Spring Tidal Cycles Research Articles

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.

The origin of neap–spring tidal cycles

The origin of oceanic tides is a basic concept taught in most introductory college-level sedimentology/geology, oceanography, and astronomy courses. Tides are typically explained in the context of the equilibrium tidal theory model. Yet this model does not take into account real tides in many parts of the world. Not only does the equilibrium tidal model fail to explicate amphidromic circulation, it also does not explain diurnal tides in low latitude positions. It likewise fails to explain the existence of tide-dominated areas where neap–spring cycles are synchronized with the 27.32-day orbital cycle of the Moon (tropical month), rather than with the more familiar 29.52-day cycle of lunar phases (synodic month). Both types of neap–spring cycles can be recognized in the rock record. A complete explanation of the origin of tides should include a discussion of dynamic tidal theory. In the dynamic tidal model, tides resulting from the motions of the Moon in its orbit around the Earth and the Earth in its orbit around the Sun are modeled as products of the combined effects of a series of phantom satellites. The movement of each of these satellites, relative to the Earth's equator, creates its own tidal wave that moves around an amphidromic point. Each of these waves is referred to as a tidal constituent. The geometries of the ocean basins determine which of these constituents are amplified. Thus, the tide-raising potential for any locality on Earth can be conceptualized as the result of a series of tidal constituents specific to that region. A better understanding of tidal cycles opens up remarkable opportunities for research on tidal deposits with implications for, among other things, a more complete understanding of the tidal dynamics responsible for sediment transport and deposition, changes in Earth–Moon distance through time, and the possible influences tidal cycles may exert on organisms

Interactions of river discharge and tidal modulation in a tropical estuary, NE Brazil

Observations of thermohaline properties and currents were undertaken in the Curimatau River estuary (6°18′S), Rio Grande do Norte state (RN), Brazil, during consecutive neap–spring tidal cycles in the austral autumn rainy season. Highly asymmetric neap tide along channel velocities (−0.4 to 0.9 m s−1) and highly stratified conditions were generated by an increase of the buoyancy energy from the freshwater input (RiE≈5.6). During the spring-tidal cycle the river discharge decreased and the longitudinal velocity components were higher, less asymmetrical (−0.8 to 1.1 m s−1) and semidiurnal, associated with moderately stratified conditions (RiE≈0.1) due to the increase of the kinetic tidal energy forcing mechanism. The overall salinity variation from surface to bottom during two tidal cycles was from 20.5 to 36.3 and 29 to 36.7 in the neap and spring tide experiments, respectively; in the last experiment, the tropical water (TW) mass intrusion was enhanced. The net salt transport reversed from down to up estuary during the neap and spring tide experiments, respectively, varied from 6.0 to –2.0 kg m−1 s−1, an indication of changes in the main forcing of the estuary dynamics. Evaluation of a classical steady analytical model, in comparison with nearly steady experimental vertical profiles of velocity, shows an agreement classifiable as reasonably fair.

Diel and tidal variations in larval fish exchange in the mouth region of the Gamtoos Estuary, South Africa

Diel and tidal variations in density of larval fishes were monitored over one neap and one spring tidal cycle in the mouth region of the warm temperate Gamtoos Estuary, South Africa. Data were collected over two 24h periods, using mixed method sampling with WP2 nets and a pushnet to sample both channel and margin waters. Larval fishes collected included 20 species from 12 families. Density varied with tide and diel state during the study period, with significantly higher catches recorded on the spring tide. Density of larval fishes peaked after the onset of darkness, predominantly associated with the nocturnal ebb tide. Large numbers of preflexion gobiid larvae were recorded on this tide state, this behaviour serving to facilitate movement out of the estuary for gobies that undergo a marine phase in their development. Spatial variations in larval fish distribution were also noted. Preflexion larvae were caught mainly in mid-channel waters, whilst postflexion larvae were more often encountered in the slower-flowing marginal waters. Data for a hydrodynamic model of tidal exchange of fish larvae in the estuary mouth region were collected concurrently with sampling and were used to model exchange of larval fishes over the neap and spring tidal cycle by extrapolating actual densities, using modelled tidal volume. Findings suggest that for some species, tidal exchange could escalate to millions of individuals over a single tidal cycle.

