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Estuaries have strong physicochemical gradients that lead to complex variability and often high rates of biogeochemical processes, and they are also often impacted by humans. Yet, our understanding of estuarine biogeochemistry remains skewed towards temperate latitudes. We examined seasonal and spatial variability in dissolved organic matter (DOM) and nutrients along a partly eutrophic, agricultural–urban estuary system in Southeast Asia: the Johor River and the East Johor Strait. Dissolved organic carbon (DOC) and coloured DOM (CDOM) showed non-conservative mixing, indicating significant DOM inputs along the estuary. The CDOM spectral slopes and CDOM : DOC ratios suggest that terrigenous, soil-derived DOM dominates along the Johor River, while phytoplankton production and microbial recycling are important DOM sources in the Johor Strait. CDOM properties were not unambiguous source indicators in the eutrophic Johor Strait, which is likely due to heterotrophic CDOM production. Nitrate concentrations showed conservative mixing, while nitrite concentrations peaked at intermediate salinities of 10–25. Ammonium concentrations decreased with salinity in the Johor River but increased up to 50 µ mol L -1 in the Johor Strait, often dominating the dissolved inorganic nitrogen (DIN) pool. Phosphate concentrations were low (<0.5 µ mol L -1) throughout the Johor River but increased in the Johor Strait, where DIN : phosphate ratios were typically ≥ 16 : 1. This suggests that the Johor Strait may experience phosphorus limitation and that internal recycling is likely important for maintaining high nutrient concentrations in the Johor Strait. Overall, our results indicate that the Johor River and Johor Strait are clearly not part of the same estuarine mixing continuum and that nutrient recycling processes must be quantified to understand nutrient dynamics in the Johor Strait. Moreover, our results highlight the need for better techniques for DOM source tracing in eutrophic estuaries. [ABSTRACT FROM AUTHOR]

The Shatt Al-Arab estuary is the lifeline of the Basrah city, as it is the source of water for various uses of irrigation, industry, agriculture and human use, in addition to being the navigational passage towards the Arabian Gulf throughout history. Nowadays, the estuary suffers from the deterioration of its water quality due to the increase in salinity values as a result of the intrusion of salinity from the Arabian Gulf due to the lack of freshwater flows from the Tigris and Euphrates rivers. To reduce the risks of water quality deterioration and the lack of natural solutions represented in the provision of freshwater, the idea of building an artificial structure to narrow the channel came to give the ability to resist the impact of the tide and the intrusion of marine salinity. Two hypothetical sites were chosen to build the structure: i.e. in Dweeb and Faw sites, to test the feasibility of this artificial estuary structure. A one-dimensional mathematical model was adopted, which is the Mike 11 software package. The model was run according to a set of scenarios using the critical water discharge values of 10 and 20 m3/s. The results showed through simulation of the model for a period of six months that the structure is useful on improving the water quality by reducing salinity in the river, in addition, there is a delay in the time of arrival of the intrusion of salts. [ABSTRACT FROM AUTHOR]

Low-inflow estuaries are naturally more susceptible to anthropogenic stressors, compared to well flushed systems, with excessive nutrient loading posing a particular threat. This study investigated the benthic diatom community structure of two eutrophic, microtidal estuaries impacted by daily wastewater effluent discharges. It was hypothesised that the community structure would be similar between the warm-temperate Hartenbos and subtropical uThongathi estuaries due to disproportionally high dissolved inorganic nitrogen (H: 38 kg DIN d−1; uT: 67.5 kg DIN d−1) and phosphorus (H: 22 kg DIP d−1; uT: 29.7 kg DIP d−1) inputs from wastewater treatment works (WWTWs). Taxa tolerant of high nutrient conditions proliferated in both systems. However, the dominant taxa differed with the brackish Halamphora coffeaeformis species occurring in the Hartenbos Estuary and the freshwater Navicula rostellata, Sellaphora pupula and Navicula gregaria species in the uThongathi Estuary. The overall benthic diatom diversity in both systems was low (H' > 0 but ≤ 1.5) and indicative of a degraded health state. Temporal differentiation driven by salinity was evident in the Hartenbos Estuary, while changes in community structure were limited to periods of increased river inflow in the uThongathi Estuary. Therefore, while the trophic status of the dominant taxa was determined by the nutrient stress (primary stressor), changes in salinity and river inflow (secondary stressors) shaped the distinct community assemblages observed in each estuary. This study provides insight into the impact of similar anthropogenic-induced pressures in different biogeographical regions and the importance of managing towards a natural dynamic state of microtidal, low-inflow estuaries. [ABSTRACT FROM AUTHOR]

