Sediment Exchange Research Articles

Morphological Modelling to Investigate the Role of External Sediment Sources and Wind and Wave-Induced Flow on Sand Bank Sustainability: An Arklow Bank Case Study

Offshore anthropogenic activities such as the installation of Offshore Renewable Energy (ORE) developments and sediment extraction for marine aggregates have been shown to disrupt current flow, wave propagation, and sediment transport pathways, leading to potential environmental instability. Due to the complexity of the interconnected sediment transport pathways in the south-western Irish Sea combined with an increase in planned anthropogenic activities, the assessment of this risk is imperative for the development of a robust marine spatial plan. Subsequently, this study uses two-dimensional morphological modelling to build upon previous studies to assess the dependency of Arklow Bank’s local sediment transport regime on external sediment sources. Additionally, scenario modelling is used to identify vulnerable areas of this offshore linear sand bank to wind and wave-forcing and to examine the nature of this impact. A sediment budget is estimated for Arklow Bank, whereby seven source and nine sink pathways are identified. New evidence to support the exchange of sediment between offshore sand banks and offshore independent sand wave fields is also provided. The areas of the bank most vulnerable to changes in external sediment sources and the addition of wind- and wave-induced flow are analogous. These high vulnerability zones (HVZs) align with regions of residual cross-flow under pure current conditions. The restriction of sediment sources off the southern extent of Arklow Bank impacts erosion and accretion patterns in the mid- and northern sections of the bank after just one lunar month of simulation. Where tidal current is the primary driver of sand bank morphodynamics, wind- and wave-induced flow is shown to temporarily alter sediment distribution patterns. Wind and wave-induced flow can both accelerate and decelerate the east-west fluctuation of the upper slopes of the bank, yet the nature of this impact is inconsistent due to the misalignment of the directionality of these two forces. The methods and new knowledge derived from this study are directly applicable to tidally-dominated environments outside the Irish Sea.

Alongshore variability in berm and sandbar migration patterns on a highly dynamic beach

Sediment exchange between the shoreface and the foreshore is investigated through the analysis of sandbar and berm migration cycles at La Misión beach in Baja California, Mexico. In general, nearshore sandbars migrate onshore during low-energy wave conditions merging with the shoreline to become intertidal sandbars that later contribute to berm rebuilding. However, significant alongshore differences were observed in the sandbar welding and berm rebuilding process, which can be attributed to alongshore wave skewness and near-surfzone flow variations. The analysis of topographic and bathymetric surveys collected monthly over a 5-year period revealed larger shoreline retreat and longer recovery times in the southern beach compared to the north. This was related to the inability of sandbars to weld to the intertidal beach during spring and summer, occurring only during extended periods of low-energy waves. In the southern beach, the berm formation process exhibited less dependence on incoming wave energy, persisting during mild-energy winters, and reforming in the presence of sandbars. Hydrodynamic measurements indicated that the southern part of the beach experienced relatively more intense net offshore flows (up to −0.2 ms−1) due to undertow, and possibly related to the presence of persistent rip currents. In contrast, the berm in the northern beach followed a seasonal erosion and rebuilding process associated with sandbar migrations driven by variations in incoming wave energy and cross-shore sediment transport. This was characterized by larger near-bed wave velocity skewness compared to the south. Net flows in the north were predominantly onshore, reaching up to 0.1 ms−1 during lower-energy periods with oblique wave incidence. This study reveals the presence of considerably distinct sandbar and berm migration cycles along a relatively short beach of 2.2 km long. It also highlights how such variability can be related to the morphodynamic feedback with sandbars affecting hydrodynamics, particularly near-bed wave skewness and near-surfzone flow variations, which in turn influence sediment transport and morphological evolution.

