Network analysis of nitrogen inputs and cycling in the Neuse River estuary,North Carolina,USA

The Neuse River estuary, North Carolina, United States, has demonstrated various symptoms of eutrophication during the past 20 yr. We contributed to an environmental assessment program, through ecological network analysis, a group of algorithms to evaluate networks of material flows within a structured system. Networks of nitrogen (N) cycling for 16 consecutive seasons were constructed based on previous field and laboratory studies. Network analysis provided understanding of the relationship between N loading and recycling, the fates of N and the expected interseasonal variation of both model inputs and outputs. Various indices indicated that recycling of imported N was very high, supporting measured observations. There was little correlation between estimates of loading and N uptake by phytoplankton, although loading of total and particulate N did correlate positively with export. Because of the high degree of recycling of N, the rate of loading of new N is a small fraction of the total processing of N or of the needs for primary production alone. We predict that on a short-term basis the controls on primary production tend to be associated with conditions in the estuary rather than import. This condition is likely to postpone easily observable responses to loading reduction over the entire estuary and in the short term, although improvements in water quality should occur over time.     

Denitrification rates measured along a salinity gradient in the eutrophic Neuse River estuary, North Carolina, USA

Denitrification rates along a salinity gradient in the eutrophic Neuse River Estuary, North Carolina, were quantified using membrane inlet mass spectrometry (MIMS) within short-term batch incubations. Denitrification rates within the system were highly variable, ranging from 0 to 275 μmol N m⁻² h⁻¹. Intrasite variability increased with salinity, but no significant differences were observed across the salinity gradient. Denitrification rates were positively correlated with sediment oxygen demand at the upstream sampling site where sediment organic carbon levels were lowest. This relationship was not observed in the more saline sampling sites. Denitrification rates were highest during winter. On an annual basis, denitrification accounted for 26% of the dissolved inorganic nitrogen and 12% of the total nitrogen supplied to the system.     

Estimation of Groundwater and Nutrient Fluxes to the Neuse River Estuary, North Carolina

A study was conducted between April 2004 and September 2005 to estimate groundwater and nutrient discharge to the Neuse River estuary in North Carolina. The largest groundwater fluxes were observed to occur generally within 20 m of the shoreline. Groundwater flux estimates based on seepage meter measurements ranged from 2.86?×?10⁸ to 4.33?×?10⁸ m³ annually and are comparable to estimates made using radon, a simple water-budget method, and estimates derived by using Darcy’s Law and previously published general aquifer characteristics of the area. The lower groundwater flux estimate (equal to about 9 m³ s^?1), which assumed the narrowest groundwater discharge zone (20 m) of three zone widths selected for an area west of New Bern, North Carolina, most closely agrees with groundwater flux estimates made using radon (3–9 m³ s^?1) and Darcy’s Law (about 9 m³ s^?1). A groundwater flux of 9 m³ s^?1 is about 40% of the surface-water flow to the Neuse River estuary between Streets Ferry and the mouth of the estuary and about 7% of the surface-water inflow from areas upstream. Estimates of annual nitrogen (333 tonnes) and phosphorus (66 tonnes) fluxes from groundwater to the estuary, based on this analysis, are less than 6% of the nitrogen and phosphorus inputs derived from all sources (excluding oceanic inputs), and approximately 8% of the nitrogen and 17% of the phosphorus annual inputs from surface-water inflow to the Neuse River estuary assuming a mean annual precipitation of 1.27 m. We provide quantitative evidence, derived from three methods, that the contribution of water and nutrients from groundwater discharge to the Neuse River estuary is relatively minor, particularly compared with upstream sources of water and nutrients and with bottom sediment sources of nutrients. Locally high groundwater discharges do occur, however, and could help explain the occurrence of localized phytoplankton blooms, submerged aquatic vegetation, or fish kills.     

Seasonal distributions of suspended particulate material and dissolved nutrients in a coastal plain estuary

