Search Results (23)

High-intensity anthropogenic activities have greatly altered the estuarine-shelf depositional processes of sediments, and the intensity and frequency of the impacts of human interventions have far exceeded the natural development of estuarine systems. Since the reform and opening up, human activities such as dams, sand mining, channel dredging, and reclamation have already caused anomalous changes in the dynamical–sedimentary–geomorphological processes of the Lingdingyang Estuary (LE). Analyzing the impact of high-intensity anthropogenic activities on sedimentary processes and the hydrodynamic environment through sedimentary records can provide a scientific basis for predicting the evolution of the estuary and the sustainable development of the Guangdong–Hongkong–Macao Greater Bay Area. The aims of this study are to reveal the impact of varying intensity human activities across different periods on depositional pattern and conduct a preliminary investigation into the spatial differences in sedimentary characteristic attributed to human activities. Two cores (LD11 and LD13) located in the LE were selected for continuous scanning of high-resolution XRF, grain size, and ²¹⁰Pb_ex dating tests, and scrutinized with the previous studies of the historical process of human activities in the LE. The results show the following: (1) The abrupt alterations in ²¹⁰Pb_ex, geochemical indices, and grain size in LD13 happened in close proximity to the 95 cm layer, suggesting a shift in the sedimentary environment during 1994. (2) In the context of the continuous reduction in water and sediment flux into the LE after 1994, the large-scale and high-intensity human activities like sand mining, channel dredging, and reclamation are responsible for the sedimentation rate increase rather than decrease, the coarsening of sediment fractions, the frequent fluctuations in Zr/Rb, Zr/Al, Sr/Fe, and Sr/Al ratios, and the increase in anomalous extremes. (3) Sedimentary records found in locations varying in anthropogenic intensities differ greatly. Compared with the nearshore siltation area, the grain size composition in the channel area is noticeably coarser and exhibits a wider range of grain size variations. The ²¹⁰Pb_ex is strongly perturbed and the vertical distribution is disturbed; the phenomenon of multiple inversions from the surface downwards is shown, making it impossible to carry out sedimentation rate and dating analysis, and the geochemical indicators have changed drastically without any obvious pattern. The evidence of the human activities can be retrieved in the sedimentary record of the estuary and provide a different angle to examine the impacts of the human activities. Full article

The Ganges–Brahmaputra estuary, located in the northern Bay of Bengal, is situated within the largest delta in the world. This river basin features a complex river system, a dense population, and significant variation in watershed vegetation cover. Human activities have significantly impacted the concentration of total suspended matter (TSM) in the estuary and the ecological environment of the adjacent bay. In this study, we utilised the Landsat series of satellite remote sensing data from 1990 to 2020 for TSM retrieval. We applied an atmospheric correction algorithm based on the general purpose exact Rayleigh scattering look-up-table (LUT) and the shortwave-infrared (SWIR) bands extrapolation to Landsat L1 products to obtain high-precision remote sensing reflectance. In conjunction with the normalised difference vegetation index (NDVI), precipitation, and discharge data, we analysed the variation and influencing mechanisms of TSM in the Ganges–Brahmaputra estuary and its surrounding areas. We revealed notable seasonal variation in TSM in the estuary, with higher concentrations during the wet season (May–October) compared to the dry season (the rest of the year). Over the period from 1990 to 2020, the NDVI in the watershed exhibited a significant upward trend. The outer estuarine regions of the Hooghly River and Meghna River displayed significant decreases in TSM, whereas the Baleswar River, which flows through mangrove areas, showed no significant trend in TSM. The declining trend in TSM was mainly attributed to land-use changes and anthropogenic activities, including the construction of embankments, dams, and mangrove conservation efforts, rather than to runoff and precipitation. Surface sediment concentration and chlorophyll in the northern Bay of Bengal exhibited slight increases, which means the limited influence of terrestrial inputs on long-term change in surface sediment concentration and chlorophyll in the northern Bay of Bengal. This study emphasises the impact of human activities on the river–estuary–coast continuum and sheds light on future sustainable management. Full article