Modelling reservoir sediment flushing through a bottom tunnel with an initially covered intake

Sediment flushing through a bottom tunnel is one of the most effective methods to alleviate reservoir sedimentation. However, the multi-physical hydro-sediment-morphological processes of reservoir sediment flushing through an initially covered bottom tunnel intake have remained poorly understood, and a physically enhanced and practically viable mathematical model is required. The present study uses an integrated model to resolve sediment flushing through an initially covered bottom tunnel intake. The proposed model couples a two-dimensional double layer-averaged model for the reservoir with a one-dimensional model for the bottom tunnel by means of numerical fluxes calculations at the inner boundary. The governing equations are solved synchronously using a well-balanced finite volume method. Several parameters related to boundary resistances and sediment exchange flux need to be determined for model closure. The model is tested against data from a series of physical experiments on reservoir sediment flushing, with sound agreement achieved between computed and measured scour hole geometries. Moreover, the present model successfully predicts the occurrence of tunnel blockage. The results prove that reservoir sediment flushing is best accomplished for high reservoir water level, small cover layer thickness, short tunnel length and steep tunnel slope. The present model facilitates reservoir design and operation to help preserve reservoir capacity.

Sediment exchange between southern yellow sea and Yangtze river Estuary in response to storm events

The pattern of sediment exchange between the Yangtze River Estuary and the southern Yellow Sea constitutes a crucial yet contentious line of scientific inquiry. This conundrum hampers our comprehensive understanding to predict the future morphological evolution of the radial sand ridges and Jiangsu tidal flats. This study investigates various processes, including tides, wind and waves, to identify the dominant factor controlling sediment exchange between the southern Yellow Sea and the Yangtze River Estuary under storm conditions, using a validated numerical model. Our results show that tide is the dominant force controlling hydrodynamics and sediment transport between the two systems. Tidally induced residual currents and sediment fluxes exhibit a consistent northwesterly trajectory, from the Yangtze River Estuary to the southern Yellow Sea. Conversely, the contributions of wind and wave-induced currents under normal wind conditions (Beaufort Scale 3, 3.4 m s−1) adjust net sediment flux by less than 10 and 46–68%, respectively. In particular, the influence of wind and waves on sediment transport varies significantly with wind direction. While southerly or southeasterly winds amplify the tide-induced northwestward sediment transport, northerly or northeasterly winds curtail it. Under strong winter storm conditions, however, the perturbation caused by northerly winds and waves supersedes the influence of tides in controlling sediment transport, leading to a net residual current and sediment flux oriented southeastwards.

SPATIAL VARIABILITY IN BEACH-FACE SLOPES FROM SATELLITE REMOTE SENSING

The beach face is the most seaward region of the dry beach. This region is the primary interface between land and ocean, and therefore has a great influence on coastal processes such as the exchange of sediment between land and sea or the reflection of wave energy at the shoreline. In particular, the slope of the beach-face, is an important parameter in coastal engineering to calculate the vertical and horizontal excursion of wave run-up (Stockdon et al., 2006). Yet, despite the importance of the beach-face slope parameter in many formulations used by coastal engineers, this quantity remains poorly mapped along the world’s sandy coastlines and the absence of large-scale datasets of beach-face slope is presently limiting our ability to deploy coastal inundation forecasting systems (O’Grady et al., 2019). This work describes a novel methodology to estimate beach-face slopes with satellite remote sensing and presents large-scale datasets of beach-face slopes along open-coast sandy coastlines around the Pacific Rim.

Sediment Exchange Across Coastal Barrier Landscapes Alters Ecosystem Extents

AbstractBarrier coastlines and their associated ecosystems are rapidly changing. Barrier islands/spits, marshes, bays, and coastal forests are all thought to be intricately coupled, yet an understanding of how morphologic change in one part of the system affects the system altogether remains limited. Here we explore how sediment exchange controls the migration of different ecosystem boundaries and ecosystem extent over time using a new coupled model framework that connects components of the entire barrier landscape, from the ocean shoreface to mainland forest. In our experiments, landward barrier migration is the primary cause of back‐barrier marsh loss, while periods of barrier stability can allow for recovery of back‐barrier marsh extent. Although sea‐level rise exerts a dominant control on the extent of most ecosystems, we unexpectedly find that, for undeveloped barriers, bay extent is largely insensitive to sea‐level rise because increased landward barrier migration (bay narrowing) offsets increased marsh edge erosion (bay widening).

