Estuarine Circulation Research Articles

Hydrographic restriction conditions in the Middle and Upper Yangtze region during the Early Silurian post-glacial transgression: Constraints from major, trace elemental geochemistry and Mo-TOC relationship

Redox-sensitive trace metals have not only been widely used as the proxy for palaeoredox potential due to the strong enrichments under reducing conditions but also provide critical information into water-mass properties, such as the degree of basin restriction. In order to reveal the Early Silurian post-glacial transgression hydrographic restriction conditions throughout the Middle and Upper Yangtze region, trace metal concentrations and organic geochemical data of the lower Longmaxi Formation were analyzed by the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Leco combustion techniques. We analyzed the hydrographic restriction conditions and redox conditions of different depocenters, including the Southern, Eastern, and Northern Sichuan Basin depocenters and the Western Hubei Region depocenter. The sedimentary Mo-TOC relationship, MoEF-UEF covariation, and upwelling intensity suggest that the Southern, Eastern Sichuan Basin depocenters and the Western Hubei Region depocenter were deep water areas with moderate hydrographic restriction, whereas the restriction conditions of Northern Sichuan Basin depocenter was relatively weak. In addition, due to the combined effect of geographical location, connectivity with the open Qinling Ocean, and estuarine circulation, restriction degrees varied in the same depocenter. The redox proxy (Corg/P) indicated that the lower Longmaxi Formation was deposited in an anoxic water column. There are significant spatial differences in redox conditions in different depocenters and different locations within the same depocenter, which may be controlled by relative sea level rise, hydrographic restriction, and upwelling intensity. Anoxic conditions were more likely to occur in areas where water column was relatively deep and restricted. In addition, the anoxic conditions in the Northern Sichuan Basin depocenter may also associated with the strong upwelling intensity and the behavior of P cycling back to water column, which can promote the primary productivity and further maintain persistent anoxic conditions. The correlation between Corg/P and Mo/TOC indicates that, in relatively weak restriction basin, the trace elements enrichment in sediments was controlled by trace elemental concentrations in seawater. In moderate restriction basin, the trace elements enrichment was mainly controlled by redox conditions. Whereas under more reducing conditions, hydrographic restriction may tend to become the dominant factor. The application of sediment Mo-TOC relationship, MoEF-UEF covariation, and upwelling can provide insights into hydrographic restriction conditions, but the Mo/TOC relationship is limited to anoxic facies, as little trace metal accumulation could also be the consequence of oxic conditions.

Characteristics and Mechanisms of Spring Tidal Mixing and Sediment Transport in a Microtidal Funnel-Shaped Estuary

Information about estuarine mixing and its control of sediment transport is crucial to elucidating the dynamics and evolution of estuaries. Here, the microtidal and funnel-shaped Zhenhai Estuary, located in the southwestern Pearl River Delta of China, is used to investigate the characteristics and mechanisms of water mixing and sediment transport based on observations from three spring tides. The results reveal that the studied estuary remains well mixed during spring tides from 2013–2022 despite its microtidal regime. Tidal stirring, which is enhanced by tidal energy convergence and benefits from the funnel-shaped geometry and shallow bathymetry, favors vertical mixing, contributing to the formation of strong mixing in the estuary. Due to the well-mixed regime, sediment transport in the estuary is dominated by the advective term, followed by a moderate tidal pumping term and minor estuarine circulation term. Accordingly, sediments within the estuary tend to be transported landward owing to the regulation of the funnel-shaped geometry, and a gradual but slow infilling trend is predictable. This paper deepens our understanding of hydrodynamics and sediment transport in microtidal estuaries.