Deteriorated river water quality affects estuarine environments due to river and estuary linkages. Thus, river water quality improvement could play a pivotal role in the maintenance of estuarine ecosystem functions. The South Korean administration opened river weirs to restore river ecosystems and announced that this helped restore habitats and improve water quality. Despite these reported positive results in the rivers, little research has been conducted on the effects of weir opening on estuaries linking to rivers. We investigated the physicochemical properties of the Geum Estuary during the initial weir opening (2018) and stabilization (2021) periods. The results indicated that the NO₃⁻ inputs from rivers to estuaries decreased from 2018 to 2021 by 19~68%; however, the trends of [NO₃⁻] in estuarine water columns in nearby estuary dams fluctuated between 2018 and 2021. No change in NO₃⁻ pollutant sources was observed from 2018 to 2021. Unlike [NO₃⁻], [PO₄⁻] and [SiO₂] were higher in 2021; thus, large amounts of previously accumulated PO₄⁻ and SiO₂ within the weirs might have been released from the river into the estuary. This supply of PO₄⁻ could enhance estuarine phytoplankton growth, where PO₄⁻ is a limiting factor for primary production due to excessive inputs of NO₃⁻ from rivers. Therefore, dredging the accumulated sediment within weirs should be considered prior to opening them to prevent the release of PO₄⁻ and SiO₂ into estuaries. Full article

The Mont-Saint-Michel is known worldwide for its unique combination of the natural site and the Medieval abbey at the top of the rocky islet. But the Mont is also located within an estuarine complex, which is considerably silting up. For two decades, large-scale works were planned to prevent the Mont from being surrounded by the expanding salt meadows. The construction of a new dam over the Couesnon River, the digging of two new channels, and the destruction of the causeway were the main operations carried out between 2007 and 2015. The remote sensing approach is fully suitable for evaluating the real impact of the engineering project, particularly the expected large-scale hydrosedimentary effects of reestablishing the maritime landscape around the Mont. The migration of the different channels and the erosion-progradation balance of the vegetation through space and time are the main features to study. Between 2007 and 2023, the erosion of the salt meadows was significant to the south-west of the Mont but more limited to the south-east. During the same period, the sedimentation considerably increased to the north-east of the Bay, which seems to be facing the same silting-up problem. At this stage, the remote-sensing survey indicates mixed results for the engineering project. Full article

River mouths are dynamic systems that can respond rapidly to both fluxes in fluvial water and sediment discharge and marine energy conditions, notably waves. On semi-arid wave-exposed coasts, the morphosedimentary behaviour of river mouths is particularly sensitive to variations in water discharge, which can be significantly influenced by climate variations, in addition to anthropogenic actions such as the construction of dams for water resource needs. In this climatic setting, an increasingly common consequence of decreasing river water discharge is the more or less prolonged closure of river mouths. Most studies have addressed river-mouth closure using analytical, parametric, numerical, or statistical models. The present study uses output from four numerical models to elucidate the hydrodynamic and sedimentary behaviour of the mouth of the Oum-Errabia River (catchment size: 35,000 km²), which debouches on the Atlantic coast of Morocco. The historical evolution of the river mouth and the impact of human interventions, such as the construction of dams, are discussed. The study also briefly discusses the impact of the recent closure of the river outlet, in response to particularly low water discharge, on the marine ecosystem and water quality. The modelling results covering a one-year simulation in this situation of closure indicate a deposition of 427,400 m³ of sediment in front of the mouth of the Oum-Errabia. Ensuring permanent river-mouth opening and tidal flushing and renewal of this river’s estuarine waters will necessitate costly regular dredging. Full article

Coastal embayments provide vital benefits to both nature and humans alike in the form of ecosystem services, access to waterways, and general aesthetic appeal. These coastal interfaces are therefore often subject to human development and modifications, with estuarine embayments especially likely to have been anthropogenically altered. Frequent alterations include damming to eliminate tidal influx, backfilling to create new land, and development for the sake of economic gain, which may cause profound damage to local habitats. By providing a record of transitions in surface waters over time, satellite imagery is essential to monitoring these coastal changes, especially on regional to global scales. However, prior work has not provided a straightforward way to use these satellite-derived datasets to specifically delineate embayed waters, limiting researchers’ ability to focus their analyses on this ecologically and economically important subset of coastal waters. Here, we created ICEDAP, a geometry-based ArcGIS toolbox to automatically delineate coastal embayments and quantify coastal surface water change. We then applied ICEDAP to the coast of South Korea, and found that coastal habitat change was particularly profound within embayed regions identified using an 8 km epsilon convexity setting (denoting a moderate distance from the coast and degree of enclosure by surrounding land areas). In the mapped coastal embayments, more than 1400 km² of coastal habitats were lost during the past 38 years, primarily due to human modification such as large-scale land reclamation projects and the construction of impoundments. Our results suggest that anthropogenic alterations have resulted in the widespread loss of more than USD 70 million of valuable coastal ecosystem services. Together, ICEDAP provides a new innovative tool for both coastal scientists and managers to automatically identify hotspots of coastal change over large spatial and temporal scales in an epoch where anthropogenic and climate-driven changes commonly threaten the stability of coastal habitats. Full article