The physical processes of sandy beach evolution under storm and non-storm wave conditions simulated in wave flume

Large waves and high water levels generated by coastal storms are predominant drivers for coastal beach erosion. This experimental study is undertaken to investigate the physical processes of beach profile evolution under storm waves by simulating step varying storm waves and variable water levels in a wave flume of 30 m long, 0.6 m wide and 1 m high and mild beach slope tanθ=1/10. The storm waves were simulated by following a conceptual storm hydrograph of three stages: at the first stage of 60 min both wave height and water level were gradually increased to peak storm wave height and water level, at the second stage of 60 min both maximum wave height and water level were kept for constant, and at the last stage of 60 min storm wave heights were gradually decreased to non-storm waves and water levels. Non-intrusive high-resolution cameras were used to record time-series of free surface water elevation, wave breaking point position, sandbar migration and location, and beach elevation profile in a spatial resolution of 1.4 mm and a temporal resolution of 0.1 s. In analyzing the collected experimental data, the physical processes of beach evolution under both non-storm and storm waves were found significantly different: under non-storm waves, the movement of sandbar always was found to be in the seaward direction, and the beach profile was eroded and accreted before and after the sandbar, respectively, but under storm waves, the sandbar location was found to migrate in either the landward or seaward direction, the beach profile was slowly accreted before the sandbar and eroded after the sandbar when the storm waves were gradually decreased to the non-storm waves. The surf zone is the main change area in beach morphology modification, and the sandbar is the important indicator for distinguishing the cross-shore sediment exchange. Under both non-storm and storm waves, large waves, high water levels, and especially great gradient of wave energy flux due to changes in wave height, period and water depth are the main factors controlling the formation and migration of sandbar, and resulting in beach erosion and accretion before and after the wave breaking point, respectively.

Role of trace metal co-limitation in cyanobacterial blooms of Maumee Bay (Lake Erie) and Green Bay (Lake Michigan)

The open waters of large lakes can sometimes become so depleted in important metals that phytoplankton communities become either growth limited or limited in some metabolic function. Metals such as Fe, Ni, Mo, and Zn are used as co-factors for enzymes by phytoplankton in core metabolic functions, as well as metabolic pathways that allow phytoplankton to use less preferred forms of N and P (e.g. nitrates, urea, and organic phosphorus). In the Laurentian Great Lakes, metal limitation has been observed primarily in waters that are isolated from tributary inputs and sediment exchange. These are situations where the supply of metals is very low relative to demand. We hypothesized that another situation where metal limitation could occur is within algal blooms, where the demand for metals is high because preferred forms of N and P are often low or absent and the phytoplankton biomass is extremely high. As a preliminary test of this hypothesis, we performed seven laboratory incubation experiments on naturally occurring phytoplankton communities from two nearshore habitats that frequently experience blooms (Green Bay in Lake Michigan and Maumee Bay in Lake Erie). Metals and labile nutrients (inorganic N and P) were often present at low concentrations or below the method detection limit. Amendments of inorganic N (5 experiments) and P (1 one experiment) resulted in increased chlorophyll in laboratory incubations, but metal amendments alone never appeared to stimulate growth. Although we attempted to sample during conditions when we hypothesized metal limitation would be most likely, we cannot rule out the possibility that metal limitation is occurring at other times in these eutrophic nearshore areas. Further, metal availability could affect other aspects of the phytoplankton community, such as the production of cyanotoxins or the interactions between different phytoplankton taxa.

