Neap Tidal Cycle Research Articles

Modeling tidal circulation and stratification in Skagit River estuary using an unstructured grid ocean model

Tidal circulation and river plume dynamics in shallow-water estuarine systems with large intertidal zones are complex. Strong asymmetries in tidal currents and stratification often occur in the intertidal zones and subtidal channels over a tidal cycle. The Skagit River is the largest estuary with respect to the discharge of a significant amount of freshwater and sediment into Puget Sound, Washington. It consists of a large intertidal zone with multiple tidal channels near the mouth of the estuary. To simulate the tidal circulation and salinity stratification accurately in the intertidal region, an unstructured grid numerical model with wetting–drying capability and the capability to accurately represent the bathymetry of tidal flats and the geometry of shallow distributary channels is necessary. In this paper, a modeling study for the Skagit River estuary using a three-dimensional unstructured grid, finite-volume coastal ocean model (FVCOM) supported by high-resolution LIDAR data is presented. The hydrodynamic model was validated with observed water surface elevation, velocity, and salinity data over spring and neap tidal cycles under low-river-flow and high-river-flow conditions. Wetting and drying processes in the intertidal zone and strong stratification in the estuary were simulated successfully by the model. Model results indicate that the Skagit River estuary is a highly stratified estuary, but destratification can occur during flood tide. Tides and baroclinic motion are the dominant forcing in the Skagit River estuary, but strong wind events can affect the currents in the intertidal zone significantly. Preliminary analysis also indicated that the salinity intrusion length scale is proportional to the river flow to the −¼ power.

A Composite View of Surface Signatures and Interior Properties of Nonlinear Internal Waves: Observations and Applications

AbstractSurface signatures and interior properties of large-amplitude nonlinear internal waves (NLIWs) in the South China Sea (SCS) were measured during a period of weak northeast wind (∼2 m s−1) using shipboard marine radar, an acoustic Doppler current profiler (ADCP), a conductivity–temperature–depth (CTD) profiler, and an echo sounder. In the northern SCS, large-amplitude NLIWs propagating principally westward appear at the tidal periodicity, and their magnitudes are modulated at the spring–neap tidal cycle. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a ∼6 m s−1 surface wind speed (i.e., 3 times greater than the actual surface wind speed). The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial structure of the NLIW. The observed average half-amplitude full width of NLIWs λη/2 is 1.09 ± 0.2 km; the average half-amplitude full width of the enhanced scattering strength λI/2 is ∼0.57 λη/2. The average half-amplitude full width of the enhanced horizontal velocity convergence of NLIWs λ∂xu/2 is approximately equal to λI/2. The peak of the enhanced surface scattering leads the center of NLIWs by ∼0.46 λη/2. NLIW horizontal velocity convergence is positively correlated with the enhancement of the surface scattering strength. NLIW amplitude is positively correlated with the spatial integration of the enhancement of the surface scattering strength within the convergence zone of NLIWs. Empirical formulas are obtained for estimating the horizontal velocity convergence and the amplitude of NLIWs using radar measurements of surface scattering strength. The enhancement of the scattering strength exhibits strong asymmetry; the scattering strength observed from behind the propagating NLIW is 24% less than that observed ahead, presumably caused by the skewness and the breaking of surface waves induced by NLIWs. Above the center of NLIWs, the surface scattering strength is enhanced slightly, associated with isotropic surface waves presumably induced or modified by NLIWs. This analysis concludes that in low-wind conditions remote sensing measurements may provide useful predictions of horizontal velocity convergences, amplitudes, and spatial structures of NLIWs. Further applications and modification of the presented empirical formulas in different conditions of wind speed, surface waves, and NLIWs or with other remote sensing methods are encouraged.