The temporal and spatial distributions of salinity, dissolved oxygen, suspended particulate material (SPM), and dissolved nutrients were determined during 1983 in the Choptank River, an estuarine tributary of Chesapeake Bay. During winter and spring freshets, the middle estuary was strongly stratified with changes in salinity of up to 5‰ occurring over 1 m depth intervals. Periodically, the lower estuary was stratified due to the intrusion of higher salinity water from the main channel of Chesapeake Bay. During summer this intrusion caused minimum oxygen and maximum NH₄ ⁺ concentrations at the mouth of the Choptank River estuary. Highest concentrations of SPM, particulate carbon (PC), particulate nitrogen (PN), total nitrogen (TN), total phosphorous (TP) and dissolved inorganic nitrogen (DIN) occurred in the upper estuary during the early spring freshet. In contrast, minimum soluble reactive phosphate (SRP) concentrations were highest in the upper estuary in summer when freshwater discharge was low. In spring, PC:PN ratios were >13, indicating a strong influence by allochthonous plant detritus on PC and PN concentrations. However, high concentrations of PC and PN in fall coincided with maximum chlorophyll a concentrations and PC:PN ratios were <8, indicating in situ productivity controlled PC and PN levels. During late spring and summer, DIN concentrations decreased from >100 to <10 μg-at l^?1, resulting mainly from the nonconservative behavior of NO₃ ^?, which dominated the DIN pool. Atomic ratios of both the inorganic and total forms of N and P exceeded 100 in spring, but by summer, ratios decreased to <5 and <15, respectively. The seasonal and spatial changes in both absolute concentrations and ratios of N and P reflect the strong influence of allochthonous inputs on nutrient distributions in spring, followed by the effects of internal processes in summer and fall.     

Importance of submarine groundwater discharge as a source of nutrients for the Neuse River Estuary,North Carolina

Seepage rate and chemical composition of groundwater discharge entering the Neuse River Estuary (NRE) were quantified over an annual cycle from July 2005 through June 2006. Lee type seepage meters were deployed at eight locations within the NRE to quantify the amount of submerged groundwater discharge (SGD) entering the system. Sediment porewater nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), and phosphate (PO₄ ⁻³) were also quantified at each of these locations to determine groundwater chemical composition. Seepage rates for the system ranged from 0.004 to 0.035 m³ m⁻² d⁻¹. Both the average and median value for the system-wide SGD were 0.01 m³ m⁻²d⁻¹. There were no significant differences between upstream and downstream seepage rates or between those at the north and south side of the estuary. Seepage rates varied greatly in time and space. Discharging groundwater was NO₃ ⁻ deplete but highly enriched in NH₄ ⁺. Porewater PO₄ ⁻³ levels varied but were usually present below Redfield values due to NH₄ ⁺ enrichment. SGD nutrient loading represented a small part of watershed nitrogen and phosphorus loading, 0.8% and 1.0%, respectively.     

Dynamics of NH4 + and NO3 − uptake in the water column of the Neuse River Estuary,North Carolina

In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^? uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2–3-wk intervals from March 1985 to February 1989. Significant dark NH₄ ⁺ uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH₄ ⁺ uptake ranged from 0.001 μmol N 1^?1 h^?1 to 4.2 μmol N 1^?1 h^?1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH₄ ⁺ uptake was significantly related to NH₄ ⁺ concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH₄ ⁺ uptake ranged from 0.1 mmol N m^?2 d^?1 to 133 mmol N m^?2 d^?1 and followed the trend of apparent uptake. Annual NH₄ ⁺ uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO₃ ^? was only 14% of maximum light uptake. Apparent NO₃ ^? uptake rates ranged from 0.001 μmol N 1^?1 h^?1 to 1.84 μmol N 1^?1 h^?1 with highest rates occurring in the oligohaline estuary. Apparent NO₃ ^? uptake was significantly related to NO₃ ^? concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO₃ ^? uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO₃ ^? uptake ranged from 0.1 mmol N m^?2 d^?1 to 53 mmol N m^?2 d^?1 and followed similar patterns of apparent uptake. Annual NH₄ ⁺ uptake was 11.39 mol N m^?2 yr^?1, 10.28 mol N m^?2 Yr^?1, 10.93 mol N m^?2 yr^?1, and 7.38 mol N m^?2 yr^?1, and 1.84 mol N m^?2 yr^?1, with the 4-yr mean being 10.0. Annual NO₃ ^? uptake was 3.12 mol N m^?2 yr^?1, 3.40 mol N m^?2 yr^?1, 1.96 mol N m^?2 yr^?1, and 1.84 mol N m^?2 yr^?1, with the 4-yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.     

Dissolved and particulate organic carbon in the North Inlet estuary,South Carolina: What controls their concentrations?

