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Oxygen depletion in coastal waters is increasing globally due primarily to eutrophication and warming. Hypoxia responses to nutrient loading and climate change have been extensively studied in large systems like the Chesapeake Bay and the Baltic Sea, while fewer studies have investigated smaller, shallower hypoxic zones. Thus, an improved understanding of the interactions of eutrophication and warming on hypoxia expansion (or reduction) in the wide variety of different estuarine environments is needed. We examined interannual controls on oxygen depletion in the Patuxent River estuary, a eutrophic sub-estuary of Chesapeake Bay where seasonal hypoxia develops annually. We conducted a spatial and temporal analysis of dissolved oxygen (DO) trends, timing, and several metrics of depletion over a long-term record (1985–2021). We found an internally generated hypoxic zone that initiates in the middle estuary, spreading upstream and downstream as the summer progresses, and that hypoxic volume days (HVD) have been increasing (0.11 per year, p = 0.03) over the record despite reduced watershed nitrogen loads and stable phosphorus loads. River flow and temperature have been increasing and are major drivers of increased HVD, with river flow explaining 40% of the interannual variation in HVD (temperature has increased 0.03 and 0.06 °C per year in summer and fall, respectively). Apparent oxygen utilization (AOU) is increasing in bottom waters in the fall, consistent with increasing trends of both water temperature and stratification strength. HVD was negatively related (r2 = 0.34, slope = −0.59*HVD) to the biomass of benthic invertebrates in the middle region of the estuary, suggesting that benthic forage for higher trophic levels will be limited by sustained hypoxia. These results indicate that current and future climate variability plays an important role in regulating oxygen depletion in the Patuxent River estuary, which reinforces the need to factor climate change into strategies for the restoration and management of estuaries. [ABSTRACT FROM AUTHOR]

Land reclamations influence the morphodynamic evolution of estuaries and tidal basins, because an altered planform changes tidal dynamics and associated residual sediment transport. The morphodynamic response time to land reclamation is long, impacting the system for decades to centuries. Other human interventions (e.g., deepening of fairways or port construction) will add more morphodynamic adaptation timescales. Our understanding of the cumulative effects of anthropogenic interference with estuaries is limited because observations usually do not cover the complete morphological adaptation period. We aim to assess the impact of land reclamation works and other human interventions on an estuarine system by means of digital reconstructions of historical morphologies of the Ems Estuary over the past 500 years. Our analysis demonstrates that the intertidal‐subtidal area ratio altered due to land reclamation works and that the ratio partly restored after land reclamation ended. The land reclamation works have led to the degeneration of an ebb and flood channel system, transitioning the estuary from a multichannel to a single channel system. We infer that the 20th‐century intensification of channel dredging and re‐alignment works accelerated rather than caused this development. The centennial‐scale observations show that the Ems estuary evolution corresponds to a land reclamation response following tidal asymmetry‐based stability theory as it moves toward a new equilibrium configuration with modified tidal flats and channels. Considering the long history of land reclamation in the Ems Estuary, it provides an analogy for expected developments in comparable tidal systems where land reclamations were recently carried out. Plain Language Summary: Reclaiming land along the margins of estuaries and tidal basins leads to loss of intertidal areas, impacting the remaining underwater landscape for decades to centuries. This influences the patterns, dimensions, and functionalities of the channels and tidal flats. Observations are usually not available for such a long period, limiting our capacity to study the impact of land reclamation. Here, we overcome this limitation by reconstructing the landscape adaptation in the Ems estuary since land reclamation accelerated in the 16th century, when flooding by storms reshaped the estuary. Historical and recent topo‐geographical sources were used to reconstruct the centennial‐scale developments of the tidal channels and tidal flats. Results show that, after reclamation works stopped, the tidal flats reduced in area and the tidal channels filled up. The tidal channel patterns and dimensions permanently changed, impacting, for example, shipping waterways. The observed estuary‐scale land reclamation response can be qualitatively predicted on the basis of descriptive parameters. This suggests that we can anticipate on the future evolution of comparable tidal systems that are recently impacted by land reclamation. Key Points: Land reclamation in the Ems estuary was followed by progressive subtidal infilling and degeneration of separated ebb‐flood channelsLoss of intertidal areas distorts the estuary‐scale channel‐flat configuration, which is partly restored by subtidal infillingTidal asymmetry‐based equilibrium theory can serve estuarine management by predicting estuary evolution in response to land reclamation [ABSTRACT FROM AUTHOR]

Predicting the dissolved oxygen concentration and identifying its driving factors are essential for improved prevention and management of anoxia in estuaries. However, complex hydrodynamic conditions and the limitations in traditional methods result in challenges in the identification of the driving factors for the low dissolved oxygen (DO) phenomenon. The objective of our study is to develop a robust deep learning model using four-year in situ data collected from an automatic water quality monitoring station (AWQMS) in an estuary, for accurate identification and quantification of the driving factors influencing DO levels. Mitigations in hypoxia were observed during the initial two years, but a subsequent decline in DO concentrations was witnessed recently. The periodicity of DO concentrations in the Pearl River Estuary reduced with the increase in the hypoxic intensity. Maximal information coefficient (MIC) and extreme gradient boosting (XGBoost) were employed to determine the significance of input variables, which were subsequently validated by using the long- and short-term memory networks (LSTMs). The driving factors contributing to the hypoxia problem were shown as temperature, pH, conductivity, and NH4+-N concentrations. Notably, the evaluation index values of the hybrid model are MAPE = 0.0887 and R2 = 0.9208, which have been improved compared with the LSTM model by about 99.34% in MAPE reduction and 16.56% in R2 improvement, indicating that the MixUp-LSTM model was capable of effectively capturing nonlinear relationships between DO and other water quality indicators. Based on existing literature, three traditional statistical methods and four machine learning models were also performed to compare with the proposed MixUp-LSTM model, which outperformed other models in terms of prediction accuracy and robustness. Overall, the successful identification of the driving factors for the deoxygenation phenomenon would have important implications for the governance and regulation of low DO in estuaries. [ABSTRACT FROM AUTHOR]

Introduction: The rapid advances in marine engineering projects are exacerbating environmental pressures on bay ecosystems. This study utilized the MIKE 21 model to evaluate the impacts of such projects in Laizhou Bay (LZB) on hydrodynamic conditions and the spread of dissolved inorganic nitrogen (DIN) from riverine inputs. Results: The results indicated an expansion of 80.77 km2 in areas with DIN concentrations surpassing 0.5 mg/L 2 months after input from the Yellow River, with increased levels in the southern Yellow River Delta. Decreased flow velocities adjacent to the wave barriers at the Xiaoqing River estuary impeded lateral DIN dispersion, resulting in a 0.93 mg/L increase in DIN concentrations at the river mouth. After the construction of marine engineering projects (2020), significant alterations in the coastline of LZB have markedly modified hydrodynamic characteristics near marine structures, altering DIN dispersion patterns. Conclusion: This study provides crucial information for the management of pollutants at estuaries, understanding dispersion mechanisms, and evaluating the feasibility and environmental compatibility of marine engineering projects. [ABSTRACT FROM AUTHOR]

Hypoxia in coastal waters is a pressing ecological problem caused by continued eutrophication and climatic change that has widespread consequences for metazoan life and biogeochemical cycles. Numerous studies have investigated the controls on seasonal hypoxia formation and persistence in many of the world's large estuaries and coastal hypoxic zones, but far fewer studies have examined the controls on short-term oxygen variability that leads to diel-cycling hypoxia in shallow-water environments. We utilized a unique, comprehensive (181 stations) record of dissolved oxygen concentrations collected at shallow water sites (primarily < 2 m) at high frequency (15 min) throughout the estuarine complex of the Chesapeake Bay and its tributaries to quantify how internal and external variables co-varied with dissolved oxygen. We used a combination of time-series analysis, harmonic analysis, and machine learning (e.g., classification and regression trees (CART)) approaches to identify spatial patterns in major controls on oxygen variability and the duration of moderate hypoxia. We found that key controls on oxygen variability varied substantially over space. For example, photosynthetically active radiation (PAR) was a strong predictor of oxygen dynamics in the majority of mesohaline waters. In more fetch-exposed stations, wind strongly controlled hypoxic duration, but in eutrophic, inshore locations, chlorophyll a, or turbidity were often better predictors. Specifically, diel oxygen variability was muted in upstream regions characterized by high turbidity. The duration of low oxygen conditions, which we defined conservatively as less than 4.8 mg O2 L−1 (156 µM), was strongly controlled by temperature, and simple projections of regional warming and CART-derived oxygen thresholds suggest that the Bay could experience a 10% increase in this type of hypoxia duration by mid-to-late twenty-first century. The ratio of tidal to biological variability in oxygen was found to increase under conditions of higher turbidity, stronger wind, and lower salinity, but biological variability was typically a factor of two higher than tidal variability. Although chlorophyll-a generated high oxygen concentrations at some locations, those stations with exceptionally high chlorophyll a (> 30 µg L−1) were the most vulnerable to hypoxia. Because conventional water quality modeling frameworks are designed to capture hypoxia on relatively long time scales, these new insights can help inform updated oxygen models to support the management of shallow-water estuaries in the face of managed nutrient reductions and climate change. [ABSTRACT FROM AUTHOR]