Engineering Regulation of the Weird Branches in a Branching Estuary and its Mechanics: Using the North Branch of the Yangtze Estuary as an Example

Weird horizontal shapes of branches, in large branching estuaries, often cause significant flood risks and environment-related problems. People usually resort to engineering methods to improve the horizontal shape of the weird branches and solve related issues. The responses of the riverbed evolution of a branching estuary to anthropogenic activity are complicated because of complex estuarine hydrodynamics and sediment transports, especially when the project locates specially (e.g., at estuary outlets). The North Branch of the Yangtze Estuary has a narrow upper reach which is almost orthogonal to the South Branch and has a trumpet-shaped lower reach with a wide outlet. The weird horizontal shape of the North Branch brings significant flood risks to cities along this branch, the shrinkage of its entrance, and other problems. In this study, a regulation of the North Branch, which is launched at Guyuan Sand (GYS) just outside the exit of the North Branch, is taken as an example. The GYS regulation aims to improve the weird horizontal shape of the North Branch by building new layouts of outlets, by which people decrease the flood risk of the surrounding cities. The GYS regulation is studied using a 2D numerical model. The riverbed evolution of the Yangtze Estuary in a typical hydrological year is simulated, while the water/sediment fluxes at cross-sections of branches in the estuary during a spring/neap tide are quantitatively calculated. It is found that the regulation changes the rotational flows near the shore, and further reshapes the estuarine circulations of mass inside the outlets, especially exchanges of water/sediment between different branches. The regulation directly changes the riverbed evolution at the outlet of the North Branch, and meanwhile has significant indirect influences on the riverbed evolution of the entrance of the North Branch. The varying riverbed evolution at the entrance of the North Branch and the varying water/sediment fluxes, under different designs of regulations, are related and analyzed. An essential improvement for the weird horizontal shape of the North Branch by an engineering method is shown to be possible, while the regulation mechanism of the engineering method and the response of estuarine riverbed evolution to the regulation are clarified. This study provides a new insight for improving estuarine branches with weird horizontal shapes, by reshaping the tidal processes and the accompanying sediment transports in a branching estuary.

Lateral Transport Controls the Tidally Averaged Gravitationally Driven Estuarine Circulation: Tidal Mixing Effects

Abstract In classic models of the tidally averaged gravitationally driven estuarine circulation, denser salty oceanic water moves up the estuary near the bottom, while less dense riverine water flows toward the ocean near the surface. Traditionally, it is assumed that the associated pressure gradient forces and salt advection are balanced by vertical mixing. This study, however, demonstrates that lateral (across the estuary width) transport processes are essential for maintaining the estuarine circulation. This is because for realistic estuarine bathymetry, the depth-integrated salt transport up the estuary is enhanced in the deeper estuary channel. A closed salt budget then requires the lateral transport of this excess salt in the deeper channel toward the estuarine flanks. To understand how such lateral transport affects the estuarine salt and momentum balances, we devise an idealized model with explicit lateral transport focusing on tidally averaged lateral mixing effects. Solutions for the along-estuary velocity and salinity are nondimensionalized to depend only on one single nondimensional parameter, referred to as the Fischer number, which describes the relative importance of lateral to vertical tidal mixing. For relatively strong lateral tidal mixing (greater Fischer number), salinity and velocity variations are predominantly vertical. For relatively weak lateral tidal mixing (smaller Fischer number), salinity and velocity variations are predominantly lateral. Overall, lateral transport greatly affects the estuarine circulation and controls the estuarine salinity intrusion length, which is demonstrated to scale inversely with the Fischer number.

Unsteady Circulation in a Glacial Fjord: A Multiyear Modeling Study of Milne Fiord