Heat budget for a shallow, sinuous salt marsh estuary

An experimental study of temperature cycles and the heat budget in the Duplin River, a tidal creek bordered by extensive intertidal salt marshes, was carried out in late summer of 2003 and spring of 2004 near Sapelo Island on the central Georgia coast in the southeastern US. Three water masses are identified with differing temperature and salinity regimes, the characteristics of which are dictated by channel morphology, tidal communication with the neighboring sound, ground water hydrology, the extent of local intertidal salt marshes and side channels and the spring–neap tidal cycle (which controls both energetic mixing and, presumably, ground water input). For the first experiment, heat budgets are constructed for the upper (warmer) and lower (cooler) areas of the Duplin River showing the diminishing importance of tidal advection away from the mouth of the creek along with the concomitant increase in the importance of both direct atmospheric fluxes and of interactions with the marsh and side creeks. The second experiment, in the spring of 2004, reexamines the heat budget on seasonal and daily averaged scales revealing the decreased importance of advective fluxes relative to direct atmospheric fluxes on this scale but the constant importance of marsh/creek interactions regardless of time scale or season. Short period temperature fluctuations which affect larval development are examined and analogies are drawn to use heat to understand the marsh as a source of sediment, carbon and other nutrients.

Photosynthetic activity, photoprotection and photoinhibition in intertidal microphytobenthos as studied in situ using variable chlorophyll fluorescence

The photosynthetic activity of microphytobenthos biofilms was studied in situ on an intertidal mudflat of the Ria de Aveiro, Portugal. Time series of physical variables characterizing the microenvironment at the sediment photic zone (incident solar irradiance, temperature, salinity), photophysiological parameters and productive biomass of undisturbed microalgal assemblages were measured during daytime low-tide periods along one spring–neap tidal cycle, with the objective of (1) characterizing the short-term variability in photosynthetic activity in situ, (2) relating it with the changing environmental conditions and (3) with the operation of physiologically (xanthophyll cycle) and behaviorally (vertical migration) based photoprotective processes, and (4) assessing the occurrence of photoinhibition. Pulse Amplitude Modulated (PAM) fluorometry was applied to measure photosynthetic activity (the effective and maximum quantum yield of photosystem II, Δ F/ F m′ and F v/ F m; the photosynthesis index EFY; rapid light-response curves (RLC)), the photoprotective operation of the xanthophyll cycle and photoinhibition (non-photochemical quenching, NPQ; quantum efficiency of open RCs, F v′/ F m′), and vertical migration (productive biomass, F o). The photosynthetic activity was found to be strongly affected by the cumulative light dose received during the morning low-tide periods. The fluorescence indices Δ F/ F m′, EFY, F v′/ F m′ and RLC parameters were more depressed under high irradiances when clear sky was present during the morning low tide than when foggy conditions reduced the light dose received during a comparable period. Productive biomass exhibited maximum values in the first hours of the morning, followed by a steep decrease when irradiance reached moderate levels, due to the downward migration of the microalgae. This photophobic migratory response appeared to display a photoprotective role, allowing Δ F/ F m′ to remain near optimum values until irradiance reached values as high as 750 μmol m −2 s −1. The response to high light also included the formation of NPQ, expected to represent mainly the operation of the xanthophyll cycle, which attained high values, above 5.9 for 1500 μmol m −2 s −1. Despite the photoprotection provided by energy-dissipation processes and photophobic behaviour, the light response of most photophysiological parameters showed a clear counter-clockwise hysteresis pattern, indicating the occurrence of photoinhibition. Hysteresis was due to the incomplete recovery of photosynthetic activity during the afternoon low tide, and its magnitude was dependent on the morning light doses.

Tidal and seasonal dynamics of a muddy inner shelf environment, Gulf of Papua

Subaqueous delta clinoforms are accretionary features created on the continental shelf where there is a large supply of sediment. Sediment is transported across the shallow topset region and accumulates rapidly in the deeper foreset region. In the Gulf of Papua, Papua New Guinea, the absence of cyclonic storms makes tidal currents and seasonal variation of wind direction primary controls on the timing and magnitude of across‐shelf sediment transport. To investigate processes that disperse sediment across the topset of the Fly River clinoform, anchor stations were occupied during spring and neap tidal cycles of the trade wind season, the monsoon season, and the transition period. During the energetic trade wind season, surface waves coupled to strong spring tide currents generated peaks in suspended‐sediment concentration (>100 g L−1 at some locations). However, seabed erosion and deposition over tidal timescales could not be discerned, indicating that sediment was being advected across the outer topset. Although seaward gravity flows occur with significant spatial heterogeneity, they are important for carrying sediment to the foreset region because the near‐bed circulation generally has a landward net flow. Seaward transport is restricted to the energetic trade wind season. During the quiescent monsoon season, 4–11 cm of erosion occurs on parts of the outer topset as fluvial sediment is temporarily deposited closer to shore. Seasonal patterns in sediment dispersal are likely impacted by El Niño–Southern Oscillation, and consideration of El Niño conditions of 2002–2003 are reflected in the interpretation of observations.