The high concentration of chlorophyll a (Chl-a) and pollutants in estuaries has led to the deterioration of marine environmental quality and a reduction in the sustainability and safety of marine ecosystems. This study analyzed the distribution characteristics of Chl-a and pollutants in the Liugu Estuary (LE) based on field survey data. By incorporating environmental factors, the water's nutritional status and nutrient limitations were evaluated. The results indicated that there were variations in the concentration range of Chl-a and pollutants between flood and non-flood seasons in the LE, with nitrogen identified as the main limiting nutrient. The correlation indicated that Chl-a and nutrient levels in the LE were influenced by a complex interplay of multiple factors. River inputs with higher pollution levels increased concentrations and reduced DO levels, limiting the rise in Chl-a concentration and the uptake of nutrients by phytoplankton. The findings of this study could provide essential information for an in-depth examination of the eutrophication characteristics of the LE and offer a scientific foundation for the restoration and management of the estuary's ecological environment. [ABSTRACT FROM AUTHOR]

Since 1958, there have been significant changes in the Yangtze River estuary. Due to extensive reclamation and construction of ports and channels, the water area has drastically decreased, resulting in corresponding changes in hydrodynamics and riverbeds at the mouth of the river. According to the analysis of measured topographic data and Delft3D-FLOW model for seven typical historical periods since 1958 at the Yangtze River Estuary, this study investigates the characteristics of riverbed evolution and tidal flow dynamics. From 1958 to 2019, driven by strong human activities, the total area of the Yangtze River Estuary decreased from 2084 km2 to 1403 km2, with a decrease of 32.7%, while the total volume of the corresponding river channel changed slightly and remained stable. Compared with 1958, the volume of the Yangtze River Estuary in 2019 only increased by 345 million m3, with an increase of about 4.1%. The tidal dynamic change of the Yangtze Estuary is closely related to the riverbed evolution of each reach, which not only shapes the estuary landform, but also is affected by the riverbed evolution. Tidal level, tidal range and water area change are closely related. With the decrease of water area in the Yangtze River Estuary, tidal range tends to increase. Tidal prism change is closely related to channel volume. In the past 60 years, the tidal volume at the mouth of the Yangtze River has decreased by 8%. The research findings will provide technical support for enhancing flood control and tide resistance measures at the Yangtze River Estuary, as well as formulating comprehensive management plans for estuaries, contributing to the protection and sustainable development of the Yangtze River Estuary. [ABSTRACT FROM AUTHOR]

Mangrove wetland restoration projects have been conducted for many years and it is important to evaluate their success. In this study, we explored the role of monitoring macrobenthos in assessing the success of mangrove restoration. In particular, we sampled soil and macrobenthos from a mangrove wetland area after restoration for one, three, and five years in the Pearl River Estuary Delta in China to explore the effects of different restoration periods on the macrobenthos community structure. The results showed that the macrobenthos collected in the study area mainly comprised Polychaete annelids and Bivalvia mollusks, and there were no significant differences in the species, biomass, richness index, diversity index, and evenness index among different mangrove wetland restoration stages. The macrobenthos biomass, species richness, diversity index, and evenness index were mainly correlated with the soil electrical conductivity, pH, total phosphorus, and soil organic carbon. Compared with mangrove restoration for one year, the average species richness, Margalef richness index, Shannon diversity index, and Pielou evenness index for macrobenthos were significantly higher after three years, thereby indicating that the macrobenthos species diversity increased significantly after mangrove wetland restoration for three years. However, the macrobenthos diversity was lower after mangrove wetland restoration for five years, which may have been due to human activities and soil pollution. The findings obtained in this study provide basic information to facilitate mangrove wetland restoration and conservation. [ABSTRACT FROM AUTHOR]

For the management of estuaries and the preservation of tidal flats it is crucial to understand the tidal flat shape and development. Previous work focused predominantly on the quasi-equilibrium shape of tidal flats along open coasts with a dominant cross-shore flow and wave exposure. This paper evaluates the shape of fringing tidal flats in engineered estuaries, where longshore velocities generally dominate. Using a long-term (20 years) topographic data set of an anthropogenically modified estuary in the Netherlands (the Western Scheldt estuary), we relate key profile shape parameters and changes over time to natural and anthropogenic processes. In an engineered estuary, the tidal flat shape depends on the estuary geometry, hydrodynamic forcings and human interventions. In contrast to open coast tidal flats, the presence of the channel and dominant longshore flow determines the available cross-shore length (accommodation space) of the tidal flat and the shape of the tidal flat. This accommodation space defines the maximum tidal flat height and opportunity for marsh development. We propose the use of the Index of Development, indicating to what extend tidal flats have space to develop. This index is not only influenced by longshore and cross-shore flow, but also (or even more) by hydraulic structures, dike realignments and channel migration. Especially the latter two strongly influence the accommodation space and thereby the maximum tidal flat height and the opportunity for marsh development. For large stretches of the Western Scheldt, the accommodation space is too small, and the majority of the tidal flats do not vertically extent to mean high water. The success of tidal flat and marsh restoration projects depends on the accommodation space. [ABSTRACT FROM AUTHOR]

Wetlands are badly damaged in many parts of the world. The wetland management of Minjiang Estuary wetland has achieved remarkable results. This provides valuable experience for wetland management in other areas. Minjiang Estuary wetland can achieve the effect, mainly because of the adoption of the water quality management, ecological restoration, and ecological tourism development. However, different management modes have their own scope of application. In order to spread its successes, three differential game modes related to Minjiang Estuary wetland management are constructed, and their equilibrium results are compared and analyzed. Finally, research shows that the amount of water quality control by governments is directly proportional to the resilience of wetlands. The amount of social organization culture is proportional to the degree of better water quality. The amount of development by the government and social organizations is directly proportional to the decrease of investment in the spontaneous tourism industry. When the loss caused by flood or the ecological restoration effect is large, the wetland should adopt the ecological restoration mode. When the increased income or reputation of ecotourism is large, the ecotourism development mode should be adopted. This provides a reference for how to manage wetlands and how to promote the management mode of Minjiang Estuary wetland to other areas more effectively. [ABSTRACT FROM AUTHOR]

Estuary reopening is a means of restoring estuarine habitats, which has recently been implemented in a few developed countries. The regeneration of a brackish zone in the Nakdong River Estuary (NRE), South Korea, were tested through a series of barrage reopening. During the same period, we conducted extensive fish surveys in the upper part of the NRE barrage on a monthly basis from 2017 to 2021, and subsequently determined whether fish populations and communities were affected by the reopening. The results showed that the reopening of the NRE hardly affected the fish community structure, as non-native species such as Erythroculter erythropterus and Lepomis macrochirus maintained their dominance. Still, we discovered that certain euryhaline species are positively affected by estuary reopening, as total 46 Japanese eels (Anguilla japonica) were captured after the reopening, which had not been detected before the reopening. By comparing the size structure of various fish species, we discovered that size distribution of native and migratory species presented more positively skewed pattern after the reopening, while size structure in non-native species remained relatively unchanged normally distributed pattern. Piecewise structural equation modelling revealed that the NRE had become more complex ecosystem, as migratory fish species abundance and biomass started to show a positive correlation with hydraulic factors such as discharge and negative correlation with seasonality after the reopening. We concluded that estuary reopening created some changes in migratory and native freshwater species but such changes were not notably detected in non-native species. Therefore, appropriate sluice operation methodologies, such as considering the migration seasons of migratory species, should be developed. Additionally, human-involved management policies are required to regulate non-native species populations. [ABSTRACT FROM AUTHOR]