Water samples have been taken daily at 1030 EST from three locations within North Inlet (South Carolina) since June of 1980 in order to evaluate the tidal, seasonal, and eventually annual variability in carbon concentrations within this system and generate hypotheses explaining the observed trends. Dissolved organic carbon (DOC) concentrations within North Inlet (South Carolina) vary inversely with salinity (r²=0.65), suggesting the main source of DOC in North Inlet is freshwater entering from the adjacent forested watershed. This assertion is supported by an observed decrease of tidal water salinity with the onset of streamflow. DOC variability is also associated with (1) groundwater advection and/or runoff and seepage from the marsh surface; (2) removal from tidal water via either physical sorption or biological uptake; (3) sampling location; and (4) origin of water mass. Particulate organic carbon (POC) concentrations vary seasonally, higher values found during the summer. POC variability is controlled by a series of physical and biological factors. Evidence suggests that in the smaller tidal creeks, POC concentrations are associated with (1) rain events scouring the marsh surface, (2) phytoplankton concentrations varying as a function of tidal stage, and (3) removal of particulate material from the marsh surface on the ebb tide. In the larger tidal creeks tidal water velocity appears to be the main factor influencing POC values.     

Phytoplankton Community Indicators of Short- and Long-term Ecological Change in the Anthropogenically and Climatically Impacted Neuse River Estuary, North Carolina, USA

Estuarine and coastal systems represent a challenge when it comes to determining the causes of ecological change because human and natural perturbations often interact. Phytoplankton biomass (chlorophyll a) and group-specific photopigment indicators were examined from 1994 to 2007 to assess community responses to nutrient and climatic perturbations in the Neuse River Estuary, NC. This system experienced nutrient enrichment and hydrologic variability, including droughts, and an increase in hurricanes. Freshwater input strongly interacted with supplies of the limiting nutrient nitrogen (N) and temperature to determine the location, magnitude, and composition of phytoplankton biomass. Multi-annual, seasonal, and episodic hydrologic perturbations, including changes in the frequency and intensity of tropical storms, hurricanes and droughts, caused significant shifts in phytoplankton community structure. Climatic oscillations can at times overwhelm anthropogenic nutrient inputs in terms of controlling algal bloom thresholds, duration, and spatial extent. Eutrophication models should incorporate climatically driven changes to better predict phytoplankton community responses to nutrient inputs and other anthropogenic perturbations.     

Paleoecological evidence of human impacts on the Neuse and Pamlico estuaries of North Carolina,USA

Sediment cores were collected from the Neuse and Pamlico River estuaries, North Carolina, at seven different sites, and the data show strong anthropogenic influence on water quality. The sediments from these cores were dated using²¹⁰Pb,¹³⁷Cs,¹⁴C, and pollen horizon techniques. Specific parameters investigated include bulk density, sedimentation rates, diatom assemblage changes, nutrient and trace metal flux, and vegetation changes as recorded in the pollen record. The greatest increases in sedimentation, nutrient and metal flux and changes in diatom assemblages have occurred in the past 50–60 yr in the Pamlico and Neuse. Diatom diversity has decreased and small planktonic forms have become dominant over time, most likely due to eutrophication and increased turbidity and sedimentation. Major changes occur before phytoplankton surveys and monitoring were initiated. Overall trends are similar to those found in Chesapeake Bay, although the time frame of major changes is more recent. Dominant small planktonic diatom species differ between Chesapeake Bay and the Neuse and Pamlico. Variance in paleoecological indicators between these mid-Atlantic estuaries may be due to geomorphology and land use history.     

Upper temperature tolerance of spot,Leiostomus xanthurus,from the cape Fear River estuary,North Carolina

The temperature tolerance and resistance times of postlarval (<25 mm SL) and small juvenile spot,Leiostomus xanthurus, from the Cape Fear Estuary, North Carolina were tested in the laboratory. Critical thermal maximum techniques were used to determine first equilibrium loss (FEL) and critical thermal maximum (CTM) end points and thermal shock methods were used to determine 96-h upper incipient lethal temperatures (LT₅₀). Acclimation temperatures ranged from 10 to 35°C and acclimation salinities were 10, 20 and 30‰. A quadratics model was fit to the CTM and FEL data; r² values were 0.924 and 0.928 respectively. Acclimation salinity, estimated weight, acclimation salinity by acclimation temperature interaction and acclimation temperature by estimated weight interaction were the significant components of the CTM model. Predicted CTM values ranged from 30°C at 10 °C and 30‰ acclimation to just over 40°C at 30 °C and 30‰ acclimation. Acclimation temperature, acclimation temperature squared, estimated weight and acclimation temperatures by estimated weight interaction were the significant components of the FEL model. Predicted FEL values ranged from around 28°C at 10°C and 10‰ acclimation to about 39°C at 30°C and 30‰ acclimation. The 96-h LT₅₀ values of spot acclimated to 20‰ increased linearly with acclimation temperature to 25°C. From about 25 to 35°C, LT₅₀ values increased very little with acclimation temperature. The ultimate upper incipient lethal temperature of postlarval and small juvenile spot was estimated at 35.2°C. Increased salinity increased resistance time but decreased LT₅₀ estimates. Thermal shock tests were better for predicting the effects of thermal addition than were CTM tests.     