Morphological change and sediment dynamics of the beach step on a macrotidal gravel beach

The morpho-sedimentary evolution of a pure gravel beach step over a tidal cycle is examined during fairweather conditions using detailed measurements of nearshore hydrodynamics, morphological and sedimentary change, and nearshore sediment transport. The characteristics of the beach step are analysed with specific reference to the concurrent dynamics of the beachface, a departure from previous studies which have treated the step as a feature isolated from the nearshore region as a whole. The step and berm are both accretionary features strongly linked to tidal stage, yet their temporal evolution is independent, the relaxation time of the berm being linked to the spring–neap tidal cycle, and that of the more transient step linked to the semi-diurnal tide. Over this time-scale the beachface is a closed sedimentary unit, although the beach step may be differentiated from the beachface using sedimentary moments. Indeed, despite the location of the step in the region of wave breaking, it has relatively stable sedimentology, remaining characteristically coarser and more leptokurtic than the swash zone. Co-spectral analysis between nearshore bed motion and cross-shore current velocity reveals that significant nearshore sediment transport occurs at sub-incident frequencies in response to wave groups. Motion of the nearshore bed is not a linear function of velocity magnitude or direction, so it is likely that there is a role for the various mechanical properties of the bed. Therefore a better description of nearshore sediment transport in the region of the beach step would require instantaneous sediment size information, allowing the use of a time-variant friction factor.

Sandwave migration in Monterey Submarine Canyon, Central California

Repeated high-resolution multibeam bathymetric surveys from 2002 through 2006 at the head of the Monterey Submarine Canyon reveal a sandwave field along the canyon axis between 20 and 250 m water depth. These sandwaves range in wavelength from 20 to 70 m and 1 to 3 m in height. A quantitative measure was devised to determine the direction of sandwave migration based on the asymmetry of their profiles. Despite appreciable spatial variation the sandwaves were found to migrate in a predominantly down-canyon direction, regardless of season and tidal phases. A yearlong ADCP measurement at 250 m water depth showed that intermittent internal tidal oscillations dominated the high-speed canyon currents (50–80 cm/s), which are not correlated with the spring–neap tidal cycle. Observed currents of 50 cm/s or higher were predominantly down-canyon. Applying a simple empirical model, flows of such magnitudes were shown to be able to generate sandwaves of a size similar to the observed ones.

Performance of a nowcast/forecast system for Prince William Sound, Alaska

A coastal ocean extended Prince William Sound nowcast/forecast system (EPWS/NFS) has been running semi-automatically for an extended domain of Prince William Sound (PWS), Alaska for 2 years. To determine the performance of this modeling system, an assessment is conducted. EPWS/NFS and PWS/NFS (viz., its predecessor) nowcasts are compared with observed time series of sea surface temperature (SST) and coastal sea level (CSL) at a few stations, and to velocity profiles from a moored ADCP. With the extension of the model domain to include the continental shelf outside PWS and forced by an operational global ocean model (Global-Navy Coastal Ocean Model (Global-NCOM)) and a 2D tidal model at the open boundary, EPWS/NFS has achieved significant improvement over PWS/NFS, which covered only PWS per se, for most of the predicted variables in this study. In both magnitude and phase, EPWS/NFS accurately predicts the coastal tide fluctuations, as well as M2 tidal currents in Central Sound, although significant errors in coastal tides exist during some spring and neap tide cycles. Other than for the tidal motions, EPWS/NFS generally produces less energetic CSL and velocity variations than those observed. In comparison, although PWS/NFS well predicts the coastal tides, it suffers from the absence of low-frequency CSL variations, as well as misprediction of M2 tidal currents in Central Sound. For 40 h low-passed PWS/NFS and EPWS/NFS velocities, significant phase error occurs during the model–date comparison period, while EPWS/NFS nowcasts generally produce less root-mean-square-error (rmse) and smaller correlations with the observations than PWS/NFS does. Both observations and EPWS/NFS have similar vertical profiles of baroclinic velocity standard deviations, but some substantial discrepancies occur in the velocity direction. Also, in the Central Sound, EPWS/NFS predicts well the SST seasonal cycle and a major cooling event during the summer 2005. However, for periods shorter than 1 week, both PWS/NFS and EPWS/NFS SST underestimated the observed fluctuations by an order of magnitude.