AbstractThe exchange of heat and freshwater between glaciers and the ocean is dictated by the circulation in glacial fjords and ice shelf cavities. Therefore, our ability to estimate and predict these fluxes depends on our understanding of the circulation mechanisms inside these polar estuaries. Here, we use an exceptionally long observational data set (8+ years) to develop and validate a high‐resolution realistic numerical model of Milne Fiord (Nunavut, Canada), a glacial fjord with an ice shelf at its mouth. Model results show the circulation inside Milne Fiord from 2011 to 2019 is highly three‐dimensional and unsteady. Three distinct circulation modes are identified (eddy, front, barotropic). The shifts between circulation modes are driven by density variations in the offshore coastal current, which restrict vertical stretching, allowing (or not) the coastal current to develop sufficient relative vorticity to enter the fjord. The unsteadiness of the system results in an overturning estuarine circulation with mean velocities ∼50 times smaller than the instantaneous field. Moreover, analysis of the model outputs suggest that at least 2 years of simulation are needed to yield reliable average heat flux estimates in this environment. This work highlights the value of combining long term (>1 year) observations with numerical modeling. This allowed us to uncover the spatial and temporal variability of the system, which impacts how heat and freshwater fluxes can be reliably estimated.

Intense lateral intrusion of offshore sub-surface waters in Halifax Harbour

The main physical processes affecting the circulation variability in Halifax Harbour (HH), including the intense lateral intrusion of offshore sub-surface waters into Bedford Basin (BB), are studied using a nested-grid modelling system (NGMS). The annual and monthly mean circulation in HH are characterized by a typical two-layer estuarine circulation, with seaward currents in the upper layer and landward currents in the lower layer, both showing strong temporal and spatial variability. The intense lateral intrusion of offshore sub-surface waters into BB occurs at time scales of a few days. The intense intrusion, in combination with winter connection, is important for the renewal of deep waters in BB. The model sensitivity experiments suggest that the persistent northwesterly winds strengthen the near-surface seaward currents and the lateral intrusion of offshore sub-surface waters into BB. The persistent southwesterly winds can also trigger the lateral intrusion, but are less effective than the northwesterly winds. The southeasterly winds, on the contrary, reduce significantly the seaward currents in the surface layer and prevent the lateral intrusion. The accumulative effects of winds and tides on the 3D hydrodynamics in HH are also examined. Local winds have dominant effects on the 3D currents and temperature/salinity, through driving vertical mixing and coastal upwelling. By comparison, tides play a secondly role in affecting the currents and temperature/salinity in BB but are important in the Narrows.

Aeolian desert dust as a primary estuarine sediment source: Fine sediment transport and dynamics in the northern Arabian/Persian gulf

Fluvial discharge and freshwater runoff are recognized as the main sediment sources in many estuaries worldwide. Precipitation is, however, limited in arid and semi-arid areas, allowing for high aeolian dust activity, and inducing limited river runoff. As such, aeolian dust could contribute to the estuarine sediment sources in such areas. Uniquely, the Northern Arabian/Persian Gulf (NG) is continuously loaded with vast amounts of aeolian desert dust annually. This study focuses on the role of desert dust in the distribution of suspended sediment in the Northern Arabian/Persian Gulf (NG), employing in-situ measurements of physical parameters (collected in 2015–2016), satellite imagery, and a flexible mesh hydrodynamic model (Delft3D-FM) coupled with a sediment transport model (Delft-WAQ). The model showed reasonable performance in representing the complex system of NG. The study revealed that more than 90% of the sediment along the NG is aeolian dust, with the remaining being fluvial. Owing to salinity-induced density gradients, estuarine circulation keeps the fluvial sediment confined to the estuarine zone of the Shatt Al-Arab even during the events of increased freshwater discharges. In the NG most of the suspended sediments have an aeolian origin. They were found in shallow zones within the 5 m isobath, including the estuary of Shatt Al-Arab where they are mixed with the fluvial sediment fractions. Tides were found to play a key role in sediment mixing and horizontal dispersion in the shallow area of the NG most of the time, while the Shamal increased the transport of fine sediment to offshore areas (seaward). Based on current knowledge, this study represents a rare instance where estuarine sediment dynamics are primarily governed by aeolian deposits from a nearby desert.