Estuaries have strong physicochemical gradients that lead to complex variability and often high rates of biogeochemical processes. The biogeochemistry of many estuaries is also increasingly impacted by human activities. Yet our understanding of estuarine biogeochemistry remains skewed towards temperate systems in the northern hemisphere, with far less research from tropical estuaries. This study examined seasonal and spatial variability in dissolved organic matter (DOM) and nutrient biogeochemistry along a partly eutrophic, mixed agricultural/urban estuary system in Southeast Asia, the Johor River and the East Johor Strait. Dissolved organic carbon (DOC) and chromophoric DOM (CDOM) properties showed non-conservative mixing, indicating significant DOM inputs along the estuary. The CDOM spectral slopes and CDOM:DOC ratios suggest that these inputs are dominated by terrigenous, soil-derived DOM along the Johor River, but that phytoplankton production and microbial recycling are more important DOM sources in the Johor Strait. Nitrate consistently showed conservative mixing, while nitrite concentrations peaked at intermediate salinities of 10–25. Ammonia decreased with salinity in the Johor River. In the Johor Strait, however, ammonia increased up to 50 µmol l-1, often dominating the dissolved inorganic nitrogen (DIN) pool. Phosphate was low (<0.5 µmol l-1) throughout the Johor River, but increased in the Johor Strait, where DIN:phosphate ratios were typically at or above 16:1. This suggests that phytoplankton in the Johor Strait may sometimes experience phosphorus limitation. Moreover, internal recycling is likely important for maintaining high nutrient concentrations in the Johor Strait. [ABSTRACT FROM AUTHOR]

Soil salinization is a widespread phenomenon leading to land degradation, particularly in regions with brackish inland aquaculture ponds. However, because of the high geographical and temporal fluctuation, monitoring vast areas provides substantial challenges. This study uses remote sensing data and machine learning techniques to predict soil salinity. Four linear models, namely linear regression, least absolute shrinkage and selection operator (LASSO), ridge, and elastic net regression, and three boosting algorithms, namely XGB regressor, LightGBM, and CatBoost regressor, were used to predict soil salinity. Cross-validation was performed by splitting the data into 30% for model testing and 70% for model training. Multiple metrics such as determination coefficient (R2), root mean square error (RMSE), mean square error (MSE), and mean absolute error (MAE) were used to compare the performances of these algorithms. By comparison, the CatBoost regressor model performed better than the other models in both testing (MAE = 0.42, MSE = 0.28, RMSE = 0.53, R2 = 0.92) and training (MAE = 0.49, MSE = 0.36, RMSE = 0.60, R2 = 0.90) phases. Hence, the CatBoost regressor model was recommended for monitoring soil salinity in India's massive inland aquaculture zone. [ABSTRACT FROM AUTHOR]

The water quality in estuaries is influenced by both physical and biochemical processes, making it difficult to identify and quantify the main controlling factors of chlorophyll-a (Chl-a) concentration. This study introduces an interpretable machine learning approach to identify and quantify the dominant control factors determining Chl-a concentration in the Yellow River Estuary (YRE). The model utilizes in situ data and MODIS data in the Yellow River Estuary, incorporating five key variables: salinity, water depth, turbidity, dissolved inorganic nitrogen (DIN), and soluble reactive phosphorus (SRP). Three types of inputs are used to predict Chl-a, including physical, biogeochemical, and physical-biogeochemical factors. The model further uses feature importance ranking and partial dependence plot to identify and quantify key control factors. The results reveal that (1) Chl-a variation is influenced by multiple factors, especially salinity and turbidity; (2) Chl-a concentration is inhibited when salinity is below 26, turbidity is lower than 16 NTU and above 30 NTU, DIN is lower than 0.314 mg/L and SRP is below 0.01 mg/L; (3) riverine inputs change turbidity and salinity in the Yellow River Estuary and Laizhou Bay, in response to temporal-spatial distributions of phytoplankton. This research can provide a valuable reference for water quality management in the Yellow River Estuary. • Interpretable XGBoost model is developed for estuary chlorophyll ɑ. • SHAP values reveal salinity and turbidity as the most important factors. • SHAP dependency plots help to delineate different estuarine ecoregions. [ABSTRACT FROM AUTHOR]

Saltwater intrusion is a natural mixture process between watershed freshwater and seawater that frequently occurs in estuaries. Station-based monitoring of saltwater intrusion is time-consuming and labor-intensive. To enable quick monitoring of saltwater intrusion, this study developed new remote sensing algorithms for water surface salinity measurement using four decision tree-based machine learning models. These models were built based on simultaneously collected in-situ salinity data from a waterway in the Pearl River Delta and Unmanned Aerial Vehicle (UAV) hyperspectral images. A 10-fold cross-validation was applied to assess the performance of the models, with XGBoost outperforming the other three models (R 2 =0.93, RMSE = 0.88 psu). Then the developed model was employed for Sentinel-2 multispectral satellite images to invert the estuarine salinity distribution at a larger spatial scale. Results displayed the high performance of the machine learning models proposed in this study for mapping the salinity distribution in river channels, making it an efficient and practical technique for monitoring saltwater intrusion in river channels at a regional scale. [Display omitted] • We developed a ML method to predict estuary salinity from UAV data. • Model's versatility extends to large-scale monitoring with satellite images. • Feature importance enhances predictor selection, boosting model practicality. • Validation underscores high accuracy of the method in estuary salinity mapping. [ABSTRACT FROM AUTHOR]

