Nutrient flux in a landscape: Effects of coastal land use and terrestrial community mosaic on nutrient transport to coastal waters

Long-term interdisciplinary studies of the Rhode River estuary and its watershed in the mid-Atlantic coastal plain of North America have measured fluxes of nitrogen and phosphorus fractions through the hydrologically-linked ecosystems of this landscape. These ecosystems are upland forest, cropland, and pasture; streamside riparian forests; floodplain swamps; tidal brackish marshes and mudflats; and an estuarine embayment. Croplands discharged far more nitrogen per hectare in runoff than did forests and pastures. However, riparian deciduous hardwood forest bordering the cropland removed over 80 percent of the nitrate and total phosphorus in overland flows and about 85 percent of the nitrate in shallow groundwater drainage from cropland. Nevertheless, nutrient discharges from riparian forests downslope from croplands still exceeded discharges from pastures and other forests. The atomic ratio of nitrogen to phosphorus discharged from the watersheds into the estuary was about 9 for total nutrients and 6 for inorganic nutrient fractions. Such a low N:P ratio would promote nitrogen rather than phosphorus limitation of phytoplankton growth in the estuary. Estuarine tidal marshes trapped particulate nutrients and released dissolved nutrients. Subtidal mudflats in the upper estuary trapped particulate P, released dissolved phosphate, and consumed nitrate. This resulted in a decrease in the ratio of dissolved inorganic N:P in the estuary. However, the upper estuary was a major sink for total phosphorus due to sediment accretion in the subtidal area. Bulk precipitation accounted for 31 percent of the total nongaseous nitrogen influx to the landscape, while farming accounted for 69 percent. Forty-six percent of the total non-gaseous nitrogen influx was removed as farm products, 53 percent either accumulated in the watershed or was lost in gaseous forms, and 1 percent entered the Rhode River. Of the total phosphorus influx to the landscape, 7 percent was from bulk precipitation and 93 percent was from farming. Forty-five percent of the total phosphorus influx was removed as farm products, 48 percent accumulated in the watershed, and 7 percent entered the Rhode River. These nitrogen and phosphorus discharges into the Rhode River, although a small fraction of total loadings to the watershed, were large enough to cause seriously overenriched conditions in the upper estuary.     

河口湾潮坪潮汐水道发育特征及地质意义——以钱塘江为例

河口湾中的砂体是有利的油气储集层,但复杂的水动力变化导致古代河口湾识别困难。通过对现代钱塘江河口湾的研究表明,河口湾潮坪环境潮汐水道可以成为识别古河口湾的一种辅助标志。基于野外实地观测,结合卫星地图对潮坪环境潮汐水道的发育特征进行了详细的描述,并探讨了它的地质意义。结果表明:1）泥坪潮汐水道横截面形态主要有“V”型及宽缓的透镜状,平面上由海向陆可划分为曲折的A段、平直的B段和树状分叉的C段;2）砂坪潮汐水道横截面形态主要有“U”型和阶梯型,平面上由海向陆可划分为曲折或平直的A'段和树状分叉的B'段,A'段的平直与曲折主要受控于砂坪潮道规模的大小;3）在砂质潮坪中常见炭屑层、包卷层理、液化流痕、垮塌构造、波痕等特殊的沉积构造;不同形态和规模的潮坪潮道主要受控于潮坪坡度、沉积物粒度、河口湾形态及相对海平面高低的影响,高坡度、低海平面有利于完整序列潮道的发育;4）不同类型河口湾潮坪潮道的特征及其伴生的特殊沉积构造可以为确定地质历史时期的古河口湾及其演化提供参考。     

Suspended particulate matter and dynamics of the Mfolozi estuary,Kwazulu-Natal: Implications for environmental management