Numerical study of tidal front with varying sharpness in spring and neap tidal cycle

The temporal variation of tidal-front sharpness (i.e., the maximal gradient of sea surface temperature (SST)) in Iyo-Nada, Japan has been investigated using SST obtained by a commercial ferryboat. Tidal-front sharpness varies in time with a period of 15 days. A numerical model approach was also adopted to investigate the temporal variation of frontal sharpness. The numerical model, which contains a restoring term to express the tidal front reconstructed fortnightly by tides, reproduces the tidal front accompanied by growing and/or decaying frontal waves. The amplitude of modeled frontal sharpness agrees well with the observation. The amplitude of sharpness is much smaller than the observed value, unless frontal waves develop along the modeled front. This therefore implies that tidal fronts are destroyed mainly due to growing frontal waves, and are restored fortnightly at spring tides. We quantitatively evaluated the subsurface intrusion of seawater into the stratified region from the mixed region by conducting passive-tracer experiments. We find that the cross-frontal transport with frontal waves is 4.9 times larger than that without frontal waves. In addition, the cross-frontal transport reaches a long distance (about 25 km) because of heton (mushroom)-type eddies developing along the front with frontal waves.

Quantitative relationship of runoff and tide to saltwater spilling over from the North Branch in the Changjiang Estuary: A numerical study

This study focuses on the quantitative relationship of runoff and tidal range to the saltwater spilling over (SSO) from the North Branch in the Changjiang Estuary. In order to determine the intensity of SSO, which is directly influenced by the upstream river runoff and downstream tidal range simultaneously, the net transectional water mass flux (NTWF) is numerically calculated on diurnal and semimonthly basis, respectively. Since a series of experiments were conducted, including river runoff ranging from 7000 to 40,000 m 3 s −1 and each experiencing throughout a spring–neap tidal cycle, the corresponding NTWFs were obtained. In the diurnal case, the NTWF can be determined by 533.1 × exp(0.361 × Isso)−533.1 (Isso ≥ 0) and 226.0 × Isso (Isso < 0), where Isso (index of SSO) is defined as TR−1.58 × exp(0.2457 × RF/10,000), TR is the tidal range (m) at the Qinglong Harbor and RF is the runoff (m 3 s −1). The Isso has the same sign as the diurnal NTWF. In the semimonthly case, the NTWF is determined by 701.89 × ln(−RF/10,000+6.2)−553.54, which decreases with the increasing of runoff and declines to zero when the runoff reaches 40,000 m 3 s −1. The tidal range and runoff are measured at Qinglong Harbor Station and Datong Station everyday, respectively. Thus, the NTWF can be easily obtained by using these formulae.

On the vertical structure of the Rhine region of freshwater influence

An idealised three-dimensional numerical model of the Rhine region of fresh water influence (ROFI) was set up to explore the effect of stratification on the vertical structure of the tidal currents. Prandle’s dynamic Ekman layer model, in the case of zero-depth-averaged, cross-shore velocities, was first used to validate the response of the numerical model in the case of barotropic tidal flow. Prandle’s model predicted rectilinear tidal currents with an ellipse veering of up to 2%. The behaviour of the Rhine ROFI in response to both a neap and a spring tide was then investigated. For the given numerical specifications, the Rhine plume region was well mixed over the vertical on spring tide and stratified on neap tide. During spring conditions, rectilinear tidal surface currents were found along the Dutch coast. In contrast, during neap conditions, significant cross-shore currents and tidal straining were observed. Prandle’s model predicted ellipse veering of 50%, and was found to be a good indicator of ellipticity magnitude as a function of bulk vertical eddy viscosity. The modelled tidal ellipses showed that surface currents rotated anti-cyclonically whereas bottom currents rotated cyclonically. This caused a semi-diurnal cross-shore velocity shearing which was 90° out of phase with the alongshore currents. This cross-shore shear subsequently acted on the horizontal density gradient in the plume, thereby causing a semi-diurnal stratification pattern, with maximum stratification around high water. The same behaviour was exhibited in simulations of a complete spring–neap tidal cycle. This showed a pattern of recurring stratification on neaps and de-stratification on springs, in accordance with observations collected from field campaigns in the 1990’s. To understand the increase in ellipticities to 30% during neaps and the precise shape of the vertical ellipse structure, stratification has to be taken into account. Here, a full three-dimensional numerical model was employed, and was found to represent the effect of de-coupling of the upper and lower layers due to a reduction of mixing at the pycnocline.