The influence of Karun river inflow on salinity intrusion from the Arabian Gulf towards the upper reaches of the Shatt Al-Arab river estuary was evaluated by using Mike11, a one-dimensional numerical modeling technique. The simulations results indicated that, during the moderate and low flow conditions of the Shatt Al-Arab river, freshwater inflow from Karun river at 10 and 40 m3/s, respectively, could be capable of keeping salinity extent to not exceeded the confluence location of these two rivers. Additionally, the results indicated that in the case of a sharp decline of Karun river inflow, additional releasing of freshwater from the Tigris river can completely compensate for the Karun inflow. While in the case of cut off of Tigris river, the Karun inflow in the range of 60 m3/s could be able to keep salinity extent beyond the Abo Flous station during the simulation period. Furthermore, the possibility of discharging water with high levels of salinity from the farming projects located at the lower basin of Karun river towards the Shatt Al-Arab river estuary was taken into account. In such cases, the results suggested that during low and moderate flow conditions, freshwater inflow by about 250 and 225 m3/s, respectively, should be released from the Karun river to remains salinity extent below the Abo Flous location. A water management policy agreement between Iraq and Iran could be an effective solution to the salinity issue both in the Shatt Al-Arab river estuary as well as Karun river. [ABSTRACT FROM AUTHOR]

Estuaries are dynamic environments which are driven by various natural processes like river discharge, tides, waves, influx of saline water and sediments, etc. These ecosystems are the most sensitive to sea level rise and fluctuations in river discharge associated with climate change. A direct response of sea level rise and river discharge can be observed in the water level of estuaries. However, existing models have not considered these parameters for forecasting water level. This paper focuses on developing a water level forecast model for the Chikugo River estuary in Japan using Nonlinear Autoregressive with Exogenous inputs (NARX Model). NARX neural network was used to do the one-step-ahead prediction of water level considering the various parameters that can very well be influenced by climate change: previous water level, river discharge, and salinity. Accordingly, three models were developed: (i) Model I considering previous water level; (ii) Model II additionally considering river discharge; and (iii) Model III additionally considering salinity. All the models showed appreciable performance in forecasting the water level. Model III had the best correlation with the water level with a cross-correlation value of 0.6030, while the river discharge had only a cross-correlation of 0.1113 indicating that the Chikugo River estuary is tide-dominated. The model was trained using different combinations of available data - previous water level, river discharge, and salinity. Cross-correlation results showed a better correlation between water level and salinity than various other combinations trained. Therefore, tidal intrusion influences the water level in the estuary, thereby depicting that sea level rise can affect the water level, and its influence can be well predicted by the developed model. The water level significantly affects the flora and fauna and the predictability of future estuarine floods can help in taking necessary mitigation strategies. • Water Level of Chikugo estuary forecasted using NARX model. • Analyzed the influence of previous water level, discharge and salinity. • Water level has more cross-correlation with salinity than discharge. • Forecasted the water level for various river discharge and salinity. • Model capable to analyse the influence of climate change on water level. [ABSTRACT FROM AUTHOR]

This study starts from a two‐layer 1D Estuary Box Model (EBM), the so called Centro Euro‐Mediterraneo sui Cambiamenti Climatici (CMCC) EBM (Verri et al., 2020, https://doi.org/10.1016/j.ocemod.2020.101587), devised to offer a proper representation of the net river release in a coupled modeling framework with hydrology models and ocean models reaching the mesoscale. Two key advances are proposed in the present study: (I) the estuary length and (II) the horizontal eddy diffusivity coefficient are no longer considered as static parameters to calibrate but two non‐dimensional equations have been introduced in order to make them time‐variable. The Goro branch of the Po river delta is considered as case study of "salt‐wedge estuary" flowing into a microtidal sea. Regarding the estuary length, a non‐dimensional equation has been tested and validated. This equation provides a time dependent estuary box volume which is more realistic choice. This time‐variable estuary length represents the length of the salt wedge intrusion. Regarding the horizontal eddy diffusivity, the static eddy coefficient adopted in the previous version of the model is a coarse assumption. Thus a non‐dimensional equation for this coefficient has been tested and validated. It makes the eddy diffusivity dependent on the velocity shear, the salinity gradient and the estuary geometry. The proposed dynamic formula is found to enhance the model capability to reproduce the salinity at the estuary mouth. Overall the high statistical performance in terms of RMSE and correlation coefficient, the short CPU time and the minimal calibration encourage to use the CMCC EBM in coupled mode with both mesoscale‐resolving ocean and hydrology models to produce operational forecasts and climate scenarios. Plain Language Summary: The Centro Euro‐Mediterraneo sui Cambiamenti Climatici Estuary Box Model is placed in the framework of developing a conceptual and numerical modeling approach to simulate the effects of river release on the coastal to open sea circulation and dynamics. The few kilometers resolution of mesoscale ocean models cannot explicitly resolve the estuary dynamics; thus, the idea of an "estuary box model" that gives reasonable values of water volume flux and salinity at the river mouth, which in turn affects the ocean dynamics. A further development of the model equations is offered in this article and is demonstrated to enhance the model capability to reproduce the salinity at the estuary mouth; moreover, the length of the salt wedge intrusion is considered a model unknown. The research is not only interesting from an academic point of view but also for its practical application: (i) introducing an information on river release into ocean models, from regional to global scale, can improve their performance and produce more realistic operational forecasts or climate scenarios; (ii) introducing an estimate of the length of the salt wedge contributes to address the pressing issue of salt water intrusion and salinization processes in coastal areas. Key Points: An Estuary Box Model is devised to represent the salt‐wedge intrusion length and the net river release at the estuary mouthTwo key advancements, that is non‐dimensional equations for time‐variable estuary length and eddy diffusivity, enhance the model performanceThe proposed model is suitable for coupling with mesoscale ocean and hydrology models to produce operational forecasts and climate scenarios [ABSTRACT FROM AUTHOR]