Longitudinal Salt and Sediment Fluxes in a Tropical Estuary: Itamaracá, Brazil

Abstract Along-channel velocity, salinity, and total suspended sediment concentration profiles were measured during 15 complete neap and spring tidal cycles in the dry and rainy seasons in four cross-sections in a tropical estuary, Itamaraca, Pernambuco, Brazil. The net longitudinal fluxes of salt and sediments were decomposed into advective, tidal, and cross-sectional shear transports. The advective transport of both salt and suspended sediments were for most parts export-directed and greater than the combined river discharge, suggesting a non-steady state. The dispersive tidal transport was decomposed into four separate mechanisms of which tidal pumping transport and tidal wave transport (similar to transport by Stokes’ drift) are the main transport mechanisms. The cross-sectional shear transport was negligibly small, and thus, vertical nor lateral effects are significant with respect to longitudinal dispersion in the case of Itamaraca.

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.

Rapid sediment deposition and fine‐scale strata formation in the Hudson estuary

A 9 month time series of tripod‐mounted optical and acoustic measurements of sediment concentration and bed elevation was used to examine depositional processes in relationship to hydrodynamic variables in the Hudson River estuary. A series of cores was also taken directly under and adjacent to the acoustic measurements to examine the relation between the depositional processes and the resulting fine‐scale stratigraphy. The measurements reveal that deposition occurs as a result of sediment flux convergence behind a salinity front and that the accumulation rates are sufficient to deposit up to 25 cm of new high‐porosity sediment in a single ebb‐tidal phase. Subsequent dewatering and erosion reduces the thickness of the initial deposit to several centimeters. These depositional events were only observed on spring tides. Ten depositional events during two spring tidal cycles produced a seasonal deposit of 18 cm, consistent with estimates of seasonal deposition from cores. A proxy for near‐bed suspended grain size variations was estimated from the combined acoustic and optical measurements, implying that the erosional processes resuspend only the finer‐grained sediments, thus leaving behind silt and very fine grained sand beds. The thickness of the deposited homogenous clayey silt beds, and the vertical separation between beds interlaminated with silt and very fine sand, are roughly consistent with the acoustic measurements of changes in bed elevations during deposition and erosion. The variability in individual bed thickness is the result of variations of processes over an individual tidal cycle and is not a product of variations over the spring neap fortnightly timescale.

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.

Swash Bar Morphodynamics in the Danish Wadden Sea: Sand Bed Oscillations and Suspended Sediment Flux during an Accretionary Phase of the Foreshore Cycle

The landward migration of an intertidal swash bar at Skallingen (Danish Wadden Sea), together with the associated hydrodynamics and suspended sediment transport, were monitored over several spring tidal cycles during an accretionary phase of the foreshore cycle. The landward migration was not a simple linear displacement of the bar form; instead, regular intra-tidal sand bed oscillations (average height ∼ 0.12 m; average spacing ∼ 24 m; average steepness ∼ 0.005) moved sand landward across the seaward slope. The estimated migration rate of the bed oscillations was ∼ 0.032 m min−1. These values compare favourably with other citations in the literature. The sediment flux associated with these bed oscillations was the suspended sediment transport dominated by a net landward transport primarily by gravity wave frequencies. Infragravity waves were important at certain times in the tidal cycle but were variable in frequency and the associated transport was highly variable spatially. The suspended sediment transport by mean currents was smaller than that induced by the waves, and in almost all cases was directed offshore. The sand bed oscillations appear to be driven primarily by the incident gravity waves, which were also the primary forcing for the landward migration of the swash bar as a whole.