Identifying the role of tides, rainfall and seasonality in marsh sedimentation using long-term suspended sediment concentration data

This study utilizes data on suspended sediment concentrations obtained daily from a terminal tidal creek over a period of a decade as part of the Long-Term Ecological Research Program (LTER) to examine the mechanisms contributing to salt marsh sedimentation. A month long time-series of tidal velocity and amplitude measurements are used to predict tidal currents, elevation and phase during the sediment data collection periods. Tidal variation of suspended sediment load and fluxes are estimated and are used to evaluate the role of spring–neap tidal cycles, rainfall and seasonality in salt marsh sedimentation. The results indicate that spring tides correspond to periods of higher sediment concentrations and sediment import while neap tides correspond to periods of lower concentrations and slight sediment export. This is due to higher current velocities during spring tide, which increase the availability of sediments within the marsh system, in combination with an increase in the duration and area of marsh inundation during spring tides. Low water rainfall is responsible for elevated suspended sediment concentration in the channels due to increased erosion of the exposed marsh surface and creek banks. These low-tide rainfall events are not strictly erosional periods but rather appear to be associated with periods of sediment redistribution within the marsh system, as sediments eroded from the marsh surface are advected into the back marsh basin and deposited during subsequent high waters. In the back marsh, sediment import rates are increased by approximately 50% during periods of rainfall as compared to sediment import rates corresponding to periods without local rainfall, suggesting increased sediment accumulation on the marsh surface during these times. Finally, a seasonal signal of suspended sediment concentrations is observed with highest average concentrations found in the summer when water temperature and biological activity are at a maximum. Despite this increased sediment availability in summer, the fall season is responsible for the majority of sediment imported annually (55%). This occurs because mean sea level is greatest during these times and leads to increased periods of marsh inundation. Therefore, the frequency and duration of marsh inundation is more important than sediment availability in controlling surface accumulation within this marsh system.

A 3D boundary-fitted barotropic hydrodynamic model for the New York Harbor region

A three-dimensional barotropic hydrodynamic model application to the New York Harbor Region is performed using the Boundary-Fitted HYDROdynamic model (BFHYDRO). The model forcing functions consist of surface elevations along the open boundaries, hourly winds, and fresh water flows from the rivers and sewage flows. A comprehensive skill assessment of the model predictions is done using observed surface elevations and three-dimensional currents. The model-predicted surface elevations compare well with the observed surface elevations at four stations. Mean errors in the model-predicted surface elevations are less than 4% and correlation coefficients exceed 0.985. Model-predicted three-dimensional currents at Verrazano Narrows show excellent comparison with the observations, with mean errors less than 11% and correlation coefficients exceeding 0.960. Model-predicted three-dimensional currents at Bergen Point compare well with the observations, with mean errors less than 15% and correlation coefficients exceeding 0.897. The surface elevation amplitudes and phases of the principal tidal constituents at nine tidal stations, obtained from a harmonic analysis of a 60-day simulation compare well with the observed data. The predicted amplitude and phase of the M 2 tidal constituent at these stations are, respectively, within 5 cm and 6° of the observed data. The model-predicted tidal ellipse parameters for the major tidal constituents compare well with the observations at Verrazano Narrows and Bergen Point. The model-predicted along channel sub-tidal currents also compare well with the observations. The semi-diurnal tidal ranges and spring and neap tidal cycles of the surface elevations and currents are well reproduced in the model at all stations. The observed currents at Bergen Point were shown to be flood dominant through tidal distortion analysis. The model-predicted currents also showed Newark Bay and Arthur Kill to be flood dominant systems. The model predictions showed the occurrence of a pronounced flow separation and formation of a pair of counter-rotating eddies in the Lower New York Bay. The model-predicted residual circulation pattern showed many gyres in the Harbor Region and Jamaica Bay.