The salinity distribution in the estuarine channel network of a partially mixed estuary was investigated by quantifying salt intrusion length and stratification with response to tidal circulation, seasonal variation, topography, and river discharge. The studied Karnafuli River estuary responds to a power-law relationship between discharges and salt intrusion lengths with exponent −0.64. In the river network, two salinity sinks were identified, that is, a river confluence and a channel bend. At the estuary, the surface salinity level during the dry period increased by 1.2–1.9 times that of the wet period due to residual salinity. On the other hand, the surface salinity level during the wet period decreased due to high river discharge. Again, a high salinity pulse was observed during flood tide and lower concentration was found in surface salinity during ebb tide. The spring–neap transition also causes salinity variation, reaching the lowest concentration at the end of the neap tide-wet period. Applying a subgrid scale Horizontal Large Eddy Simulation model in Delft3D, the spatial distribution of salinity showed that during flood tide the centrifugal force acted stronger than the opposing barotropic pressure gradient, and the centrifugal force moves the low-density water toward the river bend, which resists salinity intrusion. High salinity stratification was observed with high freshwater discharge and low stratification was found at a shallow water depth of the estuary. The surface salinity responds faster than the channel bottom salinity flux with changing forcing conditions (tide and discharge), while the bottom salinity circulation follows an unsteady process. [ABSTRACT FROM AUTHOR]

The present study aims to estimate effective diahaline turbulent salinity fluxes and diffusivities in numerical model simulations of estuarine scenarios. The underlying method is based on a quantification of salinity mixing per salinity class, which is shown to be twice the turbulent salinity transport across the respective isohaline. Using this relation, the recently derived universal law of estuarine mixing, predicting that average mixing per salinity class is twice the respective salinity times the river run‐off, can be directly derived. The turbulent salinity transport is accurately decomposed into physical (due to the turbulence closure) and numerical (due to truncation errors of the salinity advection scheme) contributions. The effective diahaline diffusivity representative for a salinity class and an estuarine region results as the ratio of the diahaline turbulent salinity transport and the respective (negative) salinity gradient, both integrated over the isohaline area in that region and averaged over a specified period. With this approach, the physical (or numerical) diffusivities are calculated as half of the product of physical (or numerical) mixing and the isohaline volume, divided by the square of the isohaline area. The method for accurately calculating physical and numerical diahaline diffusivities is tested and demonstrated for a three‐dimensional idealized exponential estuary. As a major product of this study, maps of the spatial distribution of the effective diahaline diffusivities are shown for the model estuary. Plain Language Summary: Eddy diffusivity determines how intensively concentrations in a fluid are spreading due to turbulent motion. Here, we analyze the diffusivity that spreads salt concentration (i.e., salinity) across a surface of constant salinity (the isohalines), also called effective diahaline diffusivity. A new method is presented that calculates effective diahaline diffusivities based on the specific volume between two specified isohalines, on the salinity mixing within this volume as well as on the surface area of the isohalines. We define mixing as the rate of destruction of salinity variance per unit volume due to turbulent mixing processes. The method applies to computer models of ocean dynamics on scales ranging from coastal to global. In such models, the mixing is determined by statistical mathematical equations of turbulent processes, which is the so‐called physical mixing. In models, additional (numerical) mixing occurs due to numerical inaccuracies of algorithms that move around water masses passively with the currents, a process called advection. Using our method, the total effective diffusivity determined for each isohaline surface can be accurately separated into contributions from physical mixing and numerical mixing. We demonstrate the functioning of the new method for an idealized model simulation of an estuary. Key Points: Mixing and volume per salinity class determine effective diahaline diffusivityEffective diahaline diffusivity is split into physical and numerical contributionsIn an idealized estuary, largest effective diffusivities are found in the brackish waters of the navigational channel [ABSTRACT FROM AUTHOR]