 The Mfolozi Estuary on the KwaZulu-Natal coast of South Africa is the most turbid estuary in Natal due to poor catchment management, leading to large quantities of suspended particulate matter (SPM) entering the estuary from the Mfolozi River. This paper quantities some of the solute and sediment dynamics in the Mfolozi Estuary where the main documented environmental concern is the periodic input of SPM from the Mfolozi Estuary to the St. Lucia system, causing reduction of light penetration and endangering biological productivity in this important nature reserve. Synoptic water level results have allowed reach mean bed shear stresses and velocities to be calculated for an observed neap tidal cycle. Results indicate that ebb velocities dominate the sediment transport processes in the estuary when fluvial input in the Mfolozi River is of the order of 15–20 m³ s^–1. Observed and predicted flood tide velocities are too low (<0.35 m s^–1) to suspend and transport significant amounts of SPM. Observed results indicate that although the SPM load entering the estuary is dominantly from the Mfolozi River, the Msunduzi River flow plays a major role in the composition of the estuary's salinity and velocity fields. It is calculated that the Mfolozi Estuary would fill with sediment in 1.3 years if it was cut off from the sea. The major fluvial flood events help maintain the estuary by periodically pushing sediment seawards (spit progrades seawards 5 m yr^–1) and scouring and maintaining the main flow channel in the estuary. During low fluvial flow conditions, tidal flow velocities will become the dominant control on sediment transport in the estuary. Interchange of SPM between the St. Lucia and Mfolozi estuaries under present conditions is complicated by the strong transverse velocity shear between the two systems at their combined mouth. This is creating a salinity-maintained axial convergence front that suppresses mixing of solutes and SPM between the systems for up to 10 h of the tidal cycle during observed conditions. Received: 22 May 1995 · Accepted: 31 July 1995     

Seasonal variation of the salinity in the Zuari estuary,Goa, India

The annual salt budget of the Zuari is examined. The characteristics of the estuary differ markedly from the low run off season during November–May to the heavy run off period of the southwest monsoon from June to October. During November–May the estuary is vertically mixed and the two processes controlling the transport of salt are run off induced advective transport out of the estuary, and tidally induced diffusive transport into the estuary. The magnitude of the latter is about 20% larger, leading to a salinity rise in the estuary. The diffusion coefficient has been estimated to be 233 ± 101 m²/sec. With the onset of the southwest monsoon, the run off increases dramatically, and the estuary loses about 75% of its salt during the first two months of the season. About 2/3 of this loss is recovered in the next two months when the run off decreases. Because the estuary is partially stratified during June–October, gravitational circulation is expected to play a role in addition to tidal diffusion and run off. The magnitude of its contribution has, however, not yet been determined.     

The effects of damming on the materials flux in the Colorado River delta

During the last century, the Colorado River delta (CRD) has been dramatically altered by the diversion of river water for use in human activities. This alteration has eliminated the delivery of fresh water to the Gulf of California radically transforming the former estuary into an inverse-estuary. Under the new conditions, the new materials budget was estimated at the mouth of the Colorado River in terms of salts, total suspended sediments, organic suspended matter and nutrients. The results of this study show that, because of the asymmetry of the tidal wave, the variability of seston concentration follows a sedimentation pattern of three successive stages: re-suspension (erosion at ebb flow) > dilution (during flood flow) > sedimentation (at the end of the flood stage). The tidal asymmetry during neap tides was characterized by longer ebb (at least, 30 min longer) than flood and more intense ebb currents (as much as 43% higher), hence characterizing an ebb-dominated system. The CRD is characterized by high nutrients concentrations. Maximal levels are: nitrates (41 μM), phosphates (2.6 μM) and silicates (68 μM), nitrite (15 μM). The mass balance indicates that the system acts as a net exporter of suspended sediment with rates as high as 7 tons per tidal cycle. This behavior indicates that the CRD is in a destructive stage as a result of the lack of freshwater inflow and supply of sediment into the system.     

Nutrient Budgets and Management Actions in the Patuxent River Estuary,Maryland

Multi-year nitrogen (N) and phosphorus (P) budgets were developed for the Patuxent River estuary, a seasonally stratified and moderately eutrophic tributary of Chesapeake Bay. Major inputs (point, diffuse, septic, and direct atmospheric) were measured for 13 years during which, large reductions in P and then lesser reductions in N-loading occurred due to wastewater treatment plant improvements. Internal nutrient losses (denitrification and long-term burial of particulate N and P) were measured in tidal marshes and sub-tidal sediments throughout the estuary as were nutrient storage in the water column, sediments, and biota. Nutrient transport between the oligohaline and mesohaline zones and between the Patuxent and Chesapeake Bay was estimated using a salt and water balance model. Several major nutrient recycling terms were directly and indirectly evaluated and compared to new N and P inputs on seasonal and annual time-scales. Major findings included: (1) average terrestrial and atmospheric inputs of N and P were very close to the sum of internal losses plus export, suggesting that dominant processes are captured in these budgets; (2) both N and P export were a small fraction (13% and 28%, respectively) of inputs, about half of that expected for N based on water residence times, and almost all exported N and P were in organic forms; (3) the tidal marsh-oligohaline estuary, which by area comprised ~27% of the full estuarine system, removed about 46% and 74% of total annual upland N and P inputs, respectively; (4) recycled N and P were much larger sources of inorganic nutrients than new inputs during warm seasons and were similar in magnitude even during cold seasons; (5) there was clear evidence that major estuarine processes responded rapidly to inter-annual nutrient input variations; (6) historical nutrient input data and nutrient budget data from drought periods indicated that diffuse nutrient sources were dominant and that N loads need to be reduced by about 50% to restore water quality conditions to pre-eutrophic levels.     