Inter-annual morphological evolution and volume changes of a meso- to macrotidal beach exhibiting multiple intertidal bars (MITB)

Although morphologically persistent in the long term, Multiple Intertidal Bar Systems (MITBs) display short-term, especially seasonal, morphodynamic behaviour. Analysis of high-density, monthly DGPS surveys conducted at Murlough and Ballykinler beaches, inter- and supratidal sediment volumes and hydrodynamic forcing (wave conditions and water levels), demonstrates a link between strong seasonality in wave climate and MITB beach behaviour. Summer, low-energy conditions limit cross-shore sediment exchange during which MITB beach morphology tends to stabilise throughout the season. With the onset of high-energy winter conditions cross-shore sediment exchanges occur between inter- and supratidal areas. Sediment transport is then enhanced during storm conditions that are coincident with spring tides, leading to high total water levels (TWL). Ultimately the storm sequencing, (frequency, magnitude and inter-storm interval), is the key parameter driving the beach morphological response. Major erosional patterns occur when the most energetic event, combined with spring high tide, occurs at the beginning of the winter season. Subsequent, less energetic storms then promote bar recovery until another extreme event occurs.Seasonality is also evident in alongshore dynamics. Cross-shore drainage channels that dissect the intertidal bars migrate alongshore, driving alongshore sediment transport and MITB longshore migration patterns. In summer migration of drainage channels is limited, whereas the winter high-energy forcing enhances channel migration rates and resulting sediment transport. Differences in dynamics between the two study sites are attributed to differences in local geology, beach morphology and sediment size, but in both locations, drainage channels are in fine migrating toward the inlet and associated ebb delta that divides the bay. Subsequent transport vectors are unknown, but the observations highlight the primary role of the inlet in the sediment circulation dynamics in the system as a whole.

Experimental and numerical modeling of pile-rock breakwater gap arrangement for optimal coastal erosion protection in deltaic coasts

Pile-Rock breakwaters (PRBW) have been constructed as long continuous features in the West Sea of the Mekong Delta. Optimising the gap arrangement and spatial orientation of these continuous breakwaters can result in positive impacts on the velocity field, sediment distribution, and shoreline morphology. In this study, experimental and numerical modelling was performed to investigate the effect of breakwater gaps on bedload sediment transport and nearshore bed morphology. The gap width across various hydrodynamic conditions was found to have a significant impact on bedload sediment transport and nearshore bed morphology. This study found breakwater gaps can result in high current speed and flow due to wave diffraction, which subsequently also accelerates the shoreline and bed erosion process. The gaps also have a positive impact by facilitating sediment supply behind the breakwaters, which enhances the living shoreline. The sediment exchange rate was higher in low water conditions for the emerged breakwater caused by wave circulation stirring up the sediment with higher wave amplitude. During high-water conditions, the results showed a relatively smaller area of sedimentation behind the breakwater. Under high water level conditions, the ability to stir up sediment decreases, resulting in low sediment transport. The findings from this study can be used to optimize the design of pile-rock breakwaters and their performance on the deltaic coast.

Barium isotope signatures of barite–fluid ion exchange in Equatorial Pacific sediments

The isotope composition of barium (Ba) in the mineral barite (BaSO4) is emerging as a powerful tracer of the pelagic Ba, carbon, and sulfur cycles. However, it is critical to identify and constrain processes that may alter the primary isotope composition of Ba, δ138Ba, in BaSO4, particularly during early diagenesis. To this end, we analyzed the Ba isotope composition of porewaters and co-located BaSO4 in sediments from the Equatorial Pacific and performed a series of laboratory experiments with these same BaSO4 to assess rates of Ba isotope alteration. We find that sedimentary BaSO4 exhibit Ba isotope compositions ≈+0.1‰ and are offset by ≈−0.16‰ relative to ambient porewaters. Experiments using isotope-labeled seawater show extensive and rapid transfer of Ba ions between the solid and fluid phase through coupled BaSO4 dissolution-precipitation, even at chemical equilibrium. Using published values for Ba isotope fractionation during BaSO4 precipitation and dissolution, we calculate that co-located BaSO4 and porewaters should exhibit Ba isotope offset of −0.17‰ at isotopic equilibrium, similar to the offsets observed in Equatorial Pacific sediments. Altogether, the field data, laboratory experiments, and calculations indicate that ion exchange occurs in Equatorial Pacific sediments and that this process also drives the observed Ba isotope offsets between pelagic BaSO4 and porewaters. This finding implies that ion exchange may alter the isotope composition of Ba in sedimentary BaSO4, though the degree of alteration will depend on the proportion of Ba held in the solid phase. More broadly, this study provides an example of how ion-exchange-mediated processes are widespread in marine geochemistry and that these processes likely affect other metals and minerals beyond Ba and BaSO4.