The wetland of Liao River estuary (LRE) in northeastern China is one of the best-preserved wetlands across the globe. However, it is extremely vulnerable to hydrological changes as well as other disturbances, particularly upstream river discharges and the reclamation from anthropogenic activities. In this study, a 3D hydrodynamic model was used to reproduce the flow patterns of the LRE and to explore the variation in salinity under different scenarios. Furthermore, the impact of river discharge and shoreline changes on the salinity distribution in the LRE was quantitatively analyzed and discussed through several simulation experiments. The model reasonably reconstructed the spatiotemporal variability and distribution of salinity in the Liao River estuary and the wetlands across intertidal areas. Increases in runoff were demonstrated to significantly decrease the mean salinity values of the estuary, with changes in salinity negatively correlated to the longitudinal distance from the estuary mouth. Moreover, the shoreline change caused by the construction of Panjin Port was observed to have an obvious influence on the salinity distribution of the LRE, particularly in the lower reaches of the Pink Beach wetland. Comparisons of the Liao River estuary residual flow fields under different shorelines revealed that the establishment of the port resulted in the diffusion of the runoff from the Daliao River due to the obstruction of the port body, which enhanced the tidal mixing effect and consequently weakened the dilution effect of freshwater entering the Pink Beach wetland. Overall, the current study should be helpful for offering a greater understanding of Suaeda heteroptera vegetation degradation in the LRE, China, and also provides a new perspective in investigating the degradation mechanism in other estuarine wetlands. [ABSTRACT FROM AUTHOR]

The Oujiang River Estuary (ORE) is a macrotidal estuary in southeast Zhejiang Province of China. Although the economy has developed rapidly over the past two decades, few studies have been explored on the estuarine dynamics. Based on in situ measurements, we established a 3-D numerical model to investigate salt intrusion and its response to river discharge, tide, and the salinity at river mouth. We obtained following conclusions. (1) Although salt intrusion length followed a power law with respect to overall river discharge span capturing the fundamental trend, salt intrusion length should be analysed at different ranges of river runoff, as regression accuracy increased substantially at individual range. (2) Estuarine salt transport was dominated by two components of downstream Euler flux and upstream Stokes flux, while tidal shear effects and tidal pumping contributed less. (3) Salt intrusion length not only was a function of river runoff and tidal range but also influenced, with more indicating significance, by mouth salinity. Additionally, geometry effect and nonlinearity might amplify the strong dependences of salt intrusion on mouth salinity and tidal range. (4) Results from partial least squares revealed an approximately linear relation between salt intrusion length and the determining factors of tidal range, river runoff, and river mouth salinity. The hysteretic term was found to have most important influence on salt intrusion length. The response time of salt intrusion in ORE varied with runoff from ~ 2.5 days for medium discharge to ~ 1 day for large and small discharges. [ABSTRACT FROM AUTHOR]

The wetland of Liao River Estuary in northeast China is one of the best-preserved wetlands across the globe. However, it is extremely vulnerable to hydrological changes as well as other disturbances, particularly upstream river discharges and the reclamation from anthropogenic activities. In this study, a 3D hydrodynamic model (FVCOM) was used to reproduce the flow patterns of the Liao River Estuary and to explore the variation in salinity under different scenarios. Furthermore, the impact of river discharge and shoreline changes on the salinity distribution in the Liao River Estuary were quantitatively analyzed and discussed through several simulation experiments. The model reasonably reconstructed the spatio-temporal variability and distribution of salinity in the Liao River Estuary and the wetlands across intertidal areas. Increases in runoff were demonstrated to significantly decrease the mean salinity values of the estuary, with changes in salinity negatively correlated to the longitudinal distance from the estuary mouth. Moreover, the shoreline change caused by the construction of Panjin Port was observed to have an obvious influence on the salinity distribution of the Liao River Estuary, particularly in the lower reaches of the Pink Beach Wetland. Comparisons of the Liao River Estuary residual flow fields under different shorelines revealed that the establishment of the port resulted in the diffusion of the runoff from the Daliao River due to the obstruction of the port body, which enhanced the tidal mixing effect and consequently weakened the dilution effect of fresh water entering the Pink Beach Wetland. Overall, the current study should be helpful for offering a greater understanding of Suaeda heteroptera vegetation degradation in the Liao River Estuary, China, this work also provides a new perspective in investigating the degradation mechanism in other estuarine wetlands. [ABSTRACT FROM AUTHOR]

Martdianto, R.; Muin, M.; Kusuma, S.B.; Muslim, A.B., and Alam, H.N., 2023. BOD-salinity simulation in the Sunter River Tanjung Priok port with a non-orthogonal model application. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.-S., and Lee, J. (eds.), Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 76-80. Charlotte (North Carolina), ISSN 0749-0208. Tanjung Priok port is the biggest port in Indonesia, located in the Capital City of Jakarta, equipped with two sides of a breakwater that protects the port from the direct impact of the sea. The Sunter river flow to Tanjung Priok port delivers high domestic waste as an influence of urban activities. The high content of BOD values along the Sunter river to Tanjung Priok increases the source of water pollution that needs to control correctly. Tanjung Priok also has pollution sources consisting of solid domestic waste, domestic liquid waste, and oil sludge. The Sunter river streamflow in the Jakarta drainage system is divided by Kanal Banjir Timur (KBT), changing flow rate, and water quality pattern. The study area is the Sunter river's main channel, which has a 17 km length and includes Duren Sawit, Pulo Gadung, Kelapa Gading, Plumpang, Rawa Badak, and Tanjung Priok. To describe the complexity of a river's geometric flow to an estuary along the Sunter river to Tanjung Priok, the concept of a curvilinear Non-Orthogonal model is used, which is careful in composing the river morphology shape, river-channel branching, dimension, and elevation of the river to the estuary. Efforts to improve water quality along the Sunter river by increasing saltwater's effect by carrying out a Density-Induced Current (DIC) and dredging configuration. Dredging at the mouths of rivers by considering the influence of DIC involves water currents, river flow, wind stress, and tides. The study presents several simulation scenarios using MuQual3D to assess water quality problems in the Sunter river by increasing salinity through river dredging. [ABSTRACT FROM AUTHOR]