Effects of hurricane floyd inland flooding,September–October 1999, on tributaries to Pamlico Sound,North Carolina

Hurricane Floyd in September 1999 caused disastrous flooding from South Carolina to Massachusetts in the United States, with particularly severe and prolonged flooding in eastern North Carolina resulting in record flood-flow loadings of freshwater and contaminants to Pamlico Sound, North Carolina. The inland flooding, water quality, and loadings to Pamlico Sound were determined as part of a multi-agency response to the floods and in an effort to understand the effects of the floods on the greater Pamlico Sound Basin. All major river basins draining to Pamlico Sound experienced floods at the 500-yr recurrence level. The volume of flood waters entering Pamlico Sound during September–October 1999 was estimated to be equivalent to about 95% of the volume of Pamlico Sound, meaning that flood waters could have essentially displaced most of the water present in Pamlico Sound. Nitrogen and phosphorus loads to the Pamlico River estuary and Neuse River estuary, the two principal estuaries draining to Pamlico Sound, in a 36-d period during the flooding were between 50–90% of the long term average annual loads. Pesticide concentrations in flood waters were surprisingly high, given the amount of dilution produced by the floodwaters.     

Tidal nitrogen exchanges across a freshwater wetland succession gradient in the upper Cooper River,South Carolina

Tidal freshwater sections of the Cooper River Estuary (South Carolina) include extensive wetlands, which were formerly impounded for rice culture during the 1,700s and 1,800s. Most of these former rice fields are now open to tidal exchange and have developed into productive wetlands that vary in bottom topography, tidal hydrography and vegetation dominants. The purpose of this project was to quantify nitrogen (N) transport via tidal exchange between the main estuarine channel and representative wetland types and to relate exchange patterns to the succession of vegetation dominants. We examined N concentration and mass exchange at the main tidal inlets for the three representative wetland types (submerged aquatic vegetation [SAV], floating leaf vegetation, and intertidal emergent marsh) over 18-21 tidal cycles (July 1998–August 2000). Nitrate + nitrite concentrations were significantly lower during ebb flow at all study sites, suggesting potential patterns of uptake by all wetland types. The magnitude of nitrate decline during ebb flow was negatively correlated with oxygen concentration, reflecting the potential importance of denitrification and nitrate reduction within hypoxic wetland waters and sediments. The net tidal exchange of nitrate + nitrite was particularly consistent for the intertidal emergent marsh, where flow-weighted ebb concentrations were usually 18–40% lower than during flood tides. Seasonal patterns for the emergent marsh indicated higher rates of nitrate + nitrite uptake during the spring and summer (> 400 μmol N m^-2 tide^-1) with an annual mean uptake of 248 ± 162 μmol m^–2 tide^–1. The emergent marsh also removed ammonium through most of the year (207 ± 109 μmol m^–2 tide^–1), and exported dissolved organic nitrogen (DON) in the fall (1,690 ± 793 μmol m^–2 tide^–1), suggesting an approximate annual balance between the dissolved inorganic N uptake and DON export. The other wetland types (SAV and floating leaf vegetation) were less consistent in magnitude and direction of N exchange. Since the emergent marsh site had the highest bottom elevation and the highest relative cover of intertidal habitat, these results suggest that the nature of N exchange between the estuarine waters and bordering wetlands is affected by wetland morphometry, tidal hydrography, and corresponding vegetation dominants. With the recent diversion of river discharge, water levels in the upper Cooper estuary have dropped more than 10 cm, leading to a succession of wetland communities from subtidal habitats toward more intertidal habitats. Results of this study suggest that current trends of wetland succession in the upper Cooper River may result in higher rates of system-wide inorganic N removal and DON inputs by the growing distributions of intertidal emergent marshes.     