Aging and growth of larval bay anchovy,Anchoa mitchilli,from the Newport River estuary,North Carolina

Bay anchovy (Anchoa mitchilli) larvae were hatched and reared in the laboratory from eggs collected near Beaufort, North Carolina. The first growth increment formed on otoliths on the fifth day after hatching when larvae were between 3.7 and 4.2 mm standard length. On the average, one otolith growth increment was formed per day thereafter in larvae up to 23 d posthatch. Age of wild larvae from the Newport River estuary in North Carolina was determined from otolith increment counts based on the assumption that increment deposition rates in nature are the same as in the laboratory. From their size and estimated age, it appears that the standard length (SL) of wild larvae in the estuary increases exponentially at about 4% d^?1 during their first 1.6 months, increasing from 0.24 mm d^?1 on day 12 to 1.11 mm d^?1 on day 49. Bay anchovy spawned early in the season (e.g., April–May) could grow to maturity and reach a size (>40 mm SL) that would enable them to spawn during their first summer.     

Hydrologic Variability and Its Control of Phytoplankton Community Structure and Function in Two Shallow,Coastal, Lagoonal Ecosystems: The Neuse and New River Estuaries,North Carolina,USA

Hydrologic conditions, especially changes in freshwater input, play an important, and at times dominant, role in determining the structure and function of phytoplankton communities and resultant water quality of estuaries. This is particularly true for microtidal, shallow water, lagoonal estuaries, where water flushing and residence times show large variations in response to changes in freshwater inputs. In coastal North Carolina, there has been an increase in frequency and intensity of extreme climatic (hydrologic) events over the past 15 years, including eight hurricanes, six tropical storms, and several record droughts; these events are forecast to continue in the foreseeable future. Each of the past storms exhibited unique hydrologic and nutrient loading scenarios for two representative and proximate coastal plain lagoonal estuaries, the Neuse and New River estuaries. In this synthesis, we used a 13-year (1998–2011) data set from the Neuse River Estuary, and more recent 4-year (2007–2011) data set from the nearby New River Estuary to examine the effects of these hydrologic events on phytoplankton community biomass and composition. We focused on the ability of specific taxonomic groups to optimize growth under hydrologically variable conditions, including seasonal wet/dry periods, episodic storms, and droughts. Changes in phytoplankton community composition and biomass were strongly modulated by the amounts, duration, and seasonality of freshwater discharge. In both estuaries, phytoplankton total and specific taxonomic group biomass exhibited a distinctive unimodal response to varying flushing rates resulting from both event-scale (i.e., major storms, hurricanes) and more chronic seasonal changes in freshwater input. However, unlike the net negative growth seen at long flushing times for nano-/microphytoplankton, the pigments specific to picophytoplankton (zeaxanthin) still showed positive net growth due to their competitive advantage under nutrient-limited conditions. Along with considerations of seasonality (temperature regimes), these relationships can be used to predict relative changes in phytoplankton community composition in response to hydrologic events and changes therein. Freshwater inputs and droughts, while not manageable in the short term, must be incorporated in water quality management strategies for these and other estuarine and coastal ecosystems faced with increasing frequencies and intensities of tropical cyclones, flooding, and droughts.     

长江悬浮颗粒物中磷的赋存形态

2006年9~10月在长江采集悬浮颗粒物样品,应用改进后的SEDEX法对磷的赋存形态进行了分析.结果表明,自生磷灰石磷是长江悬浮颗粒物中磷的主要赋存形态.涪陵至万州江段,碎屑磷灰石磷的含量较高,弱吸附态磷和有机磷含量较低,磷的赋存形态主要受悬浮颗粒物含量的影响;香溪至葛洲坝下江段,弱吸附态磷和有机磷含量较高,碎屑磷灰石磷含量较低,磷的赋存形态主要受浮游植物生长及泥沙粒径的影响;城陵矶至大通江段,有机磷、碎屑磷灰石磷和铁结合态磷含量较高,弱吸附态磷含量较低,磷的赋存形态主要受泥沙粒径和洞庭湖输入的影响.长江悬浮颗粒物中生物可利用磷占颗粒态磷的45.6%,长江上游水体中生物可利用磷含量较低,下游含量较高.     