Depositional setting and sequence stratigraphy of Upper Mississippian coal-bearing paralic cyclothems in Upper Silesian foreland

The upper Serpukhovian Poruba Member (c. 325–324 Ma) is a coal-bearing “paralic” succession deposited in the Upper Silesian Coal Basin located along the eastern foreland of the Moravo-Silesian segment of the Variscan fold and thrust belt. The basin formed an >150 km long, tectonically controlled embayment open to the north and northeast, with estuarine circulation, predominance of fluvial discharge and a limited tidal influence. A high-accommodation depocentre (possibly up to 1100 m/Myr) was filled by shallowing-upward successions related to the progradation of river dominated (bay-head) deltas, with subordinate fluvial and marine sediments. These strata form two orders of transgressive-regressive cycles, or genetic sequences, both overlapping with the Milankovitch band. Intervals of maximum transgression, marked by marine or brackish faunal horizons, immediately overlie coal beds, suggesting non-accretionary transgression in a low energy setting. Six medium-term genetic sequences (cyclothems) are recognized, each consisting of 4 to 6 elementary sequences and a number of smaller scale units of possible autocyclic origin. The medium-term sequences are attributed to a combined influence of relative sea-level change and changes in sediment input, both possibly as a far field response to Gondwanan glaciation through glacioeustasy and attendant changes in climatic seasonality.

Transitions between different estuarine states in a low-inflow estuary of arid coasts (Mond Estuary, Persian Gulf) with a focus on freshwater lenses

This study shows that the Mond Estuary, Persian Gulf experiences notable time-dependent longitudinal salinity variations. Monthly-seasonal variability in salinity changes from a positive estuarine system in wintertime to inverse and salt-plug estuaries during summertime. A highly unsteady state (time scale of hours-days) occurs in the wintertime named the freshwater lens estuary (FLE), in which a low-salinity water mass (lens) is confined by both marine and riverine saltwater sources. Here, FLE dynamics is investigated using realistic and idealized numerical models and two 25-hour data sets collected immediately before a hypersaline runoff (weak FLE condition) and during another one (strong FLE condition). The Péclet Number (Pe) reveals a much stronger advective transports than diffusive one (Pe>5.7) throughout the estuary. An exception from this behavior (stronger mixing with Pe=0.8) is observed at the estuary downstream, where a significant amount of freshwater mixes with seawater during the weak FLE conditions. Also, the minimum hypersaline runoff of ∼400 m3/s estimated by the numerical model can dampen tidal waves at the head of the estuary. In an idealized basin, the model shows that FLE longitudinal flows form two cells of positive estuarine circulation separated by a maximum mixing zone (the potential energy anomaly <0.75 J/m3). In the Mond Estuary, a sea-level rise of 20 cm increases salinity in the FLE state and the positive estuary during winter. Conversely, it decreases salinity in the inverse and salt-plug estuaries in summer.

Secondary estuarine circulation and the related vertical mixing at the sill of Ambon Bay, eastern Indonesia

Secondary estuarine circulation at the sill of Ambon Bay during wet season (July 2019) was investigated to add important insights into the existing knowledge on primary estuarine circulation done by a recent study. The datasets from the previous study were employed to calculate tidal-mean vertical advection as secondary estuarine circulation at the sill. Vertical mixing at the sill was also quantified using the continuity equation. The vertical profiles of vertical advection at the sill of Ambon Bay formed bow-like shapes with zero value at the surface that increased to reach maximum at the mid-depth of the sill (∼6 m depth), subsequently decreasing to zero at the seabed. Vertical advection at the sill was found to be larger during spring tide (∼2.5 × 10−3 m/s) than during neap tide (∼1 × 10−3 m/s). Vertical diffusivity, indicating the magnitude of vertical mixing at the sill of Ambon Bay, showed similar characteristics to vertical advection in terms of vertical profiles (bow-like shapes) and spring/neap tidal variation (spring tide value: 8.5 × 10−3 m2/s; neap tide value: ∼5 × 10−3 m2/s). The intense vertical mixing at the sill of Ambon Bay has the potential to create turbid water linked to resuspension of seabed sediment in the location.