大凌河口水动力数值模拟分析

采用大凌河实测长系列河道径流及水下地形等数据, 通过建立水动力数值模型, 综合考虑潮流和河道径流的影响, 对大凌河口的潮流进行模拟, 探讨大凌河口及辽东湾北部海域水动力过程的主要特征. 结果表明: 大凌河口附近海域的海流以潮流为主, 具有明显的往复性质. 潮流总的运动趋势是, 涨潮主流向为北东, 落潮主流向为南西. 大潮流速大于小潮流速, 涨潮流历时与落潮流历时几乎相当. 最大涨潮流速约为0.52 m/s, 最大落潮流速约为0.4 m/s, 潮流涨落平均潮流强度的分布大体和该海域等深线相适应.     

Circulation changes in the Sheepscot River estuary,maine, following removal of a causeway

A causeway which had restricted tidal flow in a portion of the Sheepscot River estuary was removed late in 1974. Flowmeter data from moored plankton nets fished over full tidal cycles, and salinity observations made in conjunction with the net sets, were used to evaluate the effects of causeway removal on circulation in the estuary. Tidal flows in the main channel increased by almost 50%. This increase was accompanied by substantial decreases in salinity stratification and in the strength of the gravitational circulation.     

Distortion of tidal currents and the lateral transfer of salt in a shallow coastal plain estuary (O estuário do Mira, Portugal)

We describe the tidal circulation of a coastal plain estuary across a flood tide delta located at its entrance. The area connects the downstream portion of the main estuary extending 30–40 km inland to the more complex delta reach that consists of a shallow main channel and a series of smaller side channels. The delta acts as a frictionally dominated zone that modifies the tidal wave from a simple sinusoid to one with ebb currents that accelerate to maximum early in the tidal cycle and last more than one-half of the tidal cycle. Along smaller side channels, the tidal currents favor stronger flood or ebb currents, depending upon the local surrounding morphology. The phase difference between ebb currents in the small channels relative to those in the main channel cause some of the salt to be retained thus reducing the tendency of freshwater discharge to flush salt out of the system. This mechanism of retention differs from the selective withdrawal mechanism described for this estuary in Blanton et al. (2000).     

河口海岸桥隧工程设计流速推算

跨海桥隧工程设计需要推算工程位置不同重现期设计流速,由于现场缺乏长期实测流速资料,设计流速推算存在很大困难。研究提出了采用不同重现期典型风暴潮过程推算河口海岸设计流速的数值模拟方法,对河口地区考虑洪水径流与风暴潮流的耦合。在依据澳门验潮站1925—2003年实测潮位资料分析珠江口海域风暴潮过程特征的基础上,结合潮位和潮差年极值频率分析结果构建了不同重现期典型风暴潮潮型。采用平面二维水动力数学模型模拟了不同重现期风暴潮和上游一般洪水组合条件下珠江口水域的流场,得出港珠澳大桥沿线各控制点处设计流速。     

Freshwater inundation effects on emergent vegetation of a hypersaline salt marsh

The coastal marshlands of the Nueces estuary, Texas depend upon periodic freshwater inundation to support current community structure and promote further establishment and expansion of emergent halophytes. Decades of watershed modifications have dramatically decreased freshwater discharge into the upper estuary resulting in hypersaline and dry conditions. In an attempt to partially restore inflow, the U.S. Bureau of Reclamation excavated two overflow channels re-connecting the Nueces River to the marshlands. Freshwater-mediated (precipitation and inflow) changes in tidal creek and porewater salinity and emergent marsh vegetation were examined over a 5-yr period at three stations in the upper Nueces Marsh with the aid of a Geographical Information System (GIS). Two stations were potentially subjected to freshwater inflow through the channels, while one station experienced only precipitation. Decreased tidal creek and porewater salinity were significantly correlated with increased freshwater at all stations (R²=0.37 to 0.56), although porewater salinities remained hypersaline. GIS analyses indicated the most considerable vegetation change following freshwater inundation was increased cover of the annual succulentSalicornia bigelovii. Fall inundation allowed seed germination and rapid expansion of this species into previously bare areas during the subsequent winter and following spring. The station affected by both inflow and precipitation exhibited greaterS. bigelovii cover than the station affected solely by precipitation in both spring 1999 (58. 7% compared to 27.9%) and 2000 (48.6% compared to 1.9%). Percent cover of the perennialBatis maritima temporarily increased after periods of consistent rainfall. The response was short term, and cover quickly returned to pre-inundation conditions within 3 mo. Prolonged inundation led to longterm (>2yr) decreases in percent cover ofB. maritima. Our results suggest that the timing and quantity of freshwater inundation strongly dictate halophyte response to precipitation and inflow. Brief periods of freshwater inundation that occur at specific times of year alleviate stress and promote seed germination and growth, but extended soil saturation can act as a disturbance that has a negative impact on species adapted to hypersaline conditions.     