Oujiang River Estuary is characteristic of complex geometry, strong tide and short length. To elucidate the role of turbulent closures in applications, numerical studies are carried out to investigate the effects of the k- ε and the q-ql with three typical wall proximity functions on hydrodynamics and salinity at Oujiang River Estuary. Therefore, four cases, based on the k- ε scheme, three q-ql schemes with the original wall functions suggested by Mellor and Yamada (noted as MY), a modified wall function at open channel (noted as OC) and a symmetric linear shape used by Burchard (noted as Bur) are compared with in-situ data in tidal phases, velocity, salinity and so on. MY produces relatively better salinity structures than OC and Bur compared to in-situ data. The k- ε scheme generates relatively higher water level and less bed stress compared to the q-ql schemes. The depth of the mixed layer may be over-predicted by more than 10%. The vertical eddy viscosity is almost twice as large in well-mixed or weakly stratified situations, therefore, a stronger mixing process can be produced. MY has anomalously high surface current at maximum ebb and low water level, and these findings are consistent with a previous conclusion. However, the anomalously high surface current disappears when strong stratification occurs in the upper water (<2 m in depth). Generally, MY simulates saltier water but contributes less length scale, which is due to the parabolic wall proximity function. OC and Bur calculate a roughly consistent vertical distribution of turbulent parameters, and they also simulate an identical distribution of stratification structure during both flood and ebb tide. MY based on the updated q-ql scheme shows relatively good results in a partially stratified situation. [ABSTRACT FROM AUTHOR]

Salinity in estuaries affects numerous ecological processes, and is strongly driven by river outflow interacting with the tide. We used an exponential decay function to model the relationship between salinity and freshwater flow (i.e., river discharge) to characterize and map salinity dynamics over the tidal flats of the Fraser River delta, British Columbia, Canada. The model has three parameters: the horizontal asymptote Asym that represents the average weekly salinity at very high river flows; the theoretical intercept R 0 that represents salinity conditions during very low river flows; and the natural log of the rate constant lrc that reflects the relationship between river discharge into the estuary and resultant average salinity. Using data from 47 salinity-monitoring stations deployed from 2015 to 2021, we found that Fraser River discharge had a strong and inverse relationship with average weekly salinity on the outlying tidal mudflats, which is expressed by the lrc value of −8.56. Model estimates indicate that weekly salinity (on the Practical Salinity Scale) at very low discharge rates (R 0) had a value of 22.4, which decreased at high discharge rates (Asym) to a value of 3.7. This predictable decrease in salinity for increasing river discharge means that the salinity regime of the Fraser River delta tidal flats is poikilohaline, and undergoes a regular and biologically relevant transition from brackish to nearly fresh water as the river outflow increases and abates seasonally. The resulting spatial gradients in salinity dynamics were associated with distance to river outflows and existing water diversion structures (i.e., jetties and causeways), meaning the timing and extent of this transition varied across the tidal flats. As estuaries become increasingly modified by coastal development and climate change, this simple model can be used to evaluate management interventions and river discharge scenarios that affect salinity gradients over tidal flats. • Salinity on tidal flats of the Fraser River delta transitions seasonally from marine to fresh water. • River training infrastructure has a footprint on the salinity regime of tidal flats. • A hydrological approach can be used to evaluate management of estuaries. [ABSTRACT FROM AUTHOR]

ABSTRACT Kim, N.-H.; Hwang, J.H., and Kim, D.H., 2018. Evaluation of Mixing and Stratification in an Estuary of Korea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 96–100. Coconut Creek (Florida), ISSN 0749-0208. In the estuary, flow structures depend on the relative contributions of the tidal current and the released freshwater, which determine the mixing and the stratification. To understand how artificially discharged freshwater affect the flow structure in an estuary of Korea, flow velocities, temperature, and salinity were measured in the field. During the field campaign, discharged freshwater cyclically forms the stratified layers during ebb tide and relatively well mixed water column during flood tide as in the conventional estuarine process. The gradient Richardson number evaluated such cyclic patterns quantitatively with the field measurement data, which were sampled following the tidal cycle and after the release of freshwater. In the early stage of the freshwater discharge, stratification dominates due to the large density difference, which overcomes the mixing by shear at the interface. As time goes on, the momentum of the released flow weakened and tidal modulation becomes significant and the discharged freshwater advected back and forth along the tidal currents. Different from the conventional mixing during flood, the stratification breaks down not by vertical mixing caused by the shear generated turbulence but by the vertical advection in the front of the gate. [ABSTRACT FROM AUTHOR]