Short-term dynamics and maintenance processes of headland-associated sandbanks: Shambles Bank, English Channel, UK

The short-term (over a spring tidal cycle) dynamics of a headland-associated sandbank (Shambles Bank, English Channel) are investigated, by means of field measurements (synchronous data, using an acoustic Doppler current profiler (ADCP) and repeated side-scan sonar imagery) and two-dimensional (2D) hydrodynamic and sediment transport models. The dynamics of the bank are described in terms of along- and cross-bank velocity components, sand transport pathways and bedform asymmetries. The results showed the occurrence of a net bedload convergent zone along the crest of the bank as a result of anti-clockwise veering of sediment transport towards the crest. This transport pattern is suggested to be the result of the dynamic interplay between two distinct tidally related processes acting over the sandbank, during each phase of the tidal cycle: (a) the formation and different stages of a transient tidal eddy that drives bedload movement during the flood phase of the tide and (b) the bottom-friction induced by the presence of the sandbank that governs the bedload transport dynamics during the ebb phase of the tidal cycle.

Timing of aggregation and larval release by Tanner crabs, Chionoecetes bairdi, in relation to tidal current patterns

Each spring, female Tanner crabs, Chionoecetes bairdi, form high density aggregations at 150 m depth in Chiniak Bay, Kodiak, AK. Within the aggregation, crabs form mounds containing hundreds to thousands of animals. In 1991, over 200 mounds and 100,000 crabs were present in an area of about 2 ha. Previous studies have suggested that aggregation and mating are synchronised with spring tidal cycles. In 1999, we placed a current meter near the aggregation, then observed aggregation behaviour with a video camera sled and remotely operated vehicle (ROV) over an 8-week period. Crabs were collected from the aggregation with the ROV, brought into the laboratory, and the numbers of hatching larvae were determined daily. Mound formation began in mid-April, hatching began around 7 May, and the median hatch date coincided with the strongest tides on 17 May. During this time, net tidal flow reversed for 3–4 days. In the laboratory, individual crabs released an average of 129,400 larvae over a period of 9.4 days. Over 74% of larvae hatched between 2000 and 2400 h. These data suggest that larval hatching is optimised to take advantage of tidal current patterns. It also implies that the timing of larval hatching for C. bairdi changes in monthly steps, and is decoupled from planktonic food production.

Estimation of Cu, Cd and Hg transported by plankton from a contaminated area (Ria de Aveiro)

The efficiency of plankton to sorb/incorporate anthropogenic metals discharged in a contaminated area of the Ria de Aveiro and the amount exported to the rest of the lagoon were examined. Suspended particulate matter and material collected in 63 and 200 μm plankton nets were sampled hourly on a semi-diurnal spring tidal cycle. The collected material was analysed for total Cu, Cd, Hg and Pb. Higher metal concentrations were registered in material collected during the ebb period, which indicates their incorporation into the plankton biomass previously transported into the area during the flood period. On the basis of the water volume variation, plankton biomass associated to the 63 and 200 μm nets and metal concentrations, mass balances were calculated for Cu, Cd and Hg. Clearly, metals associated with plankton were exported from the contaminated area to the rest of the lagoon. The net exported metals (63 μm net) were 223–336% of the imported metals corresponding to 2.4 g of Cd, 67 g of Cu, and 11 g of Hg, over the spring tidal cycle.

Particulate trace metal and major element distributions over consecutive tidal cycles in Port Jackson Estuary, Australia

Particulate trace metal (Cu, Cr, Ni, Pb and Zn) and major element (Fe, Mn and Al) concentrations have been determined following intensive sampling over two consecutive spring tidal cycles in the 'turbidity maximum zone' (TMZ) of the Port Jackson estuary, Australia. Salinity, temperature, pH, dissolved oxygen, suspended particulate matter (SPM) and chlorophyll a were also determined. A three-factor analysis of variance was used to test temporal variability in concentrations of particulate trace metals and major elements as a result of tidal oscillation. Estuarine master variables, such as temperature and pH, varied within a narrow range; nevertheless, the tidal signal was clear for surface and bottom waters. In surface water, no variance was detected in SPM concentrations between consecutive tidal cycles or between tidal stages (i.e. flood, ebb and slack water). In bottom water, however, SPM concentrations were significantly higher (P≤0.05) at flood tide than at slack high water and ebb tide. Concentrations of particulate trace metals and major elements in surface water do not display significant variability between tidal cycles or stages. Nevertheless, differences within each tidal stage were significant (P≤0.05) for all elements. In bottom water, only particulate Fe and Al exhibited significant differences (P≤0.05) between tidal cycles, whereas particulate Ni was the only trace element that presented significant differences (P≤0.05) between tidal stages, following the distribution of SPM, with highest concentrations at flood tide. Among the metals studied, significant variation was found at all three temporal scales examined (i.e. from hours to consecutive tidal cycles), although the patterns of variation were different for each metal. The semi-diurnal fluctuation of SPM and particulate trace metal concentrations during spring tides is interpreted as a resuspension–deposition cycle caused by cyclical oscillations of bottom currents. The results are discussed in the context of the implications of tidal cycle influence on the geochemistry and cycling of particulate trace metals in the Port Jackson estuary.