In this paper, we describe a numerical model for unsteady stratified flow without mixing at the interface (halocline), which was applied for the calculation of salinization in the Neretva River (Croatia) i.e. the first line of penetration. The mathematical model is based on the principles of conservation of mass and momentum. Since, the set of equations does not have analytical solution it is solved numerically, e.g. through finite element method (FEM). Computed variables are water discharges and elevations for each layer separately in each point of finite element mesh. Calibration of the model is based on in-situ measurements. The most important factor for salt water intrusion in an estuary is the fresh water discharge. In the Neretva estuary, salt water was found in Metkovic (22 km from the sea) for freshwater flows of less than 180 m3/s, while for flows greater than 500 m3/s salt water was completely extruded from the riverbed. The plan is to build a mobile barrier (gate) in the Neretva River to protect it against sea water penetration. Additionally, upstream of the barrier, fresh water would be provided in the riverbed that could be primarily used for irrigation. The same numerical model has been also successfully applied to the estuary of the Jadro River, a small river near Split. Since the model is general, it can be applied on any river mouth with stratified conditions. The projections of sea level changes in the Adriatic Sea shows increase of a few decimeters in 21st century. Generally, rising sea levels will increase salt water pressure on the coast, estuaries, water resources, aquifers and farmland. Therefore, the management of estuaries will require even greater engagement of experts from different areas to successfully deal with future difficulties. [ABSTRACT FROM AUTHOR]

Due to their role as primary producers, phytoplankton are essential to the productivity of estuarine ecosystems. However, it is important to understand how these nearly passive organisms are able to persist within estuaries when river inflow results in a net outflow to the ocean. Estuaries also represent challenging habitats due to a strong salinity gradient. Little is known about how phytoplankton are able to be retained within estuaries. We present a new individual-based Lagrangian model of the Elbe estuary which examines possible retention mechanisms for phytoplankton. Specifically, we investigated how reproduction, sinking and rising, and diel vertical migration may allow populations to persist within the estuary. We find that vertical migration, especially rising, favors retention, while fast sinking does not. We further provide first estimates of outwashing losses. Our simulations illustrate that riverbanks and tidal flats are essential for the long-term survival of phytoplankton populations, as they provide refuges from strong downstream currents. These results contribute to the understanding needed to advance the ecosystem-based management of estuaries. [ABSTRACT FROM AUTHOR]

The theory and practice of adaptive management and adaptive governance have been widely studied in the complex social contexts that mediate how humans interact with ecosystems. Adaptive governance is thought to enable adaptive management in such contexts. In this study, we examine four often-used principles of adaptive governance (polycentric institutions, collaboration, social learning and complexity thinking) to develop a framework for reflecting on adaptive governance of a social-ecological system—the Knysna Estuary in South Africa. This estuary is a priority for biodiversity conservation, as well as a common-pool resource central to livelihoods. We used the framework to structure dialogue on the extent to which the four principles of adaptive governance were being applied in the management of the Knysna Estuary. The dialogue included diverse stakeholders, from those who have the power to influence adaptive management to those most dependent on the resource for their livelihoods. Based on a combination of theory and current reality we then identified eight indicators that could be used to guide a transition towards improved adaptive governance of the estuary. These indicators were assessed and supported by most stakeholders. The main contributions of our research are (a) a process for combining theory and stakeholder dialogue to reflect on adaptive governance of a social-ecological system; (b) a set of indicators or conditions that emerged from our participatory process that can be used for reflexive monitoring and adaptation of adaptive governance of Knysna Estuary; and (c) a real-world example of seeking complementary links between adaptive governance and adaptive management to promote effective management of complex social-ecological systems. [ABSTRACT FROM AUTHOR]

Coastal environments are at the frontline of sea-level rise effects, and seawater intrusion constitutes one of the most important causes of salinization, changing the ecological patterns. Hence, we hypothesized that seawater intrusion would alter environmental gradients, causing either abrupt or gradual changes in the phytoplankton of Itapecuru River estuary. Physical and chemical and biological variables were collected bimonthly at six sites between 2019 and 2020. Seventy-six phytoplankton indicators were selected based on their specific functional traits and indicator value. Polymyxus coronalis was a good indicator of the limit of seawater intrusion. Multivariate analyses revealed high species dispersion among the estuarine sectors governed by variations in salinity, suspended particulate matter, cell size, and silicate. The distribution of freshwater species in the upper sector was correlated with low nutrient values and salinity. The marine species were transported between the middle and lower sectors under the opposite conditions. The seawater intrusion negatively affected the community, primarily in the dry season when the displacement of the turbidity maximum zone estuarine altered the structure, reducing its density, diversity, and biomass. The present study generated important information about seawater intrusion effects on the spatiotemporal variation in the phytoplankton community and provided a tool for the sustainable management of tropical estuaries. [ABSTRACT FROM AUTHOR]

Mangrove forests are vital for the filtration of land-derived wastewater in industrialized tropical and sub-tropical coastal regions, contributing to the preservation of marine ecosystems. However, intertidal communities currently face significant challenges. One major concern is the potential threat posed by heavy metal contamination resulting from human activities associated with rapid urbanization and industrialization. Extensive research has been conducted to understand the biotic responses of mangrove plants to heavy metal exposure. Contaminants have significant implications for estuary management and coastal ecosystem preservation. This study focused on assessing the heavy metal in the tissue of the mangrove tree Avicennia marina collected from the Nabq protectorate area, the Red Sea. The enrichment factor (EF) values of seawater in the study area provide insights into the contamination levels. Notably, all sampled locations in the investigated region displayed EF values below 1, suggesting minimal contamination by these metals. The analysis of the Metal Pollution Index (MPI) of water in the Nabq area revealed that the examined sites within the Nabq protectorate experienced a minor degree of metal pollution. The sites examined are classified in class III. This classification is based on the MPI values falling within the range of 1-2 levels for all sites within the protectorate. Metal concentrations in leaves, roots, stems, and surrounding seawater were investigated in twelve locations within the studied area. The analysis revealed that the concentrations of metals were ordered from high to low concentrations as follows: roots> leaves> stem. Manganese had the highest concentration of bioconcentration factor among the elements examined in the roots, leaves, and stem of Avicennia marina. [ABSTRACT FROM AUTHOR]

Due to their role as primary producers, phytoplankton are essential to the productivity of estuarine ecosystems. However, it is important to understand how these nearly passive organisms are able to persist within estuaries, when river inflow results in a net outflow to the ocean. Estuaries are also representing challenging habitats due to a strong salinity gradient. So far, little is known about how phytoplankton are able to be retained within estuaries. We present a new individual-based Lagrangian model of the Elbe estuary which examines possible retention mechanisms for phytoplankton. Specifically, we investigated how reproduction, sinking and rising, as well as diel vertical migration may allow for populations to persist within the estuary. We find that vertical migration especially rising favors the retention, fast sinking does not. We further provide first estimates on outwashing losses. Our simulations illustrate that riverbanks and tidal flats are essential for the long-term survival of phytoplankton populations, providing refuges from strong downstream currents. These results contribute to the understanding needed to advance ecosystem-based management of estuaries. [ABSTRACT FROM AUTHOR]

Residence time is an important parameter linked to the water quality in an estuary. In this paper, we identify and analyze the main processes that affect the residence time of the Caloosahatchee River Estuary, a micro-tidal and mixed diurnal-semidiurnal estuary located in western Florida. Multiyear validated hydrodynamic hindcast results were coupled with an offline particle tracking model to compute the residence time of the estuary, which showed a strong seasonality driven by the river discharge. The residence time reduced with increasing river flow. The wind velocity and direction also affected the residence time. The influence of the wind was dependent on the magnitude of the river discharge. In general, upstream-directed wind increased residence time, while downstream-directed wind decreased residence time. Downstream wind during the dry period reduced residence time on average by a week. Processes such as water density gradient-induced circulation and particle buoyancy also influenced the residence time of the estuary. The outcomes of this study can be used to better understand the influence of the main physical processes affecting the residence time at other similar estuaries and to help in the management of the estuaries to improve their water quality. [ABSTRACT FROM AUTHOR]