ABSTRACT Valentim, M.M.; Monteiro, S.M., and Rollnic, M., 2018. The influence of seasonality on haline zones in an Amazonian estuary. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 76–80. Coconut Creek (Florida), ISSN 0749-0208. This study relates salinity fluctuations throughout Mojuim estuary, dominated by macrotides and situated in the humid tropical region between the Pará River mouth and the Atlantic ocean (−0.72° and −48.00°), Northeastern Pará, Brazil. Water salinity in situ were measured, on the surface and at the bottom, with a multiparameter probe (Horiba U-50) for six months, corresponding to the intermediate season (July and August, 2016), dry season (November and December, 2016), and rainy season (January and February, 2017) during spring tide. Sampling stations comprehended 50 km throughout the estuary and were divided in five 10-km sectors. The sectors were classified according to the Venice System. Significant variations salinity (p>0.5) in water column were not observed, as it is a well-mixed estuary. During rainy season, were observed the lowest mean salinity (8.5±0.8 PSU), which increased to 15.99±0.37 PSU during the intermediate season, and to 22.38±0.61 PSU during dry season. Saline intrusion reached sector 5 (50 km upstream of the mouth) during dry season and was restricted to sector 3 (30 km) during rainy season. The latter showed the highest variation in haline zones in all seasons, and they varied from oligohaline during rainy season, to mesohaline during intermediate season, and polyhaline during dry season. During rainy and intermediate seasons, river domain limits tidal progression, and it is why sector 5 shows no haline variations, remaining as a freshwater zone, as opposed to dry season, when this sector shows a oligohaline zone, due to the higher tide reach and to the higher dispersion of salts in the estuary. These haline limits might vary interannually, which indicates the influence of climatological anomalies on estuary dynamics over the last years. [ABSTRACT FROM AUTHOR]

Local geometry and bathymetry set bounds on how estuarine circulation and salinity respond to river and tidal forcing. Although often considered secondary, wind can drive variations in the salinity field, as well as inducing locally strong along and across-estuary salinity and water level gradients. Here, we use observations and numerical simulations to look at the effect of winds on estuarine dynamics in the Coos Estuary in the Pacific Northwest. The small, strongly tidally-forced estuary, does not conform to the traditional funnel-shaped estuary, instead it is shaped like an inverted U. The numerical simulations use idealized forcing to separate the contribution of tides, river discharge, and winds, on subtidal salinity and velocity fields. We find that wind can lead to reversals in the out-estuary surface flow despite the tidal dominance on subtidal circulation, in accordance with the limited available observations. Northward winds pile fresher waters in the north side of the estuary, and decrease exchange flow due to the winds opposing the main channel surface outflow, which may ultimately enhance the transport of particles along estuary. Southward winds pile fresher waters on the southern sides of the estuary, where most of the flats are found, and act to enhance the loss of salt. These transient winds drive non-transient changes to salt content in the estuary: high discharge cases show a general increase of salt, while low and moderate discharge show a reduced loss of salt in the estuary after the winds are turned off. The wind-driven spatial and temporal variability quantified here in the salinity and velocity distribution underscores the importance of local geometry constraints on estuarine dynamics, especially as many estuaries continue to evolve either due to natural environmental changes or to anthropogenic impacts. [Display omitted] • Observations and numerical simulations are used to look at the effect of winds on estuarine dynamics in the Coos Estuary. • Wind can lead to reversals in the out-estuary surface flow, which can have biological implications. • Northward winds pile fresher waters in the north side of the estuary and decrease exchange flow. • Southward winds pile fresher waters on the flat-covered southern side, and act to enhance salt loss. • Transient winds drive non-transient changes to salt content in the Coos Estuary. [ABSTRACT FROM AUTHOR]

We studied the spatial and temporal distribution pattern of three marine mysids (Chlamydopleon dissimile, Metamysidopsis elongata atlantica and Mysidopsis coelhoi) from the Cananeia-Iguape Coastal System, southeast Brazil, in relation to environmental variables and zooplankton abundance. We also analysed mysid reproductive traits, and tested their tolerance to salinity gradients in laboratory experiments. Salinity was identified as a major factor influencing mysid spatial and temporal distribution, selectively limiting their distribution within the estuary. Chlamydopleon dissimile and M. coelhoi were common, but occurred at low abundances throughout the year, whereas the numerically dominant M. elongata atlantica had abundance peaks in spring, when thermohaline conditions were more favourable. Mysid brood size varied interspecifically; the occurrence of juveniles throughout the year indicated that mysids breed continuously in the region. The high biomass of M. elongata atlantica observed in spring confirms the importance of mysids in coastal regions' food webs. [ABSTRACT FROM AUTHOR]