Determining depth of closure based on time-series beach profiles and empirical formulas: A case study along the Florida coast

Depth of closure (DOC) is defined as the most landward depth seaward of which there is no significant change in bed elevation and no significant net sediment exchange between the nearshore and the offshore over a certain period of time, such as 5 to 20 years. DOC is an essential parameter used in beach and shore protection, sediment management, and many other aspects of coastal studies. Taking advantage of advancements in wave hindcast and bathymetry measurement in the past 20 years (2000-2019), this study determined the DOC at 12 locations along the Florida coast, including three from the northwest Gulf coast, three from the west Gulf coast, and six from the east Atlantic coast. The 12 sites covered a wide range of coastal morphodynamic conditions, with considerable difference in tidal ranges, incident wave heights, as well as nearshore and offshore morphology. Hindcast wave data from WAVEWATCHIII, available since 2005, were analyzed and applied to calculate the closure depth using various empirical formulas. At all the 12 study sites, time-series profiles demonstrated an apparent convergence point indicating the presences of a DOC. The bed-level change at DOC, as quantified by the standard deviation of elevation variation, ranged from 0.05 m to 0.19 m. Along the studied northwest Florida Gulf coast the DOC ranged from 9.12 m to 9.76 m. The DOC along the studied west Florida Gulf coast ranged from 1.59 m to 4.06 m and is influenced by the shallow flat inner continental shelf. Along the studied east Florida Atlantic coast, the DOC ranged from 4.35 m to 8.20 m, with considerable alongshore variation. The Birkemeier formula yielded the closest predictions to the measured values. A linear relationship between the seaward slope of the outer bar and DOC was identified. Incorporating the seaward slope of the outer bar into the Birkemeier formula improved the accuracy of DOC prediction.

Climate-driven tradeoffs between landscape connectivity and the maintenance of the coastal carbon sink

Ecosystem connectivity tends to increase the resilience and function of ecosystems responding to stressors. Coastal ecosystems sequester disproportionately large amounts of carbon, but rapid exchange of water, nutrients, and sediment makes them vulnerable to sea level rise and coastal erosion. Individual components of the coastal landscape (i.e., marsh, forest, bay) have contrasting responses to sea level rise, making it difficult to forecast the response of the integrated coastal carbon sink. Here we couple a spatially-explicit geomorphic model with a point-based carbon accumulation model, and show that landscape connectivity, in-situ carbon accumulation rates, and the size of the landscape-scale coastal carbon stock all peak at intermediate sea level rise rates despite divergent responses of individual components. Progressive loss of forest biomass under increasing sea level rise leads to a shift from a system dominated by forest biomass carbon towards one dominated by marsh soil carbon that is maintained by substantial recycling of organic carbon between marshes and bays. These results suggest that climate change strengthens connectivity between adjacent coastal ecosystems, but with tradeoffs that include a shift towards more labile carbon, smaller marsh and forest extents, and the accumulation of carbon in portions of the landscape more vulnerable to sea level rise and erosion.