Local Mixing Determines Spatial Structure of Diahaline Exchange Flow in a Mesotidal Estuary: A Study of Extreme Runoff Conditions

Abstract Salt mixing enables the transport of water between the inflow and outflow layers of estuarine circulation and therefore closes the circulation by driving a diahaline exchange flow. A recently derived universal law links the salt mixing inside an estuarine volume bounded by an isohaline surface to freshwater discharge: it states that on long-term average, the area-integrated mixing across the bounding isohaline is directly proportional to the freshwater discharge entering the estuary. However, even though numerous studies predict that periods of extreme discharge will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has yet to be investigated. Therefore, this numerical modeling study focuses on salinity mixing and diahaline exchange flows during a low-discharge and an extreme high-discharge period. To this end, we apply a realistic numerical setup of the Elbe estuary in northern Germany, using curvilinear coordinates that follow the navigational channel. This is the first time the direct relationship between diahaline exchange flow and salt mixing as well as the spatial distribution of the diahaline exchange flow is shown in a realistic tidal setup. The spatial distribution is highly correlated with the local mixing gradient for salinity, such that inflow occurs near the bottom at the upstream end of the isohaline. Meanwhile, outflow occurs near the surface at its downstream end. Last, increased vertical stratification occurs within the estuary during the high-discharge period, while estuarine-wide mixing strongly converges to the universal law for averaging periods of the discharge event time scale. Significance Statement Inside estuaries, such as river mouths, terrestrial freshwater is mixed with salty ocean water. This is accompanied by an estuarine circulation with inflow of saltwater into the estuary and outflow of brackish water toward the ocean. Here, we aim to better understand how salt mixing and estuarine circulation in a tidal estuary react to periods of extreme freshwater discharge. We find that even during extremely high or low discharge, salt mixing follows the freshwater discharge on time scales as short as days, and that estuarine circulation patterns are largely explained by the local distribution of mixing. As extreme runoff events are likely to occur more often with climate change, these findings may help to understand the dynamics inside future estuaries.

Estuarine Exchange Flow in the Salish Sea

AbstractThe Salish Sea is a large, fjordal estuarine system opening onto the northeast Pacific Ocean. It develops a strong estuarine exchange flow that draws in nutrients from the ocean and flushes the system on timescales of several months. It is difficult to apply existing dynamical theories of estuarine circulation there because of the extreme bathymetric complexity. A realistic numerical model of the system was manipulated to have stronger and weaker tides to explore the sensitivity of the exchange flow to tides. This sensitivity was explored over two timescales: annual means and the spring‐neap. Two theories for the estuarine exchange flow are: (a) “gravitational circulation” where exchange is driven by the baroclinic pressure gradient due to along‐channel salinity variation, and (b) “tidal pumping” where tidal advection combined with flow separation forces the exchange. Past observations suggested gravitational circulation was of leading importance in the Salish Sea. We find here that the exchange flow increases with stronger tides, particularly in annual averages, suggesting it is controlled by tidal pumping. However, the landward salt transport due to the exchange flow decreases with stronger tides because greater mixing decreases the salinity difference between incoming and outgoing water. These results may be characteristic of estuarine systems that have rough topography and strong tides.

Modelling assessment of how small-scale vertical movements of infectious sea lice larvae can affect their large-scale distribution in fjordic systems

