Spring-neap Cycle Research Articles

Coupling between physical processes and biogeochemistry of suspended particles over the inner shelf mud in the East China Sea

This study examines how geochemical signals regarding the source and transformation in suspended particles are transported in the marine portion of the sediment dispersal system. In the source-to-sink sedimentary continuum of a world major river, the Changjiang (Yangtze) River, is used to demonstrate the close coupling between the marine physical processes and the geochemical signals carried by suspended particles. A sediment trap mooring configured with a non-sequential sediment trap, a downward-looking ADCP, CTD, LISST-100× were deployed for 29 days in the winter of 2014 off the coast of Zhejiang Province, China in water depth of 22.5 m. The mooring rendered two types of data. Physical processes included temporal changes of water depth, currents at 11 depths, temperature at two depths, and salinity at one depth. There was 15-cm accumulation of captured particles in the trap, which rendered the total mass flux of 233.3 g/cm2/day. The trap material was analyzed for grain-size composition, total 210Pb, minerals including mica/illite, kaolinite, chlorite, k-feldspar, plagioclase, quartz, and dolomite. Organic material (TOC, TN) and carbon isotope δ13C were also measured. The range of terrestrial fraction (Ft) of the trap material was between 45–60%. EOF analysis was used to examine the covariability in both data sets. For the physical data set, 37 variables were used in the analysis. For the sediment trap material, 15 variables were used. The EOF results of the physical processes indicate the tidal flow was the most important physical forcing for the transport of suspended particles and water masses that carried marine- and terrestrial-sourced particles. Wave and current combined resuspension entrained reworked sediment off the seabed, which was then transported landward or seaward. The most important covariability of the particle properties in the sediment trap was caused by hydrodynamic sorting that separated the clay from silt and sand sizes. Geochemical signals of the particles were carried by clay and the particle mass (dry weight) was associated with silt and sand. The temporal characteristics of the sorting show the spring-neap tidal cycle. The next major distinction was the contrast between terrestrial- and marine-sourced materials. The EOF analysis further distinguish contributions from the distal Changjiang River and proximal small rivers at the tertiary level of the covariability. The coverabilities in both data sets show close correspondence. Our findings give firm evidence to show marine processes dominated the transport of land-derived fluvial sediment and the incorporation of marine-sourced particles into the dispersal system. There is complexity in both physical processes and geochemical signals of suspended particles. However, our study points out the close coupling between the two.

Contribution of sediments to stratification in a fluvial estuarine system during a low-discharge period

Changjiang River Estuary (CRE), as one of the largest fluvial estuarine systems worldwide, experiences great changes in river discharges, sediment loads and geomorphology, due to heavy anthropogenic activities. The contribution of sediments to stratification in CRE was examined by analyzing the field data of sea surface level, current, salinity, suspended sediment concentration (SSC) and sediment grain size. The data were simultaneously observed at 1-h intervals and six vertical layers at 10 stations during the low-river discharge period from January 11 to 19, 2016. During this winter period, the asymmetric tides at all the field stations had longer ebb durations. The tidal range in the north branch became the largest among all the stations. The tidal phase inside the north branch was more delayed than that in the south branch, due to the effect of the local bathymetry and coastlines. Currents and bed sediments impacted SSC through changing bottom friction and viscosity, and then impacted water stratification subsequently through modulating water density. Net sediment flux was mostly seaward during the entire spring-neap tidal cycle, because of the Eulerian velocity and tidal pumping effect. Turbidity-induced stratification accounts for more than 50% of the total stratification during spring tides.