Global Trends and Prospects of Nepheloid Layers: A Comprehensive Bibliometric Review

Nepheloid layers are widely distributed in the marine environment, and their formation and evolution pose many challenges to the current understanding of ocean dynamics and marine sedimentology. In sediment transport processes, nepheloid layers significantly contribute to the exchange of sediment between the continental shelf and the slope. In this paper, we summarize the global research trends on nepheloid layers. In total, 689 publications from 1990 to 2022 were collected from the Web of Science and analyzed using bibliographic software, including Bibliometrix, VOSviewer, CiteSpace, and CorText. Based on these publications, past and present popular research on nepheloid layers is examined and evaluated. The trends in nepheloid layer research are summarized by analyzing keywords, article references, countries, institutions, and authors. Finally, prospects and several key questions related to nepheloid layers are concluded, which can potentially guide future studies. The bibliographic analysis can provide new insights into the history of nepheloid layers. The results also provide valuable information for other researchers and programs investigating geological, geophysical, and biogeochemical processes.

The Analysis of Large Dam Impacts on Sediment Grain Size Distribution in a Tropical River System

Sedimentation is a natural phenomenon of rivers that is enhanced by modification of the river basin. The presence of dams delays the exchange of sediments, nutrients, and organisms between the terrestrial and aquatic environments. This article assesses the impact of the Selangor dam on the sediment grain size distribution and its association with river velocity and discharge. The fieldwork for sampling is conducted in the normal and rainy seasons. The samples were analyzed through a sieve analysis procedure to determine the particle size of the sediments. After the sieve analysis technique, GRADISTAT analysis was performed on the output. The GRADISTAT analysis classifies the sediments between sandy gravel and sand, and the median grain size (D50) ranges from 4.00 to 0.18 mm. The spatial distribution of the D50 shows that the bed-load sediments of the upper Selangor River are becoming fine-grained downstream. The skewness of the sediments differs from 0.86 to 8.44, which indicates that the sediments are poorly to moderately well sorted. The Spearman's correlation of the D50 and river velocity and discharge determine no association of the D50 with river velocity and discharge. The stations near Selangor Dam have high slopes and receive "sediment hungry" water that washes small-sized sediments; therefore, the upper stations have a more significant amount of gravel and large sand. Doi: 10.28991/CEJ-SP2023-09-02 Full Text: PDF

Influence of dyke-type causeway on Urmia Lake (NW Iran); insights from water physico-chemical parameters seasonal (2019) changes

The Urmia Lake in NW Iran, the world’s second-largest hypersaline lake, has been exposed to rapid water level fall in the last two decades due to water resources mismanagement, building up of dams on inlet rivers, and the causeway construction in the lake. This human intervention has divided the lake into northern and southern parts and caused an extreme disturbance in the hydro-chemical and hydrodynamic systems, namely current direction reversal.To fully understand the current situation, water depth and lake surface and deepwater physico-chemical parameters (i.e. density, temperature, acidity, electrical conductivity, and total dissolved solids) are analysed in the wet and dry seasons of 2019. In the wet season, deepwaters have a higher density than surface waters, and the northern part of the lake contains denser water compared to its southern counterpart. The formation of density gradients creates an anticlockwise direction lake current. Deepwaters show higher electrical conductivity and total dissolved solids than surface waters in the wet season. Unlike the wet season, the lake water becomes more homogenised due to wave action and evaporation in the dry season. Our study clearly demonstrates that the water passageway of the causeway does not allow for a complete yearly balance between water and sediment exchange. Results suggest that the northern and southern parts of the lake have almost independent hydro-chemical and hydrodynamic systems. In the undeniable reality of global warming trends, anthropogenic interventions into sensible ecologic systems need to be better planned if we were to prevent the ecological disasters we now have to face.

Impact of an exceptional winter flood on benthic oxygen and nutrient fluxes in a temperate macrotidal estuary: Potential consequences on summer deoxygenation