Short-term water quality variability in two tropical estuaries, central Sumatra

We examined high frequency fluctuations in water quality parameters in two tropical coastal plain estuaries in response to changing tidal flow conditions. The variability in total suspended sediments (TSS), volatile suspended solids (VSS), total organic carbon (TOC) concentrations, and indicators of water quality, including pH, temperature, salinity, and dissolved oxygen, over one spring tidal cycle during the early wet monsoon season was measured in two estuaries in eastern Sumatra. The relatively high rainfall experienced throughout the year, in combination with the recent extensive vegetation clearing and modification of the landscape, resulted in significant concentrations of TSS, VSS, and TOC being discharged to coastal waters. Maximum values are reached on the ebb tide (TSS > 1,013 mg l−1; VSS > 800 mg l−1; TOC >60 mg l−1). The influence of freshwater discharge and tidal flow on water properties of the lower estuaries is also marked by the variability in salinity, dissolved oxygen, and pH over the tidal cycle, with minimum values for each of these parameters following maximum current velocities and after the completion of the strong ebb tide. Estimation of seaward sediment fluxes, which are of significant interest in a region where rapid environmental change is occurring, would require further examination of sedimentary processes, such as resuspension and advection of sediment, as well as a consideration of neap-spring tidal variations and the effect of seasonality on estuarine circulation.

Semidiurnal Dynamics of Salinity, Nutrients and Suspended Particulate Matter in an Estuary in the Seto Inland Sea, Japan, during a Spring Tide Cycle

The physical and chemical variability of the water column at subtidal station of an estuary in the Seto Inland Sea, Japan, was studied over a 24-hour period during a spring tide (tidal range ca. 2 m) in May 1995. Surface water and several depths through the water column were monitored every one and two hours, respectively. At each occasion, water temperature, salinity and dissolved oxygen concentration were measured and water samples were collected for the determination of nutrients and suspended particulate matter (SPM). Disruptive changes in the physical and chemical characteristics of the water was produced by the tidal cycle and the mixing of water masses of different origin. These changes were highly significant both spatially and temporally, yet with varying effects on physical parameters, nutrients and the different components of SPM. Significant differences in nutrient concentrations were also observed when the data-set was divided into ebb and flood components, irrespective of the depth. Nitrate and nitrite rose to 1.8 times higher during the flood. Spatial differences of SPM were less marked than those of nutrients, only particulate organic carbon (POC) being significantly higher at the surface than in the intermediate and the lower layer. Both POC and pheopigment concentrations increased markedly through the water column, being highest shortly before the lower low tide. In contrast, suspended solid (SS) content increased sharply after the lower low tide (>40 mg l−1) and this coincided with a marked decrease of the C/SS content (<20 mg g−1). The lagtime between POC and SS tidal transport was caused by particle resuspension from the exposed intertidal sediments as the tidal level rose, and particle transport selection in relation to the tidal state.