Estuarine nekton (fishes, crabs, and shrimps) play key ecological roles and support valuable commercial and recreational fisheries. Long-term research programs focused on nekton can provide insight into community and population-level changes over time, but are uncommon due to funding and logistical constraints. We describe patterns and changes in a nekton assemblage from an intertidal creek in the North Inlet estuary, South Carolina, USA based on 19 years of observations. From 1984 to 2002, biweekly seine collections (approximately every 2 weeks, n = 469) were made in a pool isolated within an intertidal creek at low tide. The assemblage was composed of juvenile and small adult (mostly < 100 mm) fishes, shrimps, and crabs. The 10 most abundant species made up > 97% of the catch; these species included (in descending order): Leiostomus xanthurus, Litopenaeus setiferus, Fundulus heteroclitus, Mugil cephalus, Farfantepenaeus aztecus, Eucinostomus spp., Mugil curema, Fundulus majalis, Menidia menidia, and Brevoortia tyrannus. The assemblage exhibited a distinct seasonality across years, with peak total abundance in mid-spring, peak total biomass in late spring, and highest species richness in late summer and early fall. Total abundance and species richness showed evidence of significant increases across years, total biomass and the Shannon diversity index remained unchanged, and evenness significantly declined. The composition of the assemblage shifted during the sampling period, with the abundance of key year-round residents (F. heteroclitus, M. cephalus) decreasing and warm-season species (L. xanthurus) increasing. Most of the five metrics of the nekton assemblage and the abundances of the five top species were positively correlated with both water temperature and salinity. Direct and indirect effects of hurricanes and storm events on the hydrogeomorphology of the creek and pool were also recognized as influences on the long-term patterns and trends. Long-term (decadal) sampling programs like this can provide important baseline information, and thus insight into the influence of significant weather events and global climate change on nekton populations and their roles in ecosystem dynamics, as well as inform the management of estuaries and fisheries in the southeastern US. [ABSTRACT FROM AUTHOR]

This study aimed to analyze the relationship between Cygnus spp. population trends and land-use changes and artificial development activities in the Nakdonggang Estuary for the necessity of management to the buffer area outside the protected area boundary in Korea. Analysis of the 15-year population change revealed a sharp decline in the Cygnus columbianus population in the winter, which could be attributed to the Nakdonggang Estuary becoming unfeasible as their wintering site. In contrast, there was no sharp decline in the Cygnus cygnus population, although a moderate decline was identified. Over the past 30 years, farmland activities decreased by approximately 10% in the areas adjacent to the protected area resulting from the expansion of the urbanized area. The rate of decrease in the water body area was high, attributed to urban development through reclamation. The correlation analysis between the area (ha) of paddy fields and the monthly (Nov.–Feb.) population of C. columbianus indicated that the populations in December (R2 = 0.768) and January (R2 = 0.743) were more correlated with the change in paddy field areas than those in the early winter (November: R2 = 0.457) and at the end of winter (February: R2 = 0.582). In the case of C. cygnus, the population tended to increase with increases in the areas of paddy fields, but the statistical explanatory power was low. We identified a distinct difference in that C. columbianus was more sensitive to urbanization than C. cygnus in the Nakdonggang Estuary. [ABSTRACT FROM AUTHOR]

Estuaries worldwide are experiencing increasing threats from climate change, particularly from the compounding effects of sea level rise (SLR) and varying magnitude of river inflows. Understanding the tidal response of estuaries to these effects can guide future management and help assess ecological concerns. However, there is limited existing understanding on how estuarine tidal dynamics may respond to the compounding effects of SLR and altered riverine inflows in different estuaries. To partially address this knowledge gap, this study used data analysis and scrutinised idealised hydrodynamic models of different estuary shapes and boundary conditions to (i) identify broad effects of SLR on estuarine tidal dynamics under various river inflow conditions, (ii) determine how longitudinal cross-sections are impacted by these effects, and (iii) highlight some implications for environmental risk management. Results indicated that short- to moderate-length, high convergent estuaries experience the greatest and short- to moderate-length prismatic and low convergent estuaries experience the least variations in their overall tidal dynamics (i.e., tidal range, current velocity, and asymmetry). These variations were most evident in estuaries with large riverine inflows and macrotidal conditions. Compounding effects of SLR and altered riverine inflows induced spatially heterogenous changes to tidal range, current velocity, and asymmetry, with transects nearest to the estuary mouth/head and at a three-quarter estuary length (measured from estuary mouth) identified as the most and the least vulnerable zones, respectively. These findings provide an initial broad assessment of some effects of climate change in estuaries and may help to prioritise future investigations. [ABSTRACT FROM AUTHOR]

In coastal areas, to take any development plan, it is important to determine the stability of the coast and coastal islands. Therefore, the present research took an initiative to illustrate the development of coastal islands in the Feni estuary of Bangladesh through Remote Sensing (RS) and Geographic Information System (GIS) techniques. Multi-dated Landsat images of 1978, 1989, 2014 and 2020 were used in the study. The research reveals that in 1978, 1989, 2014 and 2020, the total area of various islands in the estuary was 377,650,894 and 1063 km2, respectively. The islands show cyclical growth and erosion, but their main components have existed for decades. During the period from 1978 to 1989, 1989 to 2014 and 2014 to 2020 about 330,386 and 379 km2 of new land added to the existing land mass and about 57,142 and 210 km2 of existing land eroded, respectively. Finally, from 1978 to 2020, the size of the islands increased by about 14.64 km2 yr-1. During the same period about 110 and 40 km2 lands along the coastline were eroded and accreted, in the north-western and eastern parts of the estuary, respectively. The study further reveals that the islands of the estuary have developed over the past few decades, except Sandwip. Finally, the outputs of this study will be helpful for policy makers and planners for sustainable estuary management.

Major threats to South African estuaries include freshwater flow alterations, urbanisation, habitat transformation, deteriorating water quality, increased biological invasions and resource over-exploitation. Although South African estuary restoration is in its infancy, important lessons have been learnt through past efforts undertaken in some of the most impacted systems. This review presents case studies focused on hydrological and physical restoration actions, while recognizing the need to implement urgent remedial measures to restore the populations of targeted estuary-associated marine fish species. Common interventions designed to arrest declining estuary health include artificial management of estuary mouths and the use of artificial structures to restore tidal action and remediate erosion damage. Limited natural habitat restoration actions have been conducted on the subcontinent. Fortunately, estuaries are resilient ecosystems and can recover fairly rapidly if appropriate restoration interventions are successfully applied. Restoration needs to be co-ordinated through a national strategy which is currently not in place. It is particularly important to address the root cause of deterioration through active restoration and/or rehabilitation measures rather than only managing symptoms. The implementation of existing and future conservation legislation regarding the exploitation of natural resources within estuaries needs to be prioritised, so that over-exploited angling and subsistence fish species populations can recover. [ABSTRACT FROM AUTHOR]

It is essential to make an accurate prediction of the concentration of dissolved oxygen (DO), hydrogen ion concentration (pH), and potassium permanganate (KMnO4) in order to ensure the quality of the drinking water. The lack of monitoring data and the large fluctuation increase the difficulty of predicting DO, pH, and KMnO4 in the tide-sensing estuary. In this research, improved grey association analysis (IGRA) was provided to determine the correlation between DO, pH, KMnO4, and other water quality indicators, thereby resolving the dimension disaster problem of long short-term memory (LSTM). Furthermore, LSTM based on the improved sparrow search algorithm (ISSA) was established, and five LSTM parameters—learning rate, batch size, training times, hidden layer nodes, and fully connected hidden layer nodes—are automatically optimized, which could accurately predict the concentration of DO, pH, and KMnO4. Using the data from the Qiantang River Gate Observation Station from November 8, 2020, to June 27, 2021, 70% of which were training sets and 30% of which were test sets, predicted data for day 4. The results show that the coefficient of determination (R2) of the IGRA-ISSA-LSTM model for DO, pH, and KMnO4 were 0.92, 0.93, and 0.726, respectively, which are greater than that of IGRA-BP model, IGRA-LSTM model, and IGRA-SSA-LSTM model. Therefore, this research provides technical support for water quality management in tidal estuaries. [ABSTRACT FROM AUTHOR]

