BHP concentrations and structural diversity are substantially lower than in soils, suggesting that in aquatic environments either bacterial biodiversity is lower or there is not the same requirement for hopanoid synthesis. However, different aquatic regimes vary substantially: high BHP diversity and enhanced BHP production occur in the biogeochemically dynamic Yangtze estuary, whereas BHP distribution is uniform with much less structural diversity in the oligotrophic ECS open shelf. The Rsoil index, based on the relative abundances of soil‐marker BHPs against bacteriohopanetetrol, is suggested as a new approach to trace soil organic matter input into marine sediments. The Rsoil indices decrease from the river to the ECS, correlating strongly with branched and isoprenoid tetraether indices and moderately correlating with δ13C of organic carbon values and the concentrations of higher plant biomarkers, demonstrating their ability to trace soil organic matter inputs at least to the ECS.

Seasonal Variability and Estuary–Shelf Interactions in Circulation Dynamics of a River-dominated Estuary

The long-term response of circulation processes to external forcing has been quantified for the Columbia River estuary using in situ data from an existing coastal observatory. Circulation patterns were determined from four Acoustic Doppler Profilers (ADP) and several conductivity–temperature sensors placed in the two main channels. Because of the very strong river discharge, baroclinic processes play a crucial role in the circulation dynamics, and the interaction of the tidal and subtidal baroclinic pressure gradients plays a major role in structuring the velocity field. The input of river flow and the resulting low-frequency flow dynamics in the two channels are quite distinct. Current and salinity data were analyzed on two time scales—subtidal (or residual) and tidal (both diurnal and semidiurnal components). The residual currents in both channels usually showed a classical two-layer baroclinic circulation system with inflow at the bottom and outflow near the surface. However, this two-layer system is transient and breaks down under strong discharge and tidal conditions because of enhanced vertical mixing. Influence of shelf winds on estuarine processes was also observed via the interactions with upwelling and downwelling processes and coastal plume transport. The transient nature of residual inflow affects the long-term transport characteristics of the estuary. Effects of vertical mixing could also be seen at the tidal time scale. Tidal velocities were separated into their diurnal and semidiurnal components using continuous wavelet transforms to account for the nonstationary nature of velocity amplitudes. The vertical structure of velocity amplitudes were considerably altered by baroclinic gradients. This was particularly true for the diurnal components, where tidal asymmetry led to stronger tidal velocities near the bottom.

Estimating tidal current amplitudes outside estuaries and characterizing the zone of estuarine tidal influence

Tidal currents within many estuaries are stronger than on the adjacent continental shelf. Estuarine tidal influence extends onto the shelf elevating tidal amplitudes above ambient shelf levels over a region outside the estuary. In this paper, a kinematic theory that describes tidal flow in these estuary–shelf tidal interaction zones is derived and tested. Using these results, tidal current amplitudes outside estuaries are estimated and the range and intensity of estuarine tidal influence is characterized. The estimates for the tidal current amplitudes ( u tide) of the dominant constituent are appropriate for linear or weakly nonlinear systems with predominantly barotropic tides; the ratio of tidal height to water depth is small ( η/ h≪1) in linear or weakly nonlinear regimes. Within tidal interaction zones, the tidal current amplitude is composed of an ambient shelf part ( u a) and an estuary-induced part ( u e). The characteristic shelf amplitude far from the estuary is used to set u a. Continuity arguments indicate that u e (averaged over a control volume arc centered on the mouth with radius r and a swath angle of γπ) approximately equals the ratio of the tidal volume flux through the mouth ( V m) and the cross-sectional area of any control volume arc ( u e ≈ V m / γ π r h ¯ ) . This approximation holds where the squared product of the tidal wave number and radial distance is small: ( kr) 2≪1. For a shelf with constant slope α and small coastal wall depth, the arc-averaged depth is h ¯ ≈ sr (where s=2 α/ pi for γ=1). For this shelf bathymetry, u e decays with squared radial distance from the estuary mouth. Theoretical predictions are consistent with observed tidal current amplitudes outside the Delaware Bay and Block Island Sound. In both test cases, the estimated and observed amplitudes exhibit an r −2 dependence and decay over a similar distance. The relative importance of estuarine tidal influence is assessed with the tidal interaction index ( T i= u e/ u a). The intensity of estuarine influence is characterized by the value of T i at the mouth ( T im). The extent of an estuary–shelf tidal interaction zone is measured by the interaction radius r i (where T i=1) and by r ∞ (where T i=0.1). These metrics are used to compare the tidal interaction zone characteristics for the Delaware Bay, Block Island Sound, several other major estuaries, and a small tidal inlet. The kinematic theory can be applied outside other estuaries where only limited tidal information is available or used to interpret observed patterns in tidal amplitudes and characterize the degree of estuarine influence.