The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta

Holocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea-level rise within a deeply incised bedrock-confined valley system. The present day Hawkesbury River is interpreted as a wave-dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine-dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood-tidal delta. Ocean wave energy is partially dissipated by this flood-tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river-dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone. The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole-core X-ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C-14 dates reveal four stages of incised-valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse-grained fluvial sand and silt was laid down under falling-to-lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine-grained fluvial sand and silt, muddy bayhead delta and muddy central-basin deposits developed as the incised valley was flooded during eustatic sea-level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central-basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth-bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth-bar, interdistributary-bay and bayhead-delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead-delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead-delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised-valley systems.     

Tidal variability in the water quality of an urbanized estuary

Tide and water quality data were collected concurrently in the Hackensack River estuary (HRE), a tidal tributary of the New York-new Jersey Harbor estuary. Harmonic analyses of tidal elevation data indicate that HRE tides are predominantly semidiurnal, though modulated by diurnal and fortnightly components. Nearly uniform tidal ranges (averaging approximately 1.6 m) were observed at three stations within the HRE. Periodogram estimates reveal significant tidal variability for the water quality parameter NH₄−N under dry-weather conditions. Lag correlation analyses associate NH₄−N concentration variations with water level fluctuations. Longitudinal profile plots for NH₄−N reveal a consistent pattern of tidal translations, with peak concentrations oscillating about a major wastewater discharger. These analyses suggest that the distribution of NH₄−N concentrations in the HRE is controlled primarily by major point source loadings and horizontal advection. A simplified, one-dimensional model is used to describe this distribution. Effects of tidal variability in masking water quality status and waak trends are also analyzed. These analyses highlight the potential importance of short-term water quality variability in tidal estuaries where concentration gradients are large.     

Nutrient and phytoplankton dynamics in a sewage-impacted Gulf coast estuary: A field test of the PEG-model and equilibrium resource competition theory

Eutrophication and noxious bloom events are becoming more prevalent with increasing anthropogenic activities. To lessen ecological damage, there is a need to develop phytoplankton management programs aimed at enhancing growth of beneficial algae. The success of such management schemes with be dependent on the predictability of phytoplankton succession within the target system to a controlled perturbation. Freshwater lakes appear to exhibit a degree of predictability as described by the PEG-model and Equilibrium Resource Competition theory. We investigated whether these concepts could be applied to a marine system, the Nueces River estuary, Texas. The PEG-model predicted nicely the initial occurrence of edible phytoplankton forms after a favorable nutrient perturbation. Equilibrium Resource Competition theory, however, only successfully predicted the occurrence of major phytoplankton taxa immediately after a nutrient perturbation. Systemwide correlations between N:P and cyanobacteria, green algae, and diatoms were poor. In the Nueces River estuary, succession within the phytoplankton community showed a degree of predictability to nutrient perturbations. Therefore, management of the phytoplankton community composition may be possible. The PEG-model appears to be a useful guide for a phytoplankton management scheme, while the utility of Equilibrium Resource Competition may be limited.     