Tidal and Physicochemical Effects on Phytoplankton Community Variability at Tagus Estuary (Portugal)

The Tagus Estuary is one of the largest estuaries in Europe and merges large urban and industrial areas. Understanding phytoplankton community variability is key for an appropriate assessment of the estuarine ecological status. The objective of the present study was to assess the importance of the tidal influence over the phytoplankton community and to evaluate its main drivers of variation. Weekly sampling was performed at two stations on the Tagus Estuary with different anthropogenic pressures (Alcântara and Barreiro). The sampling covered periods with different tidal amplitude. Alcântara presented both the lowest and highest concentrations of dissolved inorganic nitrogen (DIN) and orthophosphate concentration (DIP), depending on the tidal height. Such high variability in this sampling station is probably due to its proximity to a sewage treatment station outfall and to the estuary mouth. In the present study, both seasonal and tidal variations influenced the chlorophyll a concentration of which the tidal cycle explained up to 50% of the chlorophyll a variations. Chlorophyll a displayed a seasonal trend with two peaks of phytoplankton biomass between spring and mid-summer. The main drivers of chlorophyll a variation were radiation, water temperature, tidal amplitude, salinity, river discharge, and the inorganic nutrients DIN and DSi. The estuarine phytoplankton community was mainly dominated by Bacillariophyceae, especially at Alcântara. Bacillariophyceae were less important at Barreiro, where communities had a higher representation from other phytoplankton groups, such as Cryptophyceae and Prasinophyceae. The drivers of variability in the community composition were similar to those influencing the total biomass. In conclusion, the spring-neap tidal cycle strongly influenced the phytoplankton community, both in terms of biomass and community composition. Of the several tidal conditions, spring tides were the tidal condition that presented both higher biomass and higher Bacillariophyceae representativity in the community.

Simulating the role of tides and sediment characteristics on tidal flat sorting and bedding dynamics

AbstractUnderstanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand‐dominated layers (SDLs) and mud‐dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding.

Spatio-temporal variability of the salinity intrusion, mixing, and estuarine turbidity maximum in a tide-dominated tropical monsoon estuary

The hydrodynamic processes of tropical estuaries in developing Asian countries are rarely reported due to their complex shape, high tidal range, long monsoon season with high river flows, and lack of infrastructure and technology. Therefore, this study investigates the seasonal and neap-spring tidal variability of salinity intrusion, mixing and formation of estuarine turbidity maximum (ETM) in a multi-branched estuarine system, Tanintharyi River estuary, Myanmar during the neap-spring tidal cycle in both dry and wet seasons of 2019. The salinity intrusion, mixing and ETM in the estuary exhibited both spatial and temporal (neap-spring tidal and seasonal) variability. The ETM zone was located at the high salinity region during the dry season whereas it was located at the low salinity region during the wet season. Hence, the ETM zones are not associated with a singular salinity, which is a different phenomenon that has been previously reported in tide-dominated estuaries. The spatial variability of salinity intrusion, mixing, and ETM in different branches at the same time during each season can be attributed to the effect of the length and shape of each branch. Finally, these hydrodynamic processes in the estuary were influenced by the strong tidal flow during the dry season whereas the river flow slightly dominated the tidal flow during the wet season.

The Intra-Tidal Characteristics of Tidal Front and Their Spring–Neap Tidal and Seasonal Variations in Bungo Channel, Japan

The intra-tidal variations of a tidal front in Bungo Channel, Japan and their dependence on the spring–neap tidal cycle and month were analyzed utilizing high-resolution (~2 km) hourly sea surface temperature (SST) data obtained from a Himawari-8 geostationary satellite from April 2016 to August 2020. A gradient-based front detection method was utilized to define the position and intensity of the front. Similar to previous ship-based studies, SST data were utilized to identify tidal fronts between a well-mixed strait and its surrounding stratified area. The hourly SST data confirmed the theoretical intra-tidal movement of the tidal front, which is mainly controlled by tidal current advection. Notably, the intensity of the front increases during the ebb current phase, which carries the front toward the stratified area, but decreases during the flood current phase that drives the front in the opposite direction. Due to a strong dependence on tidal currents, the intra-tidal variations appear in a fortnight cycle, and the fortnightly variations of the front are dependent on the month in which the background stratification and residual current changes occur. Additionally, tidal current convergence and divergence are posited to cause tidal front intensification and weakening.

Daily ratings of mood and sleep duration over 3.3 years are associated with the lunar illumination cycle in a rapid cycling bipolar patient.