River deltaic estuaries are dynamic ecosystems characterised by linkages between tidal currents, river water discharge, and sediment from the basin. The present study is based on the application of remote data: multispectral satellite images, DEM, LULC (Land use and land cover), lithology, and hydroclimatic factors. The standardised methodology was based on the adoption of a coupled modelling approach for this work, involving the semi-distributed catchment scale hydrological Soil and Water Assessment Tool (SWAT) model and the statistical Digital Shoreline Analysis System (DSAS) for (1) identifying environmental drivers of sediment transport changes of the estuarine reach; (2) analysis of retrospective changes in shoreline configuration; (3) assessing discharge and sediment dynamics of the estuarine section, and (4) generating future projection scenarios for the estuary’s state to take action for its long-term ecological stability. Our study employs a coupled modelling framework to fill the research gap for Subarnarekha deltaic estuary. Integrating outputs derived from DSAS and SWAT, a comprehensive understanding of the changes in watershed hydrology, water diversions, and damming of rivers have altered the magnitude and temporal patterns of freshwater flow and sediment, which potentially contributed to the receding of the Digha Coast shoreline. Full article

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated. Full article

In 2015 a mine dam with Mn-Fe-rich tailings collapsed releasing million tons of sediments over an estuary, in the Southwest of Brazil. The tailings have a high concentration of metals that contaminated soil until the present day. The high contaminant concentrations possibly caused a selection for microorganisms able to strive in such harsh conditions. Here, we isolated metal(loid) and anti-biotic resistance bacteria from the contaminated estuarine soil. After 16S rDNA sequencing to identify the strains, we selected the Mucilaginibacter sp. strain for a whole-genome sequence due to the bioprospective potential of the genus and the high resistance profile. We obtained a complete genome and a genome-guided characterization. Our finding suggests that the 21p strain is possibly a new species of the genus. The species presented genes for resistance for metals (i.e., As, Zn, Co, Cd, and Mn) beyond resistance and cross-resistance for antibiotics (i.e., quinolone, aminoglycoside, β-lactamase, sulphonamide, tetracycline). The Mucilaginibacter sp. 21p description as new species should be further explored, as their extracellular polymeric substances and the potential of this strain as bioremediation and as a growth promoter in high met-al(loid) contaminated soil. Full article

Coilia grayii is the anadromous form of anchovy that is distributed in the East and South China Seas. It is a common fish species in the estuarine area of the Pearl River. Nevertheless, freshwater populations appear upstream in the Pearl River, but the migratory pathway has been mostly impeded by dam construction. Behavioral differences and constrained habitat within tributaries are suspected of promoting genetic divergence in these populations. In this study, we investigated the migratory behavior and genetic divergence of six populations of C. grayii fragmented by dams based on the otolith strontium/calcium (Sr/Ca) ratio, mitochondrial DNA, and microsatellite genotyping. All populations were in freshwater with low Sr/Ca ratios, except the estuarine population (Humen population) hatched in brackish water. Reduced nucleotide diversity corresponding to distance was observed. Populations from distant hydrological regions exhibited a decline in genetic diversity and a significant difference with the remaining populations after fitting the isolation by distance model. Pairwise fixation indices confirmed these results and moderate and significant differentiation was found between Hengxian site and downstream sites. Furthermore, STRUCTURE analyses revealed that all separated populations exhibited an admixed phylogenetic pattern except for individuals from the Hengxian locality. The upstream sites showed significantly increased resistance to gene flow from the estuarine population because of isolation by the dam. The results of the neutrality test and Bayesian skyline plots demonstrated complex demography—individuals’ experienced historical expansion and partial upper-dam populations had recently undergone a colonization, forming a new genetic structure. Accordingly, this study demonstrates differences in the migration pattern and genetic differentiation of C. grayii as a consequence of demographic history and current processes (habitat fragmentation and colonization). Full article