Zooplankton abundance and community structure in a mesohaline North Carolina estuary

The lower Neuse River Estuary is a temperate mesohaline system which forms the major southern tributary of Pamlico Sound, North Carolina. The crustacean zooplankton of this well-mixed system were sampled for a 20-month period from May 1988 through December 1989. A submersible pump was used to sample both the entire water column and the sediment surface. Seasonal dominants included the calanoid copepodsAcartia tonsa andParacalanus crassirostris in summer, the cyclopoid copepodOithona colcarva in fall, the cladoceranPodon polyphemoides in winter, and harpacticoid copepods in spring. Non-naupliar biomass over the study period consisted of 38.8%A. tonsa, 7.7%P. crassirostris, 21.2%O. colcarva 23.6% harpacticoid copepods, and 6.0% cladocerans. The remainder of the biomass consisted ofPseudodiaptomus coronatus and barnacle nauplii. Mean total copepod densities ranged from 600 m^?3 in May 1988 to 180,000 m^?3 in August 1988. Mean copepod densities for 1989 were 25,000 m^?3. Maximum densities during both years occurred during summer, with subsequent descreases throughtout the year until early spring. Abundances of total copepods, and ofAcartia tonsa in particular, were significantly correlated with water temperature, but with neither chlorophylla, phytoplankton productivity, nor any of an array of other physical or chemical variables. Regression analyses using data from this investigation, and supported by results from other regional studies, indicate that water temperature is likely the single most important variable predicting zooplankton temporal abundance in North Carolina estuaries.     

Time series analyses of suspended sediment concentrations at North Inlet,South Carolina

Daily water samples have been collected at three stations in the North Inlet (South Carolina) marshestuary system since February 1981 as part of the NSF Long-Term Ecological Research (LTER) project. As a result of this sampling regime, nearly continuous time series of inorganic and organic suspended sediment, particulate organic carbon (POC), Secchi disk, salinity, and water temperature are now available. Power spectrum analysis of these data reveals that most of the explainable variance in the inorganic suspended sediment, POC, and Secchi disk data is related to a yearly cycle that is strongly coherent with water temperature such that high turbidity is associated with high water temperature. Only a small fraction of the explainable variance is associated with frequencies that can be related to the semidiurnal tide. Simple correlation analysis also indicates that turbidity is more closely associated with water temperature than with tide height or salinity. The ratio of POC to inorganic suspended sediment shows no discernible power spectra peaks and is weakly, but inversely, correlated with temperature. From these results we hypothesize that temperature-regulated bioturbation is the main factor controlling turbidity variations in the system. The lack of a strong inverse correlation between turbidity and salinity suggests that river runoff has little immediate impact on the suspended sediment of nearshore coastal waters in systems similar to North Inlet.     

Parasites and cyclopoid predators of age-0 fish in the Roanoke River,North Carolina

Altered river flow has been suggested as a cause for the low recruitment of striped bass,Morone saxatilis, in the Roanoke River (North Carolina) because of its effect on the proximity of zooplankton and larval striped bass. This results in unsuccessful feeding and subsequent starvation, which was considered to be a major mortality factor. Other mortality factors, such as parasitism and copepod predation on age-0 fish, may also be regulated to some extent by changes in river flow. The relationship of cestode plerocercoids, trematode metacercaria, mussel glochidia, and cyclopoid copepod predators with age-0 fish was evaluated in the lower Roanoke River and western Albemarle Sound from plankton net collections made in 1984 to 1986 and 1988. Plerocercoid prevalence was higher under low river flow conditions than under high flow conditions in darters (Percidae; 16.7% vs. 9.2%), minnows (Cyprinidae; 28.8% vs. 4.7%), andMorone (1.9% vs. 0%). Gut analysis of the age-0 fish revealed that copepods (source of the plerocercoids) were a major diet component ofMorone and darters but not of minnows or herring (Clupeidae). Decreases in river flow were associated with increases in copepod density (Pearson r=?0.62; p=0.0001) and plerocercoid prevalence inMorone (Pearson r=?0.29; p=0.03). The low correlation value forMorone may be quite strong considering the complexity of the variables associated with prevalence. Metacercaria were found only inMorone and minnows, and prevalence and mean intensity were less than that found for plerocercoids. Mussel glochidia prevalence was less than 0.5% for all affected taxa, an order of magnitude less that that found in other studies. The low value may indicate that the mussel population in the Roanoke River is declining. Prevalence of attacks by the predatory copepodMesocyclops edax on age-0 fish was similar to that in Chesapeake Bay, and striped bass was the primary prey. Spatial and temporal proximity of copepods and fish prey may be the key factors in regulating copepod attacks. The low prevalence of parasites and copepod predators seen in this tudy would suggest that mortality from these sources may not be a major factor in age-0 recruitment in this system. Confirmation of these conclusions would require a more controlled experimental approach.     