Acoustic remote sensing of internal solitary waves and internal tides in the Strait of Gibraltar

High-frequency underwater acoustic transmissions across the Strait of Gibraltar are used to examine the feasibility of acoustically measuring several physical processes in the Strait, a difficult area to sample with conventional instruments. Internal undular bores propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water. As they cross the acoustic path they are recognized by their scattering effects in the acoustic record. The time between internal bore crossings is influenced more by the tidal phase of the bore release at the Camarinal Sill than by variability in the bore’s propagation time to the acoustic path. When internal bores were present, the acoustic arrival patterns could be classified as one of three types with different internal bore and internal tide amplitudes. The arrival types alternate during spring to neap tide transitions, suggesting that internal bore amplitude is not linearly related to tidal height. The sensitivity of acoustic observables to several physical parameters is investigated using a forward model, and a demonstration of inverse techniques provides estimates of several physical parameters from spring tidal cycles.

Interactive effects of wave exposure and tidal height on population structure of the mussel Perna perna

The influence of wave exposure and of tidal height on mussel (Perna perna Linnaeus) population structure (size, density, biomass and adult/juvenile correlations) was examined at 18 sites along the south coast of South Africa. Sites were classified as exposed or sheltered prior to sampling, without reference to the biota, on the basis of aspect, topography and wave regime. A single set of samples was collected from each site during three spring tide cycles. Adult mussels on these shores almost always attach directly to the rocks, and layering of mussels is virtually absent. Shore height always had a strong influence on population structure, but exposure had significant effects only lower on the shore, and almost exclusively on mussel sizes. Principal component analysis (PCA), based on size distribution data for each population, revealed a general upshore decrease in the modal size of the adult cohort. The effects of exposure on size distribution, however, varied with tidal height. PCA separated exposed zones, with larger mussels, from sheltered zones on the low-shore. Farther upshore the two shore types were increasingly confounded. The maximum size of mussels showed a similar pattern, with significant differences (ANOVA, p 15 mm) densities decreased up the shore, with low-, mid- and high-shore zones being significantly different from one another (ANOVA, p 15 mm) densities in all zones and for both exposure regimes ( p < 0.05 in all cases), but there was considerable variability and extremely low predictability in these relationships (r2 generally <0.2). Predictability tended to be greater towards the high-shore, where adults were more clumped. As with density, biomass was not affected by exposure, but decreased upshore as mean size and density decreased. A reduction in the influence of exposure farther upshore may be caused by greater emersion overriding the effects of exposure. The presence of free space within mussel beds and significant correlations between recruit and adult densities suggest that these mussel populations are recruit limited.

Development and application of elemental fingerprinting to track the dispersal of marine invertebrate larvae

The early life history of many marine benthic invertebrate and fish species involves a planktonic larval stage that allows exchange of individuals among separated adult populations. Here, we demonstrate how natural and anthropogenic trace elements can be used to determine larval origins and assess bay‐ocean exchange of invertebrate larvae. Trace elements can be effective site markers for estuaries because run‐off and pollutant loading often impart distinct elemental signatures to bay habitats relative to nearshore coastal environments. Crab larvae originating from San Diego Bay (SDB) were distinguished from those originating in neighboring embayments and exposed coastal habitats by comparing multiple trace‐element concentrations (“fingerprint”) in individuals. Discriminant function analysis (DFA) was used to characterize stage I zoeae of the striped shore crab, Pachygrapsus crassipes, of known origin (reference larvae) via trace‐elemental composition (i.e., Cu, Zn, Mn, Sr, Ca). Linear discriminant functions were used to identify the origin and characterize the exchange of stage I P. crassipes zoeae between SDB and the nearshore coastal environment during one spring tidal cycle. Elemental fingerprinting revealed that most (87%) of the stage I larvae collected at the bay entrance during the flood tide were larvae of SDB origin that were reentering the bay. Nearly one third of zoeae sampled (32%) at the entrance during ebb tide were coastal larvae leaving the bay and returning to open water. The observed bidirectional exchange contrasts with the unidirectional transport of zoeae out of the bay predicted from stage I vertical migratory behavior. Because P. crassipes zoeal survivorship is lower in SDB than in coastal waters, bay‐ocean exchange has significant implications for the dynamics of P. crassipes populations. Trace‐elemental fingerprinting of invertebrate larvae promises to facilitate investigations of many previously intractable questions about larval transport and dynamics.