It is well-known that climate change significantly impacts ecosystems (at the macro-level) and individual species (at the micro-level). Among the former, estuaries are the most vulnerable and affected ecosystems. However, despite the strong relations between climate change and estuaries, there is a gap in the literature regarding international studies across different regions investigating the impacts of climate change and variability on estuaries in different geographical zones. This paper addresses this need and reviews the impacts of climate change, variability and extreme weather on estuaries. It emphasises the following: (i) a set of climate parameters governing estuarine hydrology and processes; and (ii) a sample of countries in Asia (Bangladesh), Europe (Portugal) and South America (Uruguay). We reviewed the influences of the climatic drivers of the estuarine hydrology, ecological processes and specific species in estuarine communities across the selected geographical regions, along with an analysis of their long-term implications. The key results from the three estuaries are as following: (i) Hilsa fish, of which the catches contribute to 10% of the total earnings of the fishery sector (1% of GDP), are affected by climate-forced hydrological and productivity changes in the Meghna; (ii) extreme droughts and short-term severe precipitation have driven the long-term abundance and spatial distribution of both fish larvae and juveniles/adults in the Mondego; and (iii) the river inflow and fluctuations increases since the early 1970s have contributed to variations in the salinity, the stratification, the oxygen, nutrient and trophic levels and the spatial pattern for the life stages of planktonic species, fish biomass and captures in the Rio de la Plata. The results suggested that immediate action is needed to reduce the vulnerability of estuaries to climate stressors, mainly the changing river flows, storms and sea-level rise. As a contribution to addressing current problems, we described a set of adaptation strategies to foster climate resilience and adaptive capacity (e.g., early-warning systems, dam management to prevent overflows and adaptive fisheries management). The implications of this paper are two-fold. Firstly, it showcases a variety of problems that estuaries face from changing climate conditions. Secondly, the paper outlines the need for suitable adaptive management strategies to safeguard the integrity of such vital ecosystems. Full article

It is well known that estuarine systems are significantly affected by hydrodynamic conditions such as river discharge, storm surges, waves and tidal conditions. In addition to this, human interferences through developmental projects have the capability of disrupting the natural morphological processes occurring at estuaries. In West Africa, the goal to improve standards of living through large-scale dam construction, offshore ports and coastal erosion countermeasures has triggered alarming changes in the morphodynamics of estuarine systems. The estuaries at the Volta River mouth (Ghana) and “Bouche du Roi” inlet (Benin), located along the Bight of Benin coast, West Africa, were selected as two case study sites to examine their long-term morphodynamics and sandspit evolution. In this study, we primarily analyzed estuarine morphology using remotely sensed images acquired from 1984 to 2020. We further estimated the longshore sediment transport for this region using results from the image analysis and the depth of active sediment motion. Our results reveal that the longshore sediment transport rates for this region are in the magnitude of 10⁵–10⁶ m³/year. Comparative analysis with other estuaries and sandy coasts suggests that the longshore sediment transport along this coast has one of the largest rates estimated in the world. Full article

This study reviews the monsoonal Yangtze and the arid Nile deltas with the objective of understanding how the process–response between river-basin modifications and delta-estuary ecological degradation are interrelated under contrasting hydroclimate dynamics. Our analysis shows that the Yangtze River had a long-term stepwise reduction in sediment and silicate fluxes to estuary due to dam construction since the 1960s, especially after the Three Gorges Dam (TGD) closed in 2003. By contrast, the Nile had a drastic reduction of sediment, freshwater, and silicate fluxes immediately after the construction of the Aswan High Dam (AHD) in 1964. Seasonal rainfall in the mid-lower Yangtze basin (below TGD) complemented riverine materials to its estuary, but little was available to the Nile coast below the AHD in the hyper-arid climate setting. Nitrogen (N) and phosphate (P) fluxes in both river basins have increased because of the overuse of N- and P-fertilizer, land-use changes, urbanization, and industrialization. Nutrient ratios (N:P:Si) in both delta-estuaries was greatly altered, i.e., Yangtze case: 75:1:946 (1960s–1970s), 86:1:272 (1980s–1990s) and 102:1:75 (2000s–2010s); and Nile case: 6:1:32 (1960s–1970s), 8:1:9 (1980s–1990s), and 45:1:22 (2013), in the context of the optimum of Redfield ratio (N:P:Si = 16:1:16). This led to an ecological regime shift evidenced by a long-term change in phytoplankton communities in the Yangtze estuary, where silicious algae tended to lose dominance since the end of the 1990s, when more toxic dinoflagellates began to emerge. In the Nile estuary, such a regime shift was indicated by the post-dam dramatic reduction in zooplankton standing crop and fish landings until the early 2000s when biological recovery occurred due to nutrient inputs from anthropogenic sources. Although the Yangtze had higher human impacts than the Nile in terms of population, industrialization, and fertilizer application, N concentrations in the Nile estuarine waters surpassed the Yangtze in recent decades. However, eutrophication in the Yangtze estuary is much more intensive than in the Nile, leading to the likelihood of its estuarine water becoming more acidic than ever before. Therefore, ecological degradation in both delta-estuaries does not follow a linear trajectory, due not only to different climate dynamics but also to human forcings. The comparative insights of this study should be incorporated into future integrated coastal management of these two important systems. Full article