Silica Mass-Balance and Retention in the Riverine and Estuarine Scheldt Tidal System (Belgium/The Netherlands)

An annual budget for dissolved silica (DSi) and biogenic silica (BSi) was constructed for the Scheldt estuary and for the entire riverine and estuarine Scheldt tidal system (Belgium/The Netherlands) using previously published silica concentrations and fluxes for the period 2003–2005. The annual estuarine DSi mass-balance was established, based on seasonal fluxes estimated using measured DSi concentrations and (fully transient) model simulations of conservative transport. The annual BSi mass-balance was deduced from measured BSi contents in the suspended particulate matter and annual mud fluxes taken from the literature. The Scheldt estuary acted as a net sink not only for the BSi carried by the tidal river as well as that produced by diatoms in the estuary, but also for large amounts of BSi imported from the coastal zone. This results in the retention of dissolved and biogenic silica higher than that of DSi alone, which is in contrast with the classical consideration that rivers act as a source of BSi for the coastal zone. DSi and silica (DSi + BSi) retentions amounted to, respectively, 28 and 64 % in the estuary, and 33 and 66 % in the entire tidal system. This study highlights thus the predominant role of the estuary in the entire Scheldt tidal system when dealing with silica dynamics, as well as the importance of including BSi when investigating estuarine silica retention.     

珠江口的沉积作用和沉积相

珠江口为华南沿海的重要河口,是珠江水(系(西江、北江、东江、流溪河)的受水盆地。珠江流域每年有8000余万吨悬沙。大约3000万吨溶解质和少量底沙进入珠江河口区域,它们在迳流、潮流和其他海洋动力因素共同影响下,沉积作用甚为活跃。但是珠江口水域宽阔,地形复杂,各河口、河口湾或湾外区域的动力背景及泥沙运动不尽相同,因而其沉积作用和沉积特征也具有区域差异。     

An ephemeral turbidity maximum generated by resuspension of organic-rich matter in a macrotidal estuary, S.W. Wales

An ephemeral estuarine turbidity maximum (ETM) occurs at high water in the macrotidal Taf estuary (SW Wales, United Kingdom). A new mechanism of ETM formation, due to resuspension and advection of material by flood tidal currents, is observed that differs from classical mechanisms of gravitational circulation and tidal pumping. The flood tide advances across intertidal sand flats in the main body of the estuary, progressively entraining material from the rippled sands. Resuspension creates, a turbid front that has suspended sediment concentrations (SSC) of about 4,000 mg I⁻¹ by the time it reaches its landward limit which is also the landward limit of salt penetration. This turbid body constitutes the ETM. Deposition occurs at high slack water but the ETM retains SSC values up to 800 mg I⁻¹, 1–2 orders of magnitude greater than ambient SSC values in the river and estuarine waters on either side. The ETM retreats down the estuary during the ebb; some material is deposited thinly across emergent intertidal flats and some is flushed out of the estuary. A new ETM is generated by the next flood tide. Both location and SSC of the ETM scale on Q/R³ where Q is tidal range and R is river discharge. The greatest expression of the ETM occurs when a spring tide coincides with low river discharge. It does not form during high river discharge conditions and is poorly developed on neap tides. Particles in the ETM have effective densities (120–160 kg m⁻³) that are 3–4 times less than those in the main part of the estuary at high water. High chlorophyll concentrations in the ETM suggest that flocs probably originate from biological production in the estuary, including production on the intertidal sand flats.     

A modeling study of the Satilla River estuary,Georgia. II: suspended sediment

A three-dimensional (3-D) suspended sediment model was coupled with a 3-D hydrodynamic numerical model and used to examine the spatial and temporal distribution of suspended sediments in the Satilla River estuary of Georgia. The hydrodynamic model was a modified ECOM-si model with inclusion of the flooding-drying cycle over intertidal salt marshes. The suspended sediment model consisted of a simple passive tracer equation with inclusion of sinking, resuspension, and sedimentation processes. The coupled model was driven by tidal forcing at the open boundary over the inner shelf of the South Atlantic Bight and real-time river discharge at the upstream end of the estuary, with a uniform initial distribution of total suspended sediment (TSS). The initial conditions for salinity were specified using observations taken along the estuary. The coupled model provided a reasonable simulation of both the spatial and temporal distributions of observed TSS concentration. Model-predicted TSS concentrations varied over a tidal cycle; they were highest at maximum flood and ebb tidal phases and lowest at slack tides. Model-guided process studies suggest that the spatial distribution of TSS concentration in the Satilla River estuary is controlled by a complex nonlinear physical process associated with the convergence and divergence of residual flow, a non-uniform along-estuary distribution of bottom stress, and the inertial effects of a curved shoreline.