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MysidsTenagomysis chiltoniandT. novaezealandiaeare abundant in southern New Zealand estuaries. Size-specific oxygen consumption rates of adult females ofT. chiltoniandT. novaezealandiaewere measured under salinities 0–1, 5, 10, 15, 20, 25, 30 and 33–34 at 5 °C and 20 °C in relation to body size. We predicted that, irrespective of salinity variations, the larger individuals would show lower size-specific oxygen consumption rates than smaller ones. Body size and the changes in salinity were not significantly related to the size-specific oxygen consumption rates ofT. chiltoniandT. novaezealandiae, probably because of the small size range of individuals measured. At 20 °C, size-specific oxygen consumption rates were significantly higher than those at 5 °C forT. novaezealandiae, but not forT. chiltoni. Within the tolerable range of salinity variation, small differences in body size are possibly not important for energetics of adultTenagomysisspp. [ABSTRACT FROM PUBLISHER]

Long-term (20+ years) water quality datasets for estuaries are rare, especially for smaller systems. Monitoring of salinity, temperature, pH, dissolved oxygen and turbidity has been undertaken since 1991 in the intensively utilised, modified, and managed temporarily open/closed Touw Estuary and its associated three interconnected estuarine lakes, Eilandvlei, Langvlei and Rondevlei, of the Wilderness Lakes System, South Africa, a national park and Ramsar site. Spatial variability of salinity, pH and turbidity was pronounced in the Touw Estuary but largely absent in the lakes. Significant differences in median salinity, pH, dissolved oxygen and turbidity occurred between lakes, with reverse salinity and pH gradients frequently occurring. Seasonal variability in temperature and dissolved oxygen occurred in all waterbodies. Significant long-term declines in salinity have occurred in the more inland lakes, with decreases in turbidity and pH also occurring in some waterbodies. Water chemistry of the Wilderness Lakes is changing from that of an estuarine to a lacustrine system. Both biological and physical features were driving water quality changes, including reductions in river inflow, reduced marine connectivity, constriction of flow between waterbodies, and declines in submerged plant biomass. Management actions are proposed relating specifically to addressing the apparent causes for water quality changes. [ABSTRACT FROM AUTHOR]

Abstract: The quantity of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC) in three Finnish estuaries (Karjaanjoki, Kyrönjoki and Kiiminkijoki) was investigated, with respect to predicting DOC concentrations and loadings from spectral CDOM absorption measurements. Altogether 87 samples were collected from three estuarine transects which were studied in three seasons, covering a salinity range between 0 and 6.8, and DOC concentrations from 1572 μmoll−1 in freshwater to 222μmoll−1 in coastal waters. CDOM absorption coefficient, a CDOM(375) values followed the trend in DOC concentrations across the salinity gradient and ranged from 1.67 to 33.4 m−1. The link between DOC and CDOM was studied using a range of wavelengths and algorithms. Wavelengths between 250 and 270 nm gave the best predictions with single linear regression. Total dissolved iron was found to influence the prediction in wavelengths above 520nm. Despite significant seasonal and spatial differences in DOC–CDOM models, a universal relationship was tested with an independent data set and found to be robust. DOC and CDOM yields (loading/catchment area) from the catchments ranged from 1.98 to 5.44gCm−2 yr−1, and 1.67 to 11.5 a CDOM(375) yr−1, respectively. [Copyright &y& Elsevier]

Abstract: Large-scale river diversions on the lower Mississippi River are considered to be an important component of wetland restoration efforts in coastal Louisiana. Diversions are used primarily for salinity control but increasingly proposed also as a major way to deliver sediments and nutrients to coastal wetlands impacted by the construction of flood control levees. We used a coupled hydrology–hydrodynamics model of the Barataria estuary, a site of the Davis Pond Diversion – the world''s largest river diversion project, to examine salinity variations under different diversion discharge scenarios. Discharge scenarios were selected based on actual freshwater discharges in different years and management alternatives that included a scenario with several new diversions. The model results indicate that river diversions strongly affect salinities only in the middle section of the Barataria estuary. The upper parts of the estuary are fresh most of the time and so the excess fresh water from river diversions has only a minor impact on salinity in this region. Also, the Davis Pond diversion has little impact on salinities in the coastal section of the estuary because of strong marine influence in this area adjacent to the Gulf of Mexico. Interestingly, the predicted salinity differences between different model scenarios can be as high as 10 in some months and places. These differences can be biologically significant depending on the salinity tolerance of different species and could cause a shift in community composition within the affected region. [Copyright &y& Elsevier]

The problems of studying river mouth areas and their parts, namely, estuaries and deltas are discussed. The step is taken to combine the concept of river mouth area and mouth processes developed in our country with the concept for estuary and estuarine processes widely spread among foreign scientists. In this connection, the article offers new definitions of a river mouth area and estuary as well as new schemes of regionalization of river mouth areas and typification of estuaries. The types of river mouth areas and estuaries are illustrated by case studies. [ABSTRACT FROM AUTHOR]

Abstract: Climate change has had significant impacts on the hydrologic cycle of the planet. Of particular concern are estuarine environments, such as San Francisco Bay (USA) which is fed by diminishing snow pack