Fate of the Evros River suspended particulate matter in the northern Aegean Sea

Evros River is the most important river flowing into the North Aegean Sea (eastern Mediterranean) in terms of freshwater discharge, and the second largest one of Eastern Europe after the Danube River. Salinity and temperature measurements, together with suspended particulate matter concentrations were obtained in various depths at 14 stations in the adjacent Alexandroupolis Gulf during four seasons (June 1998, September 1998, February 1999 and March 2000) in order to investigate the particle dynamics and distributions in the northern Aegean Sea. Analysis of the collected data, together with particle observations under the scanning electron microscope and study of satellite images showed that, under certain circumstances driven by the hydrological and wind regime of the area, the Evros River particulate matter, with the associated pollutants, can be transferred far away from the estuary and implicitly comprise a hazardous factor for the environmental status of the northern Aegean Sea. This fact, combined with the future construction of the Burgas-Alexandroupolis pipeline, may cause a negative impact on the studied natural ecosystem.     

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.     

Seasonal and tidal monthly patterns of particulate matter dynamics in the Columbia River estuary

We investigated seasonal and tidal-monthly, suspended particulate matter (SPM) dynamics in the Columbia River estuary from May to December 1997 using acoustic backscatter (ABS) and velocity data from four long-term Acoustic Doppler Profiler (ADP) moorings in or near the estuarine turbidity maximum (ETM). ABS profiles were calibrated and converted to total SPM profiles using pumped SPM samples and optical backscatter (OBS) data obtained during three seasonal cruises. Four characteristic settling velocity (W _s) classes were defined from Owen Tube samples collected during the cruises. An inverse analysis, in the form of a non-negative least squares minimization, was used to determine the contribution of the four,W _s-classes to each, total SPM profile. The outputs from the inverse analyses were 6–8 mo time-series ofW _s-specific SPM concentration and transport profiles at each mooring. The profiles extended from the free surface to 1.8–2.7 m from the bed, with 0.25–0.50 m resolution. These time series, along with Owen Tube results and disaggregated size data, were used to investigate SPM dynamics. Three non-dimensional parameters were defined to investigate how river flow and tidal forcing affect particle trapping: Rouse numberP (balance between vertical mixing and settling) trapping efficiencyE (ratio of maximum SPM concentration in the estuary to fluvial source concentration), and advection numberA (ratio of height of maximum SPM concentration to friction velocity). The most effective particle trapping (maximum values ofE) occurs on low-flow neap tides. The location of the ETM and the maximal trapping migrated seasonally in a manner consistent with the increase in salinity intrusion length after the spring freshet. Maximum advection (high values ofA) occurred during highly stratified neap tides.     

Hydrodynamics of suspended particulate matter in the tidal freshwater zone of a macrotidal estuary (the Seine Estuary, France)

The effects of fortnightly, semidiurnal, and quaterdiurnal lunar tidal cycles on suspended particle concentrations in the tidal freshwater zone of the Seine macrotidal estuary were studied during periods of medium to low freshwater flow. Long-term records of turbidity show semidiurnal and spring-neap erosion-sedimentation cycles. During spring tide, the rise in low tide levels in the upper estuary leads to storage of water in the upper estuary. This increases residence time of water and suspended particulate matter (SPM). During spring tide periods, significant tidal pumping, measured by flux calculations, prevents SPM transit to the middle estuary which is characterized by the turbidity maximum zone. On a long-term basis, this tidal pumping allows marine particles to move upstream for several tens of kilometers into the upper estuary. At the end of the spring tide period, when the concentrations of suspended particulate matter are at their peak values and the low-tide level drops, the transport of suspended particulate matter to the middle estuary reaches its highest point. This period of maximum turbidity is of short duration because a significant amount of the SPM settles during neap tide. The particles, which settle under these conditions, are trapped in the upper estuary and cannot be moved to the zone of maximum turbidity until the next spring tide. From the upper estuary to the zone of maximum turbidity, particulate transport is generated by pulses at the start of the spring-neap tide transition period.