Late-Holocene evolution of the Mahakam delta, East Kalimantan, Indonesia

The late-Holocene Mahakam delta, located along the tropical eastern shore of Kalimantan, Indonesia, is considered to be a textbook example of a mixed tide–fluvial dominated delta system. The delta prograded about 60 km during the past 5000 years, which led to the development of a distinct network of distributary and tidal channels. Wave action is low due the limited fetch in the narrow strait of Makassar. Mahakam River discharge is about a quarter of the Mississippi River discharge and is characterized by absence of flood surges. Therefore, natural levees, crevasse splays and avulsions are absent in the delta plain. For the past four decennia, both modern and ancient Mahakam delta deposits have been studied in detail in order to better understand subsurface Miocene and Tertiary Mahakam deposits, which host large volumes of hydrocarbons. This study focuses on the dynamics and stratal patterns of delta plain, delta-front platform deposits and suspended sediments. Due to the predominance of semi-diurnal tides and the associated flow reversals, depositional patterns are highly variable which has resulted in the formation of characteristic sand–mud couplets. The distribution of the sand–mud couplets found in this study differs from previously proposed conceptual models. They are limited to the fluvial domain and form in the distributary channels (lateral channel bar) or at the fluvial dominated delta-front platform, which flanks the mouth bar deposits in offshore direction. The sand–mud couplets which formed as delta-front platform and lateral channel bar deposits are similar and can only be identified based on their 14C age. The sand content decreases significantly towards the tidal dominated areas due to limitation in transport capacity. Turbidity measurements taken in front of the river mouth also show rapid settlement of river plume sediments. Some 22 new AMS 14C dates show that late Holocene sea level history resembles the eustatic sea level curve giving a first approximation of the Late Holocene sea level history for East Kalimantan. The dates suggest that the progradational delta system evolved under conditions of slowly rising sea level, which compares well to the eustatic sea level curve. In addition, calculated averaged deposition rates of the sand–mud couplets indicate that deposition is driven by the spring–neap tide cycles instead of the daily tidal cycle.

Time-series observations during the low sub-surface oxygen events in Narragansett Bay during summer 2001

A series of automated water column time-series measurement systems has been established in Narragansett Bay, Rhode Island. These systems measure near surface and near bottom temperature, salinity, oxygen, pH, chlorophyll and pressure at 15-min intervals. The data obtained from two buoy sites during the period of July through September 2001 reveal the occurrence of episodic surface phytoplankton blooms followed by subsurface hypoxic events, particularly in the upper portions of the estuary including the area known as the Providence River. Three hypoxic events occurred at monthly intervals in July, August and September. Their timing, and that of the phytoplankton blooms that preceded them, is linked to the periodic weak neap tidal cycles that occur alternately with somewhat stronger neap tidal cycles. The connection between surface blooms/subsurface hypoxia and tidal range is attributed to water column stratification and the ability of moderate changes in tidal amplitude to reduce stratification through vertical mixing. During the summers of 2002 and 2003 we predicted and subsequently observed hypoxic events in the Upper Bay. Based on these findings we can project into future years the times when summer blooms and hypoxia are most likely to occur in the upper portions of Narragansett Bay. The observations from summer 2001 suggest that the oxygen consumption and renewal in the subsurface waters is delicately balanced. Further increases in inputs of nutrients, organic matter, or oxygen-consuming substances could shift this balance from hypoxic to anoxic with substantial impacts on fish and other marine organisms.

Modelling trace metal concentration distributions in estuarine waters

Details are given of a numerical model study of the fate and transport of trace metals in estuarine waters, with particular application to the Mersey Estuary, located along the northwest coast of England. A dynamically integrated model was first developed, including a two-dimensional depth-integrated model and a one-dimensional cross-sectional averaged model. This model was then refined to predict the hydrodynamic and sediment and trace metal transport processes in the Mersey Estuary. Details are given of the development of a governing equation of the total trace metal transport, including both dissolved and particulate metals. The model was first calibrated against field data, collected during spring and neap tidal cycles, for water levels, salinity and suspended sediment. The calibrated model was then used to investigate the trace metal transport processes in the Mersey Estuary, with the partition coefficient between the dissolved and adsorbed particulate phases being modelled as a function of salinity. Comparisons were also made between the model predictions and field-measured data along the estuary. Reasonable agreement between the model results and field data has been obtained, indicating that the novel approach to model metal concentration distributions is capable of representing the fate and transport of trace metals in estuarine environments and can be used as computer-based tool for the environment management of estuarine waters.