Despite 20 years of control on eutrophication, episodes of summer hypoxia still occur in the Loire estuary, impacting water quality and posing a key scientific and management challenge. This work aimed to quantify the contribution of the benthic compartment to hypoxia in the Loire estuary by direct measurement of water–sediment fluxes and an in-depth understanding of the seasonal variations of oxides and phosphorus stocks. During the summer’s low-discharge period, results show that the iron oxide-rich deposit is stable under hypoxic conditions, limiting the release of dissolved phosphorus into the overlying water column. The high nitrate content of the water column appears to be an important oxidizer of iron during hypoxic periods, limiting dissolved phosphorus leakage and aggravation of hypoxia. During the exceptional winter flood, significant sediment erosion associated with bubbling phenomena (attributed to methane efflux) created severe fractures in the sediment and stimulated water–sediment exchange. During the following months, these fractures were progressively filled, which decreased the intensity of benthic fluxes. However, due to the high residence time in the water during the summer period, a simple model demonstrated that benthic contributions were sufficient to directly (by direct oxygen consumption) or indirectly (by promoting ammonia oxidation) affect the oxygen stock in the water column during the low-discharge period. Our study demonstrates the importance of the benthic compartment in the occurrence of hypoxia and the obvious lack of knowledge to illustrate and model the biogeochemical functioning of the estuary.

Straits and seaways: controls, processes and implications in modern and ancient systems

Abstract Straits and seaways are connections between basins, key areas for exchange and transfer of water, heat, biota and sediments, and they can influence regional and global climate. A better understanding of strait/seaway dynamics, their evolution and preserved deposits is, therefore, fundamental to reconstructing the palaeoecology, palaeogeography and stratigraphy of interconnected basins. Straits and seaways are also important for understanding climate dynamics in Earth's past, and for safely exploiting energy resources (fossil and renewable). This volume aims at improving the knowledge on this topic, and at providing a comprehensive collection of state-of-the-art works bringing together complementary disciplines. Even though studies on modern and ancient straits and seaways exist, they are not necessarily conceived towards a better geological understanding of these features as depositional systems in their own right. In this introduction, we emphasize the geological importance of straits and seaways, summarizing the content and key findings of the contributions on this topic. The articles included in this volume explore four main research themes related to straits and seaways: (1) occurrence and classification; (2) morphological features, facies and stratigraphic variability, sedimentary processes and dynamics; (3) tectonic and climatic controls, their feedback with climate changes; and (4) palaeogeographical reconstructions and preservation of associated deposits.

Substantial increase in P release following conversion of coastal wetlands to aquaculture ponds from altered kinetic exchange and resupply capacity

The reclamation of wetlands and its subsequent conversion to aquaculture may alter regional nutrient (im)mobilization and cycling, although direct assessments of phosphorus (P) cycling and its budget balance following wetland conversion are currently scarce. Here, parallel field experiments were conducted to investigate and compare the availability and mobilization mechanisms of P from natural coastal wetlands and the adjacent converted aquaculture ponds based on high-resolution diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques and the DGT-induced fluxes in sediments (DIFS) model. The study found that the conversion of wetland to pond strongly reduced the sediment P pool by changing its forms and distribution. High-resolution data showed that concentrations of labile P and soluble reactive P across the sediment–water profiles were markedly enhanced by the converted aquaculture pond, although they exhibited large spatiotemporal heterogeneity. Moreover, the synchronous distribution of labile P, iron (Fe) and sulfur (S) across profiles in coastal wetlands indicated that the dissolution of Fe (III) oxyhydroxide–phosphate complexes coupled with sulfate reduction were the main mechanisms regulating sediment P mobilization in coastal areas. However, the converted aquaculture pond weakened or even reversed this dependence by decoupling the Fe-S-P reactions by changing the sediment structure and nutrient balance. Substantial increases in labile P, Fe and S fluxes in the pond suggested the conversion of wetland to aquaculture facilitated the internal release of P, Fe and S from sediment into water. The high resupply parameter (R) and desorption rate (k-1) combined with low response time (Tc) in the pond, as fitted by DIFS model, indicated the strong resupply capacity and fast kinetic exchange of sediment P across the sediment–water interface, which is consistent with the high P diffusion fluxes recorded in the pond. It was concluded that converted aquaculture ponds act as an important source of P release in coastal areas, potentially exacerbating water quality degradation and eutrophication. Specific initiatives and actions are therefore urgently needed to alleviate the internal P-loading during aquaculture.

Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems Estuary: the EDoM campaign

Abstract. An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).