Previous reports from a single research group have described changes in mood in rapid cycling bipolar patients that are associated with the lunar cycle. In many cases, there is more positive mood around the time of the full moon, and more negative mood around the time of the new moon, in accordance with the lunar illumination cycle period of ~29.5 days (the equivalent of two spring-neap cycles).

Optimal operation of tidal plants based on nonlinear model predictive control strategy

Tidal range barrage is a well-recognized approach to extract energy from tides in a large scale. Nevertheless, the complexity of tide oscillation poses a challenge to efficaciously regulate this type of plants. Optimization of the process control may result in enhancement of energy yield with low influences on environment. This paper presents a framework where the whole tidal-electric system is modelled and coupled with a nonlinear model predictive control (NMPC) strategy to determine the optimal state of turbo-generators over a complete spring-neap cycle in a month. The method allows the system model to be nonlinear and dynamic under tight constraints, which would be extremely appropriate for the deployment of this highly predictable but frequently varied energy. A potential bay in Norway is chosen to be a proposal site to verify the benefits of the control strategy, by comparing the energy extraction between a simple barrage without sluices and a conventional one with sluices assembly. Results show the conventional barrage type is superior with respect to energy generation, while keeping the turbine blade diameter as as factor. For excessively large turbines, though, the revenue may be curtailed by costs due to environmental factors, and turbine construction.

Application of a multibeam echosounder to document changes in animal movement and behaviour around a tidal turbine structure

AbstractChanges in animal movement and behaviour at fine scales (tens of metres) in immediate proximity to tidal stream turbine structures are largely unknown and have implications for risks of animal collision with turbine blades. This study used upward-facing multibeam echosounder data to detect and track animal movement comprising fish, diving seabirds, and marine mammals. Measurements over spring-neap tidal cycles at a turbine structure (no blades present) are compared to a neighbouring reference area with no structure and comparable conditions, with measurements consecutive in time to maximize comparability.The majority of tracked animals (93.4% around turbine structure and 99.1% without turbine structure) were observed swimming against the flow, with 87.5% and 97.8%, respectively, making ground and showing capability of manoeuvring in tidal stream flow speeds. Track tortuosity increased around the turbine structure compared to the reference site, particularly in the wake and at low flow speeds, indicating animal station-holding or milling behaviour. These data also evidence the benefits of multibeam echosounders to measure animal movement through larger measurement volumes rather than relying on single-beam echosounders to measure animal presence alone, including to avoid large biases overestimating the size of schools swimming against the flow measured by time-in-beam.

Internal Tide Structure and Temporal Variability on the Reflective Continental Slope of Southeastern Tasmania

AbstractMode-1 internal tides can propagate far away from their generation sites, but how and where their energy is dissipated is not well understood. One example is the semidiurnal internal tide generated south of New Zealand, which propagates over a thousand kilometers before impinging on the continental slope of Tasmania. In situ observations and model results from a recent process-study experiment are used to characterize the spatial and temporal variability of the internal tide on the southeastern Tasman slope, where previous studies have quantified large reflectivity. As expected, a standing wave pattern broadly explains the cross-slope and vertical structure of the observed internal tide. However, model and observations highlight several additional features of the internal tide on the continental slope. The standing wave pattern on the sloping bottom as well as small-scale bathymetric corrugations lead to bottom-enhanced tidal energy. Over the corrugations, larger tidal currents and isopycnal displacements are observed along the trough as opposed to the crest. Despite the long-range propagation of the internal tide, most of the variability in energy density on the slope is accounted by the spring–neap cycle. However, the timing of the semidiurnal spring tides is not consistent with a single remote wave and is instead explained by the complex interference between remote and local tides on the Tasman slope. These observations suggest that identifying the multiple waves in an interference pattern and their interaction with small-scale topography is an important step in modeling internal energy and dissipation.