The effects of semi-lunar spring and neap tidal change on nutrients cycling in the intertidal sediments of the Yangtze estuary

During a semi-lunar tidal cycle from full moon till new moon, tide is characterized by the periodic change in spring and neap tide. Under the in situ conditions of light and temperature, the influence of a semi-lunar spring and neap tidal cycle on nutrient cycling in intertidal flat of the Yangtze estuary was simulated in the laboratory in July 2002. Lab experiments showed that NH 4 + and PO 4 3− were always released into overlying waters, while NO 3 − was directed into sediments in the permanently waterlogged systems, suggesting that the long-term waterlogged sediment acts as a significant source for NH 4 + and PO 4 3− , and a sink for NO 3 − in water columns. In contrast, reflooding of intertidal sediments after long-term desiccation promoted the considerable effluxes of NH 4 + , NO 3 − and PO 4 3− into overlying waters, reflecting that the long-term exposed sediment is an important source for nutrients in overlying waters. In addition, the semi-lunar tidal cycle led to the intricate depth distribution patterns of nutrients in intertidal sediments. During long-term exposure, NH 4 + in sediments was quickly transformed into NO 2 − and NO 3 − , while organic P pool might be converted to slightly adsorbed and iron-bound P. Therefore, it is considered that the semi-lunar tidal circulation has the significant influence on the biogeochemical cycle of nutrients in intertidal systems.

Monitoring and Analysis of Water, Salt and Nutrient Fluxes at the Mouth of a Lagoon

An intensive sampling program of physical and chemical parameters has been undertaken at Vassova lagoon, northeast Greece, in order to examine water circulation and estimate the instantaneous and residual fluxes of water, salt, nutrients and chlorophyll-a through the entrance canal of this lagoon. Field data of hydrographic and water quality parameters were collected under neap and spring tidal cycles, and under winter and summer conditions, to account for the fortnight and seasonal effects in the lagoon. The analysis showed that the Eulerian residual transport is positive in direction during both tidal cycles, thus pushing water, salt, nitrate-nitrogen, phosphate-phosphorus and chlorophyll-a inward Vassova lagoon. Tidal pumping was mostly positive during the neap tide and negative during the spring tidal cycle, thus pushing water, salt, phosphorus and chlorophyll-a outwards of Vassova lagoon.

A Prescriptive Bulk Model of Periodic Estuarine Stratification Driven by Density Currents and Tidal Straining

A prescriptive bulk model of periodic estuarine stratification is developed using potential energy anomaly arguments and a bulk vertical eddy viscosity model derived a priori. In the model, vertical mixing is dominated by tidal stirring except around slack water when other weaker ‘residual’ mixing mechanisms, such as due to the shear induced by the gravitational circulation and wind stirring, dominate. The model under certain conditions exhibits bi-stable solutions corresponding to the well-mixed and stratified states found on either side of a tidal mixing front. In the transition from spring and neap tidal cycles, a stratification state change can occur depending on the strength of residual mixing. The model can account for the phase lag observed between changes in tidal stirring and the onset and persistence of stratification, and leads to new theoretical criteria for the position of tidal mixing fronts.

Combined tidal velocity and duration asymmetries as a determinant of water transport and residual flow in Paranaguá Bay estuary

Net transports of water, salt and suspended particulate matter (SPM) for a cross-section in front of the Paranaguá Harbour (Paranaguá Bay, Brazil) are presented for eight distinct tidal cycles. Data include measurements over single spring and neap tidal cycles, during both wet and dry seasons. The main drive forces of circulation and SPM dynamics are identified. Advective transport dominated under moderate to high vertical salinity stratification and weak currents, while mixing processes dominated under well-mixed conditions generated by high currents and low freshwater input. Under partially mixed conditions, both advective and mixing processes were important. The tide-induced residual circulation dictated the magnitude and direction of residual currents and net transports of water and salt, but not of SPM transport. The SPM dynamics was intrinsically related to cyclical processes of erosion, resuspension and deposition driven by tidal currents. The turbulent mixing intensity conditioned the vertical mixing of SPM. Resuspension and vertical mixing were conspicuous in spring cycles, while the horizontal advection preponderated in the neaps. Lags between maximum currents and SPM peak concentrations occurred, with more pronounced hysteresis during ebb periods.