The Sacramento-San Joaquin Delta (Delta) is a case-study of the Anthropocene "great accelerations," with exponentially increasing temperatures and sea level over time, leading to rapid change in other ecosystem components. In nearly all these interconnected changes and across scales, primary producers play a major role, with diverse effects that mitigate or exacerbate the rapid change induced by climate or other human-driven perturbations. Through this anthropocentric lens, primary producers can be viewed as performing numerous ecosystem services--which ultimately benefit humans--as well as ecosystem disservices, which negatively affect human communities. For example, through carbon sequestration, wetlands can perform ecosystem services of mitigating warming at a global scale and combating relative sea-level rise at a local scale, while generating food that supports regional food webs and fisheries. On the other hand, invasive aquatic vegetation (IAV) can trap sediment before it reaches wetlands, exacerbating local subsidence and relative sealevel rise while incurring great costs to recreation, fishing, and agencies tasked with its control. Effectively managing these ecosystem services and disservices requires understanding how they are connected. For example, wetland restoration often creates opportunities for IAV, which may inhibit sediment deposition on the wetland and out-compete native species. As the Delta science community works toward a more integrative understanding of how different components of the Delta interact as a whole and across scales, the pervasive effects of the ecosystem services and disservices of primary producers serve as foundational knowledge. In this topically themed edition of State of Bay-Delta Science, we review these effects. Individual contributions focus on the historical ecology of the primary productivity of aquatic vegetation, the ecology and control of invasive aquatic vegetation, harmful algal blooms, carbon sequestration and subsidence reversal by wetlands, and remote sensing methods for quantifying the ecosystem services and disservices of Delta primary producers. [ABSTRACT FROM AUTHOR]

Since the late 1960s, large-scale coastal embankments have been constructed along the Qiantang Estuary (QE, China) covering both the Qiantang River (QR) and the Hangzhou Bay (HZB) to reduce the tidal prism and thus stabilize the channel. While most previous studies have reported their relevant impacts on hydrodynamic and sediment transport in the HZB, a more comprehensive investigation covering the whole QR + HZB system remains lacking, so as a quantitative analysis of the tidal prism and the channel stability. Therefore, numerical case studies have been conducted to examine evolutions of hydrodynamics and sediment transport of the QR + HZB system in response to different coastal embankments (i.e., no embankment as in 1958; QR embankment as in 1984; QR embankment + part of HZB embankment as in 2004; both QR and HZB embankment as in 2016). It is shown that the flood tidal asymmetry in the QR and the tidal range in the HZB are significantly increased, implying an intensified tidal distortion and thus a strengthened river role after embankments. While this intensification does not change the direction (from HZB to QR) of the net sediment transport, all of the sediment exchange rates [e.g., the landward (during flood tides) and seaward (during ebb tides) sediment transport volumes, as well as the net sediment transport rate between them] exhibits obvious decreasing trends. In addition, strong deposition is observed in the Jianshan (JS) reach, which has led to further increase of the underwater giant sandbar. The constructions of embankments have facilitated the initial goal of reducing tidal prism in most part of the QR + HZB system, except some local parts of the HZB, where increased tidal prism can be observed as the effect of the enlarged tidal range overwhelms that of embankments. Moreover, the channel stability was greatly improved as the mainstream has been mostly converged to the reach centerline and the width-depth ratio of the channel has been considerably reduced. The present investigation of the QE regulation could be indicative for the management of other estuaries in the world. • Embankment effects in the Qiantang Estuary (QE) are comprehensively investigated. • The achievements of the QE regulation goals are quantitative assessed. • Two main goals of reducing tidal prism and stabilizing main channel are mostly achieved. • Embankments weaken sediment exchanges between Hangzhou Bay and Qiantang River. • The Jianshan reach showed increased deposition due to embankments. [ABSTRACT FROM AUTHOR]

Understanding the alterations in spatial–temporal water level dynamics caused by natural and anthropogenic changes is essential for water resources management in estuaries, as this can directly impact the estuarine morphology, sediment transport, salinity intrusion, navigation conditions, and other factors. Here, we propose a simple triple linear regression model linking the water level variation on a daily timescale to the hydrodynamics at both ends of an estuary. The model was applied to the upper Yangtze River estuary (YRE) to examine the influence of the world's largest dam, the Three Gorges Dam (TGD), on the spatial–temporal water level dynamics within the estuary. It is shown that the regression model can accurately reproduce the water level dynamics in the upper YRE, with a root mean squared error (RMSE) of 0.061–0.150 m seen at five gauging stations for both the pre- and post-TGD periods. This confirms the hypothesis that the response of water level dynamics to hydrodynamics at both ends is mostly linear in the upper YRE. The regression model calibrated during the pre-TGD period was used to reconstruct the water level dynamics that would have occurred in the absence of the TGD's freshwater regulation. Results show that the spatial–temporal alterations in water levels during the post-TGD period are mainly driven by the variation in freshwater discharge due to the regulation of the TGD, which results in increased discharge during the dry season (from December to March) and a dramatic reduction in discharge during the wet-to-dry transitional period. The presented method to quantify the separate contributions made by changes in boundary conditions and geometry to spatial–temporal water level dynamics is particularly useful for determining scientific strategies for sustainable water resources management in dam-controlled or climate-driven estuaries worldwide. [ABSTRACT FROM AUTHOR]

In coastal areas, to take any development plan, it is important to determine the stability of the coast and coastal islands. Therefore, the present research took an initiative to illustrate the development of coastal islands in the Feni estuary of Bangladesh through Remote Sensing (RS) and Geographic Information System (GIS) techniques. Multi-dated Landsat images of 1978, 1989, 2014 and 2020 were used in the study. The research reveals that in 1978, 1989, 2014 and 2020, the total area of various islands in the estuary was 377, 650, 894 and 1063 km², respectively. The islands show cyclical growth and erosion, but their main components have existed for decades. During the period from 1978 to 1989, 1989 to 2014 and 2014 to 2020 about 330, 386 and 379 km² of new land added to the existing land mass and about 57, 142 and 210 km² of existing land eroded, respectively. Finally, from 1978 to 2020, the size of the islands increased by about 14.64 km² yr-1. During the same period about 110 and 40 km² lands along the coastline were eroded and accreted, in the north-western and eastern parts of the estuary, respectively. The study further reveals that the islands of the estuary have developed over the past few decades, except Sandwip. Finally, the outputs of this study will be helpful for policy makers and planners for sustainable estuary management. [ABSTRACT FROM AUTHOR]

Dikes are the conventional means of flood defence along rivers and estuaries. However, dikes gradually lead to the superelevation of waterbodies compared to the subsiding embanked areas, resulting in a rapidly increasing unstable situation under sea-level rise. Therefore, future flood management requires new, sustainable strategies that not only minimise flood risk, but also steer land-level rise. An example is a transitional polder, where a dike-protected area is temporarily reopened to the tide to capture sediment until it has risen well above mean sea-level, after which it could be returned to its original function. This study explores how the sequence of opening transitional polders affects sediment capture and large-scale estuary dynamics through 2D modelling in Delft3D. To this end, different opening sequences were tested along a large estuary, using the Western Scheldt (NL) as an example. Findings show land-level rise in all permutations. However, polders opened later in an opening sequence temporarily experience a lag in muddy sediment capture, most likely due to a deficit in fines. Opening more upstream located polders alone or at the start of an opening sequence generally causes a stronger reduction in mean tidal range than opening more downstream located polders. This is explained by increased friction due to (1) locally added intertidal width and (2) shallowing of the main channels because of increased flood dominance. An upstream-to-downstream opening sequence caused the greatest reduction in mean tidal range, but this is negligible compared to the increase in tidal range due to historic dredging within the estuary for navigational purposes. Further work is needed to determine how dredging, closure of transitional polders and storm surges may negate this benefit to flood safety.