Sea lice are ectoparasites that can be found in high numbers in and around salmon farms, where they are a threat to fish health and can induce high aquacultural costs. Large numbers of suitable hosts facilitate the infection and subsequent release of their planktonic larvae in the surrounding environment where they can potentially infect wild salmonids, including migrating juvenile fish (smolts). Investigating sea lice spatial distribution is generally done using coupled hydrodynamic and particle tracking models. The quality of these numerical tools is critical to identify areas of higher infection risk to valuable wild salmon populations and thus support sustainable salmon aquaculture. While the transport of salmon lice is mainly affected by physical processes, biological behaviours such as vertical swimming can also play an important role. However, a review of previous sea lice studies shows no clear consensus on their swimming abilities and the parameters implemented in sea lice dispersion modelling. Here, we focus on the Diel Vertical Migration (DVM) behaviour, a vertical migration that sea lice perform within a daily cycle. Our sensitivity study of the infectious copepodid phase of sea lice highlights how their retention potential and spatial distribution within a water body are affected by their vertical swimming velocity and maximum swimming depth. In a fjordic system (Loch Linnhe on the West Coast of Scotland), the vertical position of sea lice can affect their horizontal trajectory due to the two-layer exchange flow of the estuarine circulation. At the surface, transport is mainly due to the wind driven circulation, and the residual seaward current. Lower in the water column, the saltier shelf water is drawn into the sea loch. This can potentially increase the retention of sea lice if they dive deep enough to reach those opposing subtidal currents. Therefore, lack of confidence in sea lice DVM parameterisation may introduce inaccuracy in modelled sea lice distribution. Effective sea lice management in aquaculture would benefit from more observational data like farm sea lice count, sea lice swimming laboratory observations, field observation of sea lice vertical distribution and accurate sea lice quantification in the field. This would help to reduce uncertainties derived from sea lice vertical swimming behaviour parameterisation in lice dispersal models.

Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea

Abstract. Turbulent diapycnal mixing is important for the estuarine circulation between basins of the Baltic Sea as well as for its local ecosystems, in particular with regard to eutrophication and anoxic conditions. While the interior of the basins is overall relatively calm, stratified flow over steep bathymetric features is known as a source of strong turbulent mixing. Yet, current in situ observations often cannot capture the spatio-temporal development of dynamic and intermittent turbulent mixing related to overflows over rough bathymetry. We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the sparsely sampled Southern Quark located in the Åland Sea, connecting the northern Baltic Proper with the Bothnian Sea. Our data were acquired during two 1-week cruises on R/V Electra in February–March 2019 and 2020. We collected high-resolution broadband acoustic observations of turbulent mixing together with in situ microstructure profiler measurements, and current velocities from acoustic Doppler current profilers. We found that a temporally reversing non-tidal stratified flow over the steep bathymetric sill created a dynamic and extremely energetic environment. The observed flow reversed during both cruises on timescales of a few days. Saltier, warmer, and less oxygenated deep water south of the sill was partly blocked, the reversing flow was at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill, and high spatial variability occurred in the surface layer on small scales. Dissipation rates of turbulent kinetic energy, vertical turbulent diffusivities, and vertical salt flux rates were increased by 3–4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow with average dissipation rates near the sill between 10−7 and 10−6 W kg−1, average vertical diffusivities of 0.001 m2 s−1 in the halocline and up to 0.1 m2 s−1 below the halocline, and average vertical salt flux rates around 0.01 g m−2 s−1 in the halocline and between 0.1 and 1 g m−2 s−1 below the halocline. We suggest, based on acoustic observations and in situ measurements, that the underlying mechanism for the highly increased mixing across the halocline is a combination of shear and topographic lee waves breaking at the halocline interface. We anticipate that the resulting deep- and surface-water modification in the Southern Quark directly impacts exchange processes between the Bothnian Sea and the northern Baltic Proper and that the observed mixing is likely important for oxygen and nutrient conditions in the Bothnian Sea.

A 3D Numerical Study on the Tidal Asymmetry, Residual Circulation and Saline Intrusion in the Gironde Estuary (France)

A full 3D numerical model is used for studying tidal asymmetry, estuarine circulation, and saline intrusion in the Gironde estuary. The model is calibrated and verified using the data measured during two field surveys in the Gironde estuary. Harmonic analysis of numerical results is proposed to understand how the superposition of M2, M4 and M6 components generate a complex estuarine circulation and salinity intrusion in the Gironde estuary. The numerical results show that the M6 component plays a significant role as important as the M4 one in modifying the nature of tidal asymmetry, especially in the Gironde upper estuary. In this case, the use of the phase lag between M2 and M4, neglecting M6, to predict the tidal asymmetry nature could produce errors. The effect of asymmetrical tides on saline intrusion and residual circulation is specifically discussed here.