Second order tidally induced flow in the inlet of a coastal lagoon

Current meter data obtained in Indian River Inlet and Indian River Bay, Delaware are analyzed to compute second order low-frequency tidal flow and tidally induced mean flow in the system. Results from least-squares harmonic analysis show that nonlinearly induced M 4 currents in the inlet and bay occur at order 10 −1 of the M 2 amplitudes, indicating weak nonlinearity in the system. Tidally rectified mean flow computed from M m and M sf is ∼3 cm s −1, which is of the same order of magnitude as the observed mean current. The estimated low-frequency tidal flow and the tidally induced mean flow agree well with scalings computed for the inlet and with results found by Münchow et al. [Münchow, A., Masse, A.K., Garvine, R.W., 1992. Astronomical and nonlinear tidal currents in a coupled estuary shelf system. Continental Shelf Research 12, 471–498] in Delaware Bay.

The Influence of Fluvial Discharge on Pelagic Production in the Gulf of Papua, Northern Coral Sea

The influence of river discharge on the behaviour of dissolved and particulate nutrients, and patterns of primary and bacterial production, were investigated in the Gulf of Papua, northern Coral Sea. Inshore, close to the mouths of the Fly and Purari Rivers, buoyant plumes of warmer river water, marked by high total suspended solid concentrations and floating plant debris, overlay cooler saline waters, and their movement with the tides results in marked short-term changes in salinity profiles. Mixing of silicate was conservative, but the behaviour of other nutrients (nitrate, DOC, phosphate) indicated biological uptake and/or production and release from particles. DIN:DIP ratios increased slightly into the gulf indicating possible phosphorus limitation to phytoplankton growth in mid-shelf and outer-shelf waters. At the mouth of the Fly Delta and in inshore waters, decreases in phosphate concentration corresponded to increases in standing stocks of chlorophylla.Primary production rates were <20mgCm−2day−1in waters of salinity <4, but ranged from 133 to 942mgCm−2day−1in river delta and inshore waters. Bacterial production was highly variable (range: 162–886mgCm−2day−1), but correlated very significantly (r=0·97) with primary production. Rates of bacterioplankton production nearly equal or exceed primary production rates, indicating that pelagic bacteria in the gulf are utilizing other carbon sources (e.g. riverine DOC and POC) and that gulf waters are net heterotrophic.The pattern of primary production across the estuary—shelf salinity gradient off the southern Papua New Guinea coast is similar to that observed off the Amazon. However, the much lower volume of water exiting the Papuan rivers restricts maximum production to the inshore (<50km from the coast) rim of the Gulf of Papua. This pattern is mirrored in benthic secondary production indicating close benthic–pelagic coupling.

Astronomical and nonlinear tidal currents in a coupled estuary shelf system

We analysed the tidal content of current time series from 53 current meters at 31 mooring locations on the inner continental shelf in the vicinity of Delaware Bay, U.S.A. We distinguish between an astronomically forced short period flow field at diurnal and semi-diurnal frequencies and a nonlinearly generated long period tidal flow field. The latter operates at the difference frequencies of the former. We found short period tidal currents to vary from 90 cm s −1 at the mouth of the estuary to 8 cm s −1 only 30 km to the north. Tidal volume flux through the mouth generates intensified tidal currents on the shelf as far as 50 km away to the southeast. At periods longer than 2 weeks we detected tidal currents with amplitudes from 1 to 3 cm s −1 and phases indicative of a horizontally coherent flow field. We also estimated mean rectified tidal flow fields of about 2–7 cm s −1. These thus contribute substantially to the overall mean currents on the shelf.

Suspended-sediment transport in the Seine estuary, France: Effect of man-made modifications on estuary—shelf sedimentology

Man-made modifications of the Seine estuary have produced large geometric changes with important hydrographic and sedimentological consequences affecting estuary—shelf interrelationships. The estuary once had shallow, meandering ebb and flood tidal channels and broad tidal flats. Today, after 130 years of landfill, marsh reclamation, jettying and dredging, there is only a narrow, deep, jettied channel extending to the sea. Most of the tidal flats and marshes have been filled. These modifications have concentrated river discharge and ebb flow into a narrow jettied channel which, in turn, has produced a seaward migration of the salt intrusion and the turbidity maximum. As the estuary volume was reduced, river flow dilution was also reduced and its mean residence time in the estuary was shortened. At the same time, sedimentation and landfill have reduced the area available for sedimentation of fluvial sediments. This change has amplified seaward transport of fluvial suspended sediment and increased the size of shelf mud zones off the estuary mouth. Man has thus changed the geologic role of the estuary from a sink for fluvial and marine sediment, to a source of fluvial sediment for the shelf.