Estuary health indicators sit alongside metrics for air, groundwater, lakes and rivers, supporting a more integrated view of estuary health. Another milestone in estuary management is the development of an estuary health module on the Land Air Water Aotearoa (LAWA) website.[4] The inability to measure estuarine ecological health in a standardised manner has been a major impediment to estuarine management in Aotearoa. Estuaries are unique ecosystems where fresh and salt waters mix, connecting land to the ocean. [Extracted from the article]

Why have mangroves and their ecosystems been so hotly contested over the last quarter century in Aotearoa New Zealand's northern waters? Central to 'mangrove mania' are multiple, competing and often antagonistic perceptions of the perceived risks posed by mangrove presence, their removal and efforts to restore them. Not surprisingly this has led to a chaotic mangrove knowledge space, with significant gaps in knowledge required to understand risks associated with mangrove management. In a socio-ecological investigation of risk questions relating to the mangrove debate, we reveal how localised contestations of mangroves have been 'arbitrated' – in different directions. This state of affairs has resulted from several critical threads: from many sources, pressures and intentions, but all involving the often ill-specified lens of public opinion, science practice, environmental management procedures, Māori knowledge, community views, land and inshore development, and consenting processes. We systematically examine the trajectory of these critical threads and their interactions, underlying the mangrove debate and the resulting variability in how risks are portrayed. Our evidence suggests that perceived risk of mangrove expansion, and of mangrove removal impacts, is contested and linked to different desired and imagined futures. [ABSTRACT FROM AUTHOR]

Many estuaries are squeezed between sea-level rise, coastal hardening and adverse cumulative effects from unsustainable catchment activities. In Aotearoa New Zealand, there is little planning to provide future accommodation space for estuaries. A complex knot of policies and plans, and accompanying poor implementation also affect ecological sustainability. We examine these issues for Brooklands Lagoon/Te Riu o Te Aika Kawa, a tidal lagoon northeast of Christchurch/Ōtautahi. It is within the takiwā of Ngāi Tahu iwi and Ngāi Tūāhuriri hapū, and the jurisdictions of Christchurch City Council and Environment Canterbury. The estuary is influenced by three rivers, and surrounding land use is managed under three different statutory resource management plans, along with several non-regulatory strategies and organisational management plans. However, these plans are poorly integrated and estuarine ecological health is compromised. The incoming tide of resource management and local government reform will add complexity, but also an opportunity to accommodate and enhance the estuary as a blue carbon sink, and to restore cultural and ecological values. This requires specific recognition of estuaries in the proposed managed retreat and spatial planning laws, and within the replacement resource management statute. Legal recognition of Indigenous customary rights could also produce novel governance models to improve management. [ABSTRACT FROM AUTHOR]

Shrimp aquaculture has expanded rapidly in coastal zones worldwide over the past few decades. Saline water stored in shrimp farm ponds can infiltrate into the underlying aquifer causing groundwater salinization and increased submarine groundwater discharge (SGD) to coastal water. However, little research has assessed salinization resulting from these shrimp ponds. To understand the impacts of shrimp farm irrigation on groundwater salinization and SGD, we numerically simulated a series of aquaculture management scenarios in a two‐dimensional conceptual coastal aquifer using a coupled surface‐subsurface approach. We characterized sensitivities to pond water salinity, pond water depth, and farm width. Salinization was assessed by three indicators (salinized area, infiltrated salt mass, and recovery rate), and three SGD indicators were evaluated (fresh SGD, saline SGD, and saltwater circulation rate). Our results show that pond water depth is the primary control on the mass of saltwater infiltration while farm width is the primary control for recovery rate. Pond water salinity and depth affect both fresh and saline SGD. We show that aquaculture is a previously unrecognized mechanism of salinization affecting coastal aquifer vulnerability and SGD. A regional graphical information system analysis shows transformation into aquacultural ponds could introduce considerable SGD variability spatially and temporally. These findings will enable coastal managers to better evaluate groundwater vulnerability in regions with expanding onshore aquaculture and demonstrates the impact of aquaculture on coastal groundwater resources and the need for further study to understand the impact of aquaculture across Asia and the globe. Plain Language Summary: Understanding coastal groundwater salinity and flow is important for managing limited fresh groundwater resources and protecting precious estuarine environments, especially in densely populated coastal areas. Coastal groundwater studies have shown the impacts of diverse natural hydrogeological driving forces and settings. Recently, global expansion of coastal aquaculture ponds has raised environmental concerns about contaminants discharging to surface waters, but less attention has focused on groundwater flow. We developed conceptual models of coastal shrimp ponds to simulate how aquaculture impacts groundwater salinity and discharge into nearby coastal waterbodies. Our results show that coastal aquaculture ponds can contaminate surficial aquifers with saline water and increase discharge of groundwater and solutes to the ocean. This work improves the understanding of human‐induced saltwater intrusion with scientific implications for coastal groundwater practitioners and estuary management. Key Points: Coastal shrimp pond farms cause vertical saltwater intrusion and increased submarine groundwater dischargePond depth controls vertical extent of saltwater intrusion; farm width controls recovery ratesPond water salinity and depth affect character of both fresh and saline submarine groundwater discharge [ABSTRACT FROM AUTHOR]

Community coalescence related to bacterial mixing events regulates community characteristics and affects the health of estuary ecosystems. At present, bacterial coalescence and its driving factors are still unclear. The present study used a dataset from the Chesapeake Bay (2017) to address how bacterial community coalescence in response to variable hydrochemistry in estuarine ecosystems. We determined that variable hydrochemistry promoted the deterioration of water quality. Temperature, orthophosphate, dissolved oxygen, chlorophyll a , Secchi disk depth, and dissolved organic phosphorus were the key environmental factors driving community coalescence. Bacteria with high tolerance to environmental change were the primary taxa accumulated in community coalescence, and the significance of deterministic processes to communities was revealed. Community coalescence was significantly correlated with the pathways of metabolism and organismal systems, and promoted the co-occurrence of antibiotic resistance and virulence factor genes. Briefly, community coalescence under variable hydrochemical conditions shaped bacterial diversity and functional traits, to optimise strategies for energy acquisition and lay the foundation for alleviating environmental pressures. However, potential pathogenic bacteria in community coalescence may be harmful to human health and environmental safety. The present study provides a scientific reference for ecological management of estuaries. • Variable hydrochemistry of the Chesapeake Bay was visualized. • Bacterial community coalescences and its driving factors were revealed. • Community coalescences alleviated environmental pressures. • Community coalescences promoted the threat of potential pathogenic bacteria. [ABSTRACT FROM AUTHOR]

Reliable predictions of future inundation extent within estuaries require a precise evaluation of future extreme sea levels and the application of accurate numerical models that account for the physical processes driving estuarine hydrodynamics. In this study, a methodology that integrates the estimation of local extreme sea levels with high-resolution numerical modeling was applied to assess the future inundation extent in five estuarine systems located on the Portuguese Coast. The main findings obtained were compared with available results from the popular bathtub approach, that disregards the physical processes driving estuarine hydrodynamics and therefore provide imprecise predictions of inundation extent and associated socio-economic impacts. The inundation extent is revealed to be highly dependent on the extreme sea levels and on the estuarine geomorphology, which controls the propagating long-wave. As the long-wave height is highly attenuated within estuaries that have adjacent low-lying areas, restricted inlets, or extensive tidal flats, the results of this study revealed that the extent of inundation is considerably smaller than that obtained by the bathtub approach. The uncertainties associated with mean sea level rise and the estuarine geomorphological evolution constitute the greatest difficulty in assessing the extent of flooding, posing major challenges to the efficient and sustainable management of estuaries. [ABSTRACT FROM AUTHOR]

Understanding the alterations in spatial-temporal water level dynamics caused by natural and anthropogenic changes is essential for water resources management in estuaries, as this can directly impact the estuarine morphology, sediment transport, salinity intrusion, navigation conditions, and other factors. Here, we propose a simple triple linear regression model linking the water level variation 5 on a daily timescale to the hydrodynamics at both ends of an estuary. The model was applied to the Yangtze River estuary (YRE) for examining the influence of the world's largest dam, the Three Gorges Dam (TGD), on the spatial-temporal water level dynamics within the estuary. It is shown that the regression model can accurately reproduce the water level dynamics in the YRE, with a root mean squared error (RMSE) of 0.063-0.151 m seen at five gauging stations for both the pre- and post-TGD periods. This confirms the hypothesis that the response of water level dynamics to hydrodynamics at both ends is mostly linear in the YRE. The regression model calibrated during the pre-TGD period was used to reconstruct the water level dynamics that would have occurred in absence of the TGD's freshwater regulation. Results show that the spatial-temporal alterations in water levels during the post-TGD period are mainly driven by the variation in freshwater discharge due to the regulation of the TGD, which results in increased discharge during the dry season (from December to March) and a dramatic reduction in discharge during the wet-to-dry transitional period. The presented method to quantify the separate contributions made by changes in boundary conditions and geometry on spatial-temporal water level dynamics is particularly useful for determining scientific strategies for sustainable water resources management in dam-controlled estuaries worldwide. [ABSTRACT FROM AUTHOR]