Though the number of sluices and dams in coastal areas has increased rapidly in recent years, the influence of their construction on phytoplankton in estuary areas is hardly known. This paper aims to provide a reference for quantitative research on the ecological influence of sluice construction and give ecological justifications for the setting of environmental standards in the estuary areas. The survey data gained at the lower reach of the Yongjiang River and its estuarine areas in June 2015 were used in MIKE21 software (Danish Hydraulic Institute (DHI), Denmark)) for establishing a two-dimensional numerical model to simulate the salinity field distribution after sluice construction. Based on the simulation results, the salinity gradient changes caused by the construction were analyzed. The one-dimensional Gaussian model was applied to calculated the phytoplankton’s ecological threshold interval over the salinity changes, which helped predict the influence of salinity changes on phytoplankton cell density. The study shows that salinity in the Yongjiang estuary increases obviously, beyond the phytoplankton ecological threshold, after sluice construction without water discharge. Salinity will become a restriction factor to phytoplankton growth after sluice construction in the study area, which may cause a sharp decrease of certain phytoplankton species. Full article

Temporarily closed estuaries require seasonal opening to tidal flows to maintain normal
ecological processes. Each estuary has specific environmental flow (EFlow) requirements based
on the relationship between freshwater inflow, coastal dynamics, rate of sandbar formation,
and the open/closed state of the mouth. Key abiotic processes and ecosystem services linked
to mouth state were highlighted. We reviewed completed EFlow requirement studies for temporarily
closed estuaries in South Africa and found that the formulation of these requirements should
consider the timing and magnitude of flows in relation to the morphology of an estuary, its mouth
structure, catchment size, and climate. We identified ten key principles that could be adapted to
similar systems in equivalent climatic settings. Principle 1 recognizes that each estuary is unique in
terms of its EFlow requirements because size, scale, and sensitivity of core elements to freshwater
inflow are specific for each system; EFlows cannot be extrapolated from one estuary to another.
Principle 2 highlights the importance of baseflows in keeping an estuary mouth open because a small
reduction in flow can cause the mouth to close and alter essential ecological processes. Principle 3
outlines the role of floods in resetting natural processes by flushing out large volumes of sediment
and establishing the equilibrium between erosion and sedimentation. Principle 4 emphasizes the need
for open mouth conditions to allow regular tidal flushing that maintains water quality through
reducing retention times and preventing the onset of eutrophic conditions. Principle 5 advises
artificial breaching to be practiced with caution because execution at low water levels encourages
sedimentation that reduces the scouring eect of flushing. Principle 6 holds that elevated inflow
volumes from wastewater treatment works or agricultural return flows can increase the frequency of
mouth opening and cause ecological instability. Principle 7 states that water released from dams to
supply the environmental flow cannot mimic the natural flow regime. Principle 8 specifies the need
for short- and long-term data to increase the confidence levels of EFlow assessments, with data to
be collected during the open and closed mouth states. Principle 9 advocates the implementation of
a monitoring program to track the achievement of EFlow objectives as part of a strategic adaptive
management cycle. Finally, Principle 10 recommends the adoption of a holistic catchment-to-coast
management approach underpinned by collaboration with regulatory authorities and stakeholders
across a range of sectors. These principles can be used to guide the formulation and management
of EFlows, an essential strategy that links the maintenance of estuarine ecological integrity with
social well-being. Full article