Seasonal changes in submarine melting mechanisms controlling frontal ablation of Hansbreen, Svalbard

Abstract We describe the annual pattern of frontal ablation driven by submarine melting mechanisms at the Hansbreen terminus: these are reflected in the intensity and spatial distribution of calving events. Analysis of time-lapse images of the Hansbreen front in conjunction with oceanographic and meteorological data shows that calving intensity is driven primarily by seawater temperature. Regression analysis also highlights the importance of air temperature, which we take to be a proxy for surface ablation and subglacial discharge. This, combined with seasonal changes in ice cliff tortuosity and the increasing significance of wave motion outside the ablation season, enabled us to determine seasonal changes in the mechanisms of ice cliff undercutting by submarine melting. While submarine melting controlled by estuarine circulation primarily drives frontal ablation in summer, wave-driven melting at the waterline is more important outside the ablation season. During winter, ice cliff undercutting by melting is suspended by low seawater temperature, negligible subglacial water discharge and sea-ice cover. The most intense frontal ablation, recorded in summer, was related to higher sea temperature and vigorous estuarine circulation.

A Tampa Bay coastal ocean model (TBCOM) nowcast/forecast system

As a partially mixed estuary, Tampa Bay is influenced both by its connections to the adjacent Gulf of Mexico (GOM) and what occurs locally within the estuary. To assist in addressing the many scientific questions arising from various environmental factors, a very high resolution Tampa Bay Coastal Ocean Model (TBCOM) is modified to downscale from the deep GOM, across the continental shelf and into Tampa Bay to provide daily, automated nowcasts and forecasts. Veracity tests are provided for sea levels and currents forced by tides, synoptic weather variations and for extreme events. The model is also demonstrated to reproduce the net estuarine circulation through comparisons between in situ observations and model simulations. With demonstrated accuracy, TBCOM forecast sea levels are provided online as a reference for navigation support and for extreme events such as hurricane storm surge. Model simulations, even with a perfect model, are subject to errors by the forcing functions. For Tampa Bay, the NOAA NAM winds used to force the model are found to underestimate the actual winds, suggesting that additional wind observations for assimilation into operational weather forecast models may offer further improvements. This finding highlights the need for further coordination between coastal ocean observing systems and the ocean and atmosphere modeling communities. With coastal ocean and estuary material properties determined largely by the circulation, most ecological applications require accurate and timely circulation information, which the TBCOM Nowcast/Forecast System for Tampa Bay endeavors to provide.

Calibration of redox thresholds in black shale: Insight from a stratified Mississippian basin with warm saline bottom waters

Depositional models for black shale formation rely on a detailed understanding of redox conditions and how they relate to basin development. Here, we calibrate multiple redox proxies (Fe speciation, U and Mo systematics, and pyrite framboid distributions) in the Bowland Shale, a major hydrocarbon unit in the Mississippian basin of northern England, and develop a depositional model for black shale deposition in basins adjacent to extensive carbonate platforms. The transition from deep ramp carbonates to basinal mudrocks initially occurred under oxic conditions before anoxic conditions began to expand from basin-center locations. By the end of Bowland Shale deposition, ∼10 m.y. later, black shale deposition extended from the basin, where platform-edge carbonates were deposited, into shallow-water settings. By this stage, euxinic conditions were widespread throughout the Bowland Shale. The prolonged persistence of euxinia during younger Bowland Shale deposition, and the sharp transition to fully oxygenated facies at the basin margin, suggest there was a well-developed water-column pycnocline. Black shale development in silled basins is traditionally interpreted to form beneath a halocline with a positive water balance caused by freshwater run-off (estuarine circulation). This model is not considered appropriate in this case. Bowland Shale deposition was terminated by the onset of turbidite deposition supplied by a major deltaic system; but for most of its history, the basin was surrounded by carbonate platforms that are unlikely to have experienced brackish conditions. The encroachment of the clastic system saw the rapid improvement of basinal oxygenation in the uppermost Bowland Shale, and even minor turbidite sandstones within the black shales coincided with a weakening in the intensity of euxinia, which suggests that fresher, sediment-laden waters flushed out and oxygenated the basin. We therefore suggest a warm saline bottom-water model for black shale deposition, with basinal waters generated on the adjacent carbonate platforms due to evaporation. Such a scenario is likely more broadly applicable to basinal black shales that developed adjacent to shallow-water carbonate successions.