Salt intrusion in surface waters endangers freshwater availability, influences water quality, and affects estuarine ecosystem services with high economic and social values. Salt transport and the resulting salinity distributions result from the non‐linear interactions between salt and water dynamics. Estuaries are often considered under (quasi)‐steady assumption or by focusing on specific timescales. Our understanding of their temporal multiscale response to transient forcing is limited, which hinders the implementation of effective mitigation strategies. We apply wavelet analyses to quantify the variability of salt intrusion from hourly to seasonal timescales and unravel the temporal variability of its response across scales. We focus on an estuary that undergoes significant transient forcing, the Modaomen estuary in the Pearl River Delta, and apply the wavelet analyses to year‐long data generated by a coastal ocean numerical model. Our results show that this estuary responds to changes in tidal and riverine forcing throughout the year over interwoven timescales. Our results highlight the temporal variability of the salt intrusion response time both within a given regime and for the transition between regimes. They also suggest that tides control the response time more strongly than river discharge, even though river discharge determines the magnitude of the salt intrusion, and thus modulates the evolution of the salt intrusion response time. We propose a broadly applicable framework to calculate response times with simple data. These results can provide a first‐order guidance for design and implementation of estuarine management strategies and mitigation measures that ensure water access and facilitate sustainable development. Plain Language Summary: With more and more people living by coast, more water resources are needed, and the fragile equilibrium of estuaries and coastal ecosystems is at risk. In estuaries and deltas, the salty water from the ocean meets the fresh water from the river and can propagate upstream. We call this salt intrusion. One of the solutions used to control salt intrusion is releasing water to push the salt intrusion back, but for this to be effective it needs to be well timed. We use a computer model to understand how the changes in river and ocean affect how far and when the salty water reaches upstream. We show that the river determines how far it reaches but it is mostly the tide that determines when this will happen. Since the ocean moves at different scales, the salt intrusion also responds at multiple scales with different time delays, and it is difficult to distinguish one from another because they overlap. The time delays are not constant and evolve throughout the year. We need to consider the multiple overlapping scales and the evolving time response to ensure the sustainable management of estuaries. Key Points: We analyze the multiscale temporal response of salt intrusion to transient river and ocean forcing during dry and wet seasonThe salt intrusion response time evolves over time throughout the year both within regimes and across regimesThis response time is mainly controlled by tidal forcing and indirectly modulated by river forcing [ABSTRACT FROM AUTHOR]

Numerical models are of fundamental usage for estuarine and coastal sciences. Although numerical simulations are widely applied, analyzing and improving them are often challenging tasks given their large volume and huge parameter space. In this study, a novel data‐driven framework is introduced to study the Minjiang River Plume (MJRP). The framework combines Self‐Organizing Map (SOM) clustering with a Hidden Markov Model (HMM). A three‐dimensional Regional Ocean Model System for MJRP is first configurated with realistic atmospheric, oceanic, and riverine forcings. By applying SOM clustering to the modeled sea surface salinity (SSS) with ∼2,000 2‐day averaged records from 2010 to 2020, we identify six major patterns of MJRP. Each pattern exhibits distinct circulation and plume structures. These MJRP patterns contain not only seasonal signals, but also rich short‐term variabilities driven by the riverine inputs and oceanic dynamics. Then, the SOM‐HMM method was applied to predict the future of the hidden state (i.e., patterns of MJRP) from the observable states (wind and river runoff). With a hypothetic SSS product from a geostationary satellite as the ground truth, we show that the SOM‐HMM method can predict MJRP patterns considerably high prediction accuracy and computational efficiency. Further, these patterns were translated back to SSS with high forecast skills. Combining a conventional numerical model with a data‐driven method, this approach can be promisingly applied in the short‐term marine forecast to support the utilization and management of other estuaries. Plain Language Summary: Analyzing and improving numerical models for estuaries are challenging tasks given the volume and uncertainties of model simulation. By combining a sophisticated machine‐learning method learned from large‐volume model outputs, we show that the performance and efficiency of forecasting the patterns of a river plume can be improved. The method can be further applied to predict the surface salinity of the river, which is a vital property of the river estuary. The prediction can be rapidly done with considerably high accuracy. This study highlights the potential of combining a numerical model with a data‐driven method as a new method to forecast the complex dynamics of a coastal environment. Key Points: A data‐driven framework combining Self‐Organizing Map with Hidden Markov Model was developed for predicting states of river plumesIt was applied to a representative subtropical estuarine environment: the estuary of Minjiang RiverValidated with hypothetic and model surface salinity, this method can indeed predict river plume states and surface salinity [ABSTRACT FROM AUTHOR]

Estuaries receive substantial anthropogenic nitrogen loading and are mainly considered net nitrogen sinks. While several studies have identified diverse diazotrophic communities in estuarine sediments, the role of pelagic diazotrophs in these systems is not well understood. We investigated the links between diazotrophic community composition, nitrogenase (nifH) gene expression, N2 fixation, and environmental conditions in Narragansett Bay (USA). Pelagic N2 fixation rates ranged between 0.02 and 9.41 nmol N L−1 d−1 and correlated significantly with fluctuations in diazotroph community composition. These fluctuations were also correlated with temperature, salinity, and mean sea level. The dominant sequences in our pelagic samples were related to sequences previously detected in the bay's sediments and were dominated by nifH gene Clusters I and III. We interpret this as a coupling between sediment and pelagic diazotroph communities and speculate that resuspension plays an important role for pelagic N2 fixation in shallow estuarine environments such as Narragansett Bay. For instance, the finding of active sulfate reducers in the oxygenated water illustrates that the sediment‐pelagic coupling can impact nutrient cycling in shallow environments. The pelagic N2 fixation measured during our study period showed only a minor contribution (< 1%) to the total estimated nitrogen load to Narragansett Bay. However, with intensifying nitrogen management in estuaries, the need to constrain the rates of pelagic N2 fixation in these systems will be essential for estimating nitrogen fluxes within the bay and to the adjacent coastal ecosystems. [ABSTRACT FROM AUTHOR]

Mosquito populations associated with the heavily urbanised Cooks River and Wolli Creek estuaries, Sydney, Australia, were investigated to assist in identifying potential pest and public health threats and assist local authorities in developing management strategies. Adult mosquitoes were sampled using carbon dioxide baited light traps at five locations, including two sites alongside Cooks River (Steel Park and Gough Whitlam Park), two sites alongside Wolli Creek (Jackson Track and Turella Reserve) as well as an additional location, Barton Park (Muddy Creek). These locations represented at least one of three distinct landscape characteristics, dominated either by bushland, parkland, or estuarine wetland habitats. Three trap sites were established at each location and sampling adult host-seeking mosquitoes took place on a weekly basis for 4 weeks. All mosquito specimens were identified to determine differences in relative abundance and community composition. There was a distinct difference in the abundance and community composition of mosquitoes collected between the five locations with the differences due to the presence and abundance of Aedes vigilax, Culex sitiens, Aedes notoscriptus Those locations close to estuarine wetlands were dominated by A. vigilax while other sites were dominated by mosquitoes associated with urban habitats such as stormwater infrastructure or backyard water-holding containers (e.g. A. notoscriptus and Culex quinquefasciatus). Sites adjacent to urban bushland contained a higher species richness, but not necessarily greater abundance, of mosquitoes than other sites. This study provided an insight into mosquito populations within a previously unstudied area of Sydney and highlight that local authorities should carefully consider wetland management and urban planning strategies in the future to assist in identifying potential risks of nuisance-biting, mosquito-borne disease and in determining the need for further public health action. [ABSTRACT FROM AUTHOR]

Community-based monitoring programs (CBMPs) are a cost-effective option to collect the long-term data required to effectively monitor estuaries. Data quality concerns have caused some CBMP datasets, which could fill knowledge gaps for aquatic ecosystems, to go unused. The Community Aquatic Monitoring Program (CAMP) is a CBMP that has collected littoral nekton assemblage data from estuaries in the southern Gulf of St. Lawrence since 2003. Concerns with the CAMP sampling design (station placement and numbers) have prevented decision-makers from using the data to inform estuary health assessments. This study tested if CAMP's sampling design that accommodates volunteer participation provides similar information as a scientific sampling approach. Six CAMP stations and six stations selected using a stratified random design were sampled at ten estuaries. A permutational-MANOVA revealed nekton assemblages were generally not significantly different between the two sampling designs. The current six CAMP stations are sufficient to detect the larger differences in species abundances that may indicate differences in estuary condition. The predicted increase in precision (2%) with twelve stations is not substantive enough to warrant an increased sampling effort. CAMP's scientific utility is not limited by station selection bias or numbers. Furthermore, well-designed CBMPs can produce comparable data to scientific studies. [ABSTRACT FROM AUTHOR]