Tidal and Near-Inertial Internal Waves over the Reykjanes Ridge

AbstractInternal waves in the semidiurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6°–59.1°N, 28.5°–33.3°W). Continuous measurements of horizontal velocity and temperature for more than 2 years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semidiurnal energy density and the tidal energy conversion at the spring–neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4–5 days on the ridge top versus 7–9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near-local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semidiurnal kinetic energy by a factor of 2–3 but is much more variable in time. It features a strong seasonal cycle with a winter intensification and subseasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13–15 days, which is consistent with the short time scales of observed variability of near-inertial energy. In the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase relative to cyclonic structures, illustrating the funneling effect of anticyclones.

Morphodynamics of intertidal dune field in a mixed wave-tide environment: Case of Baie de Somme in Eastern English Channel

Understanding the response of the intertidal dunes to the varying hydrodynamic conditions in mixed wave-tide environments is of high complexity. Field measurement is one of the most useful approaches to investigate the dune dynamics at different time (semi-diurnal and lunar cycles) and spatial (dunes and associated superimposed bedforms) scales. High-resolution laser scanner data for five successive surveys over a full neap-spring cycle (27 January – 9 February 2014), together with hydrodynamic measurements, have been explored to investigate the morphodynamics of dunes, their superimposed bedforms and their migration in a mixed wave-tide environment (case of the Baie de Somme, Eastern of the English Channel), by the use of a stochastic-physical coupling approach. The dunes are ~12 m wavelength, 0.4 height and oriented orthogonally to the shoreline with angles varying between - 45° and + 45° corresponding to clockwise and counter-clockwise changes, respectively.A stochastic approach, based on spectral analysis, exhibits a series of morphological defects modulating the dune changes following several scale ranges in response to various energy conditions. Such defects can be classified into four categories: (1) limited structures of ~1.6 m wavelengths locally developed along the dune during flood drainage; (2) small-scale superimposed bedforms of ~3.2 m wavelengths built-up across the dunes after high hydrodynamics and proliferated when the energy deceases; (3) large-scale superimposed bedforms of ~6.4 m resulting from the morphological growth of the smaller bedforms; (4) crestline sinuosity with ~25 m wavelength and induced by changes in the orientation of the dunes when they are exposed to high tide-wave activity. The evolving dune defects over the neap-spring cycle is strongly related to the physical mechanisms of sediment transport supplying the migrating dunes and favorable for their self-organization.The physical study of the migrating dunes with the wave-tide driven sediment transport highlights that the long-dune migration components are significantly important during high-energy episodes with a mean explained variance of 65% of the total migration while they decrease to 32% for moderate wave-tide conditions. Good matches of the cross- and long-migration components with the along-shore and the cross-shore sediment transport quantities are recorded with R2 coefficients higher than 70%. These coefficients increase for the offshore dunes where their morphological response to the time-averaged sediment transport is faster while onshore dunes respond at slower timescales resulting in lower R2 coefficients.

The role of advection and density gradients in driving the residual circulation along a macrotidal and convergent estuary with non-idealized geometry

Due to variations in channel depth, width, and lateral bottom profile, estuarine residual flows can exhibit significant variation in magnitude and transverse structure along macrotidal and convergent estuaries. This article explores the along-channel residual flow (magnitude and transverse structure), forcing mechanisms and their variations along the Gironde estuary in France. With emphasis on the role of density gradient and the advective accelerations in the along channel momentum balance, the study outlines the along-channel residual flows and forcing mechanisms over the neap-spring tidal cycle during high and low river discharge conditions. The results demonstrate that the density-driven flow contribution to total residual flows is, approximately, 75% (along-channel averaged) during neap tide and 18% during spring tide for both high and low river discharge scenarios. Owing to the complex lateral variation in the channel depth and the constriction near the mouth, advective accelerations play a major role in altering the residual flow lateral structure. However, the relative importance of advection reduces in the main body of the estuary where the channel is widened with poor lateral variation in the bottom depth. The results suggest advection and the baroclinic pressure gradient produce a laterally sheared along-channel residual flow with inflow in the channel and outflow over the shoals during neap tide. During spring tide, this lateral structure is produced due to advection. The results show that even in a homogenous system, advection can induce a flow with a structure that mimics the density-driven flow. The article shows that along macrotidal estuaries the presence of complex morphological features can affect the residual flow dynamics. As such, the residual flow in these systems should be schematized not only by considering the lateral variation of bathymetry, but also the along channel complexity.

A Lagrangian-to-Eulerian Metric to Identify Estuarine Pelagic Habitats

Estuaries are among the world’s most productive ecosystems, but recent natural and anthropogenic changes have stressed these ecosystems. Tools to assess estuarine pelagic habitats are important to support and maintain healthy ecosystem function. In this work, we demonstrate that estuarine pelagic habitats can be identified by a simple ratio, termed the LE ratio, that takes into account the tidal excursion along a channel (a Lagrangian length scale) and the distance along that channel (an Eulerian length scale). To develop and assess this concept, numerical simulations of the 1D advection–dispersion equation of a conservative tracer and tidal excursion estimates based on data were used to formulize a conceptual model and to define exchange zones within a tidal channel. This conceptual model was then used to predict the extent of pelagic habitats in a terminal channel network in the Sacramento–San Joaquin Delta. Exchange zones mapped onto these channels were found to be in good agreement with independent estimates of residence time. Sensitivity analyses of the numerical model suggest that productive pelagic habitats can be expanded by a factor of 2 by either increasing dispersion or increasing spring–neap variability in mean tidal velocity. Such changes can also enhance flushing in upper channel reaches. These findings are relevant for tidal marsh restoration projects that aim to expand beneficial aquatic habitats by varying exchange or residence time over the spring–neap cycle, because this variability may interact synergistically with varying rates of phytoplankton growth due to spatiotemporal changes in environmental conditions.

Bardsey – an island in a strong tidal stream: underestimating coastal tides due to unresolved topography

Abstract. Bardsey Island is located at the western end of the Llŷn Peninsula in northwestern Wales. Separated from the mainland by a channel that is some 3 km wide, it is surrounded by reversing tidal streams of up to 4 m s−1 during spring tides. These local hydrodynamic details and their consequences are unresolved by satellite altimetry and are not represented in regional tidal models. Here we look at the effects of the island on the strong tidal stream in terms of the budgets for tidal energy dissipation and the formation and shedding of eddies. We show, using local observations and a satellite-altimetry-constrained product (TPXO9), that the island has a large impact on the tidal stream and that even in this latest altimetry-constrained product the derived tidal stream is under-represented due to the island not being resolved. The effect of the island leads to an underestimate of the current speed in the TPXO9 data in the channel of up to a factor of 2.5, depending on the timing in the spring–neap cycle, and the average tidal energy resource is underestimated by a factor up to 14. The observed tidal amplitudes are higher at the mainland than at the island, and there is a detectable phase lag in the tide across the island; this effect is not seen in the TPXO9 data. The underestimate of the tide in the TPXO9 data has consequences for tidal dissipation and wake effect computation and shows that local observations are key to correctly estimating tidal energetics around small-scale coastal topography.

Seasonal and fortnightly variability of the hydrodynamic regime at Babitonga Bay, Southern of Brazil

This paper provides the first comprehensive observational results of the seasonal variability of hydrography and currents in Babitonga Bay, a microtidal subtropical estuary, and one of the largest estuaries in Brazil with economic and ecological importance. Measurement campaigns were performed during the higher and lower rain conditions in austral summer and winter, respectively. Time series of current velocity and salinity–temperature were obtained near the bottom at the head estuary for each season. Along-estuary hydrographic sections and tidal-cycle cross-estuary transect surveys of current velocity and hydrographic profiles at the mouth were performed in the spring and neap tides. The M2 is the main harmonic constituent. The third-diurnal principal lunar (M3) suffers the highest amplification (∼115%), followed by the shallow water overtides of principal lunar (M4) with ∼82%. The tide mainly controls the variability of salinity, density, and longitudinal currents with no variation between the two seasons. The water temperature presents irregular daily variations associated with daytime heating (summer) and nighttime cooling (summer and winter). The along-estuary salinity distribution characterizes a weakly stratified estuary with small intratidal, fortnightly, and seasonal variations. At the mouth, the flood currents start to inflow laterally in the shallow flank of the estuary, in both spring and neap tides. The maximum velocities measured along-channel are similar for neap and spring tides for both summer and winter seasons. The density induced exchange flow pattern is vertically and horizontally sheared in the spring tides and vertically sheared in the neap with no seasonal variation. The intratidal and the spring–neap cycle are more important than the seasonal scale in governing the hydrodynamic at Babitonga Bay.

Energetics and spatio-temporal variability of semidiurnal internal tides in the Bay of Bengal and Andaman Sea

Energetics of semidiurnal barotropic and internal tides in the Bay of Bengal (BoB) and Andaman Sea (AS) are studied using the simulations from a high-resolution ocean general circulation model. Barotropic M2 tide, which is the largest tidal constituent in this region, loses about 65.5 GW of energy in the BoB and AS. Most of this barotropic energy (46.5 GW) is dissipated by means of bottom friction in the shallow regions, such as the head of the bay and the Gulf of Martaban. Due to the interaction of barotropic tides with bottom topography, about one-third of the barotropic M2 energy (19.1 GW) gets converted into internal tides and most of this conversion (84%) occurs over the Andaman–Nicobar (AN) Ridge. On the contrary to the estimates in earlier studies, our results show that a large part (59%) of this internal tide energy in the BoB and AS gets dissipated far away from the generation sites. Compared to the BoB, energy dissipation rates are found to be large over the entire AS and it could result in enhanced vertical mixing in this region. Further, analysis of the temporal variability of conversion within the model domain shows that energy conversion in the semidiurnal band varies between 11 to 27 GW over a spring–neap cycle. Seasonal variability in the stratification due to river runoff significantly changes the conversion of tidal energy at the generation sites in the northern BoB and AS. Our study revealed that westward radiating internal tides from AN Ridge undergo path alteration (refraction) due to the time-varying mesoscale circulation and such path alterations cause large intraseasonal variability in internal tide activity and redistribution of internal tide energy available for mixing in the remote areas such as the continental margins of the western BoB.

Internal tides as a major process in Amazon continental shelf fine sediment transport

The description of hydrodynamics associated with the extensive reef system on the shelf break adjacent to the Amazon River is still a challenge for ocean sciences. Despite the discharge of more than one billion tons of cohesive sediment per year, the outer continental shelf of the world's largest river presents very low concentrations of suspended sediment near the bottom and an absence of modern fine sediment deposits nearly one hundred kilometers before the shelf break. The offshore limit of the subaqueous delta consists of a sigmoidal clinoform standing between 40 and 70 m in depth, a depositional feature that cannot be explained solely by estuarine-like gravitational circulation. This paper aims to test the hypothesis that internal tides have a major role in the control of offshore fine sediment transport. For that, we implement a set of tridimensional, non-hydrostatic, and high-resolution (up to 2 m, vertical, and 2 km, horizontal) Delft3D models. The experiments showed that even disregarding river plume buoyancy, wind drag, superficial waves, and ocean currents, the exclusive interaction between barotropic tidal currents, bathymetry, and the stratification structure of the ocean is capable of generating asymmetrical current patterns compatible with modern deposition. The maximum shelf slope and the relative depth between the outer shelf and the pycnocline represent the main factors influencing the generation and shoreward propagation of internal tides. Over time, spring-neap cycles are eventually capable of reverting cross-shore subtidal transport tendencies, while seasonal variability in ocean stratification modulates the intensity of baroclinic processes.

The impact of temporal variability in light-climate on time-averaged primary production and a phytoplankton bloom in a well-mixed estuary

Phytoplankton primary production (PP) in turbid estuaries is often limited by light-availability. Two important factors altering light-climate are solar irradiance at the water surface and exponential light-extinction coefficient within the water column. Additionally, the depth of the water body changes the light-climate and corresponding PP by altering the ratio of the euphotic and mixing depth in a well-mixed estuary. These three parameters are highly variable yet are often assumed to be constant by both experimental scientists and modelers because of a lack of data or to reduce complexity. Because assuming constant parameters introduces an error, we utilize an idealized model of depth-integrated primary production to analyze the (individual) impact of temporal variability in these three parameters. We only consider the main tidal and solar constituents in temporal variability of the forcings and apply a second-order moment approximation to analyze the bias introduced to time-averaged PP estimates by neglecting temporal fluctuations. We demonstrate that the sign and magnitude of this bias are system-specific and depend on two non-dimensional parameters that characterize the system. The first is equivalent to the ratio of mixing and photic depth. The second accounts for typical incident irradiance and the photosynthetic parameters of the phytoplankton population present. To demonstrate the applicability of our approach, we apply the model to two cases in the Scheldt estuary (Belgium) in the brackish and freshwater part. In the first application, we study the impact of fluctuations on phytoplankton in dynamic equilibrium, where biomass is assumed to be constant. We show that variability in solar irradiance has the largest impact on time-averaged PP in dynamic equilibrium, resulting in a 30 percent decrease compared to time-invariant forcing. By comparing with a numerical integrator, we show that a second-order moment approximation correctly predicts the order of magnitude of the impact of temporal variability of the individual parameters. In the second application, we study the impact of fluctuations on unbounded exponential phytoplankton growth. Also here, fluctuations in solar irradiance have the largest impact and lead to a significant decrease in exponential growth. In this case study, we show that temporal fluctuations delay the onset of the biomass by two weeks and decrease the biomass by a factor 14 after two weeks compared to time invariant forcing. Additionally, we show that the temporal fluctuations induce low-frequency variability in phytoplankton biomass with similar periodicity as the spring-neap cycle, making it difficult to observe these phenomena in real-world time series.

Evaluation of along-channel sediment flux gradients in an anthropocene estuary with an estuarine dam

While estuarine dams can develop freshwater resources and block the salt intrusion, they can result in increased sediment deposition in the estuary. The mechanism of increased sediment deposition in an estuary with an estuary dam is not well understood. To fill this knowledge gap, 7 ADCP measurements of flow and suspended sediment concentration (SSC) were collected along-channel in an estuary with an estuarine dam over a neap-spring cycle. Flow and SSC were used to calculate the sediment flux and sediment flux gradients, which in turn were decomposed into mean-flow fluxes (which occur on timescales longer than a tidal cycle) and correlation fluxes (which occur on the tidal timescale). Then the fluxes were compared with the external forcing to determine the relative importance of tides, dam discharge, and waves in driving the fluxes. The results indicated that the cumulative sediment fluxes at all stations were directed landward. The along-channel sediment flux gradient was negative, which indicated deposition along the channel. The landward mean-flow fluxes were dominant in the deep portion of the channel near the estuary mouth, whereas landward correlation fluxes were dominant in the shallow portion of the channel near the estuarine dam. The tides were the dominant forcing driving the sediment fluxes throughout the estuary. Waves and dam discharge were locally important forcing however. In particular, the dam discharge resulted in seaward mean-flow fluxes for 7 km seaward of the estuarine dam. This distance corresponded approximately to a tidal excursion seaward from the dam. It was the distance a freshwater mass discharged during an ebb tide could propagate before the currents returned landward during the following flood tide. Overall, the along-channel sediment flux gradient in an estuary with an estuarine dam was different from a similar estuary without an estuarine dam because the mean-flow fluxes in the inner estuary are forced by episodic dam discharge which was limited in seaward extent by the tidal excursion. This imposed a limitation for the seaward mean-flow fluxes which counteract the landward mean-flow and correlation fluxes due to the tides. With restricted seaward flushing, the tides could dominate, which could in turn result in increased depositional rates for an estuary with an estuarine dam.