Analysis of the effects of dredging on current circulation and salinity of the Itajaí-Açú estuary

The Itajaí-Açú river is the largest source of drainage in the state of Santa Catarina (Brazil). At the mouth of this river is located the Itajaí-Açú estuary, which is a highly stratified estuary, dominated mainly by river discharge. The port of Itajaí is located at the mouth of this estuary, one of the most important ports in the country's south. Due to the presence of the port, the Itajaí-Açú estuary is constantly dredged. Therefore, the main objective of this paper is to analyse the effects of the dredging of 2011, which deepened the channel from 11m to 14m, on the estuary circulation of the Itajaí-Açú river in comparison to the estuary circulation in 2008. Thus, data on river discharge, recorded by a hydrometric station, and current velocity vertical profiles, recorded by an Acoustic Doppler Current Profiler (ADCP) anchored in the channel of the estuary were analysed. A salinity analysis was also performed with data recorded by a CTD to compare the salinity stratification of the water column in the study period. Results showed an increase in current velocity in the estuary after the 2011 dredging. In both study periods (2008 and 2011), currents were ebb-directed. Results suggest that the dredging has modified the estuarine circulation as the currents intensified. Salinity results show an increase in values after the dredging (2011), being the water column clearly stratified.

Wind-induced switch of estuarine residual circulations and sediment transport in microtidal bay

To reveal the mechanism for the transport of water and suspended sediments by wind-induced estuarine residual circulations, in situ moorings of acoustic Doppler current profilers were conducted in microtidal Onsan Bay. Water circulation within the bay was characterized by freshwater discharge from the river and free exchange with the open sea, showing current seaward outflow and landward inflow in the western (M1) and eastern (M2) flanks of the navigation channel, respectively. This circulation was influenced by the intrusion of alongshore currents strengthened (weakened) by the northerly (southerly) winds. In M1, seaward outflow was dominant, except for surface disturbances caused by winds. Meanwhile, the classical estuarine circulation (72% of the entire period) in M2, which prevailed under the southerly winds, was switched into the reversed estuarine circulation when the stress of the northerly wind persistently exceeded 0.005 Pa (> 5 h). The water transport (Trw) in M1 did not significantly vary depending on the wind direction, but the Trw in M2 could be reduced by 53% within 10 m above the bed as the southerly winds were transited into northerly winds. Over the entire period, Trw tended to be mutually compensated for and balanced by the circulation system between M1 and M2. On average, however, the sediment transport (Trsed) in M2 was 37% higher than M1, indicating that the suspended sediments were preferentially deposited within the bay. Most of the suspended sediments causing the deposition within the bay were mainly transported by estuarine residual circulation rather than tidal pumping. In M2, the frequent resuspension events of the fine-grained sediments (d50 = 6 μm) due to the high variability of the along-channel current velocity could maintain a sediment concentration of 18 mg l−1 in the bottom layer. These suspended sediments responded mainly to landward current in bottom layer (classical estuarine circulation) rather than seaward current (reversed estuarine circulation), contributing to increase in Trsed. Considering that the prevailing winds from 2013 to 2020 in Onsan Bay mainly create a favorable condition for the development of the classical estuarine circulation, the sedimentation is expected to persist.