Nutrient loads delivered to estuaries via submarine groundwater discharge (SGD) play an important role in the nitrogen (N) budget and eutrophication status. However, accurate and reliable quantification of the chemical flux across the final decimeters and centimeters at the sediment–estuary interface remains a challenge, because there is significant potential for biogeochemical alteration due to contrasting conditions in the coastal aquifer and surface sediment. Here, a novel, oxygen‐ and light‐regulated ultrasonic seepage meter, and a standard seepage meter, were used to measure SGD and calculate N species fluxes across the sediment–estuary interface. Coupling the measurements to an endmember approach based on subsurface N concentrations and an assumption of conservative transport enabled estimation of the extent of transformation occurring in discharging groundwater within the benthic zone. Biogeochemical transformation within reactive estuarine surface sediment was a dominant driver in modifying the N flux carried upward by SGD, and resulted in a similar percentage of N removal (~ 42–52%) as did transformations occurring deeper within the coastal aquifer salinity mixing zone (~ 42–47%). Seasonal shifts in the relative importance of biogeochemical processes including denitrification, nitrification, dissimilatory nitrate reduction, and assimilation altered the composition of the flux to estuarine surface water, which was dominated by ammonium in June and by nitrate in August, despite the endmember‐based observation that fixed N in discharging groundwater was strongly dominated by nitrate. This may have important ramifications for the ecology and management of estuaries, since past N loading estimates have generally assumed conservative transport from the nearshore aquifer to estuary. [ABSTRACT FROM AUTHOR]

How the partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and N2 production responds to hypoxia in seasonally hypoxic estuaries has not been well understood. In this study, 15N tracer incubations were used to investigate benthic denitrification, anammox and DNRA rates for three years (2016–2018) across seasonally hypoxic areas off the Changjiang Estuary. Dissolved oxygen contents in bottom water were significantly lower in July than in February/March. Concentrations of NH4+ and NOx– in bottom water throughout the sampling sites did not vary between seasons. Rates of benthic denitrification (0.45–6.55 nmol N g−1 h−1), anammox (0.02–0.30 nmol N g−1 h−1) and DNRA (0.06–0.59 nmol N g−1 h−1) varied largely along the estuary. Denitrification, anammox and DNRA rates did not vary significantly between inshore and offshore sites. However, denitrification rates were significantly lower and DNRA rates were significantly higher in consistently than transiently hypoxic sites. Anammox rates were slightly lower in consistently than transiently hypoxic sites. Ratios of DNRA to N2 production were in a range of 0.015–0.53, and were significantly higher in consistently than transiently hypoxic sites, but did not vary significantly between inshore and offshore sites. The lower temperature sensitivity of denitrification and anammox compared to their Q10 values further indicated that hypoxia may inhibit N2 production. These results suggest that high incidence hypoxia can decrease denitrification and anammox rates but increase DNRA rates with important implications on N retention versus removal, thereby exacerbating eutrophication and hypoxia in estuarine and coastal environments. Plain Language Summary: This study investigated benthic denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) rates for three years (2016–2018) across the consistently and transiently hypoxic areas off the Changjiang Estuary. Denitrification rates were significantly lower, whereas DNRA rates were significantly higher in consistently than transiently hypoxic sites. Anammox rates were slightly lower in consistently than transiently hypoxic sites. The ratios of DNRA to N2 production (including denitrification and anammox) were significantly higher in consistently than transiently hypoxic sites, but did not vary significantly between inshore and offshore sites. Therefore, it is concluded that high incidence hypoxia can decrease N2 production but facilitate nitrogen retention, which may further exacerbate eutrophication and hypoxia in estuarine and coastal environments. Key Points: Denitrification dominated over anammox and dissimilatory nitrate reduction to ammonium (DNRA) throughout the sampling seasons and sitesDenitrification and anammox rates were lower, whereas DNRA rates were higher in consistently than transiently hypoxic sitesHigh incidence hypoxia favored N recycling rather than N removal off the Changjiang Estuary [ABSTRACT FROM AUTHOR]

While freshwater inflow has been a major focus of resource management in estuaries, including the upper San Francisco Estuary, there is a growing interest in using focused flow actions to maximize benefits for specific regions, habitats, and species. As a test of this concept, in summer 2016, we used a managed flow pulse to target an ecologically important region: a freshwater tidal slough complex (Cache Slough Complex-CSC). Our goal was to improve estuarine habitat by increasing net flows through CSC to enhance downstream transport of lower trophic-level resources, an important driver for fishes such as the endangered Delta Smelt Hypomesus transpacificus. We used regional water infrastructure to direct 18.5 million m2 of Sacramento River flow into its adjacent Yolo Bypass floodplain, where the pulse continued through CSC. Simulations using a 3-D hydrodynamic model (UnTRIM) indicated that the managed flow pulse had a large effect on the net flow of water through Yolo Bypass, and between CSC and further downstream. Multiple water quality constituents (specific conductivity, dissolved oxygen, nutrients [NO3 + NO2, NH4, PO4]) varied across the study region, and showed a strong response to the flow pulse. In addition, the lower Sacramento River had increased phytoplankton biomass and improved food quality indices (estimated from long-chain essential fatty acids) after the flow pulse. The managed flow pulse resulted in increased densities of zooplankton (copepods, cladocerans) demonstrating potential advection from upper floodplain channels into the target CSC and Sacramento River regions. This study was conducted during a single year, which may have had unique characteristics; however, we believe that our study is an instructive example of how a relatively modest change in net flows can generate measurable changes in ecologically relevant metrics, and how an adaptive management action can help inform resource management. [ABSTRACT FROM AUTHOR]

Understanding saltwater intrusion processes is crucial for both ecosystem health and water resource management in river estuaries. With the development of the global coastal economy, many estuaries worldwide have undergone shipping engineering constructions. Channel deepening in coastal areas adversely impacts the quality of freshwater required for ecological purposes, human consumption, and agricultural and industrial activities. Many studies have focused on saltwater intrusion processes in artificial canals capable of high and medium runoff characteristics; however, little is known about seawater intrusions in canals with low runoff and ship locks located far away from the estuary. This study focuses on the saltwater intrusion process in such a canal, using the Pinglu Canal of Qinjiang River Estuary in China as a prototype example. A three-dimensional (3D) numerical saltwater intrusion model using the standard k–ε turbulence closure was developed to simulate the saltwater intrusion process pre- and post-canal construction. The verification was conducted by comparing the simulated time series of the tidal level, velocity magnitude, and salinity with observations, and the impact of channel deepening on the saltwater intrusion in the Qinjiang River was analysed based on various numerical experiments. After channel deepening, mixing decreased following the decrease in flow velocity, the saltwater intrusion extent could reach the Youth Ship Lock after an increase of 5 km compared to the extent under natural conditions during high tide, and seawater remained in the channel during ebb tide, with freshwater discharge being Q < 20 m3/s. Furthermore, during the dry season, salinity in the approach channel downstream the Youth Ship Lock exhibited a trend of cumulative increase toward a stable value, and the correlation with the tide range cycle decreased from 0.62 under natural conditions to 0.09, whereas the correlation with the upstream inflow increased from 0.10 under natural conditions to 0.51. Results further indicated that the sensitivity of the saltwater intrusion to the flow rate was at a maximum, and its sensitivity to the tidal range was of minor significance after canal construction. Accordingly, a dimensionless function was constructed to quantify freshwater discharge from the Youth Hydro-hub to reduce the saltwater intrusion under variations in the tidal range and salinity during the low-water discharge period. This study can aid water resource management and suggest methods for mitigating saltwater intrusions in this type of artificial canal. • Salt intrusion process is investigated in an artificial canal with low runoff. • Saltwater intrusion extent can reach the ship lock far away from the estuary. • The sensitivity to water discharge is the strongest downstream the ship lock. • The sensitivity to tidal range decreased as increasing of the distance from estuary. • An empirical function is developed to quantify salinity reduction freshwater. [ABSTRACT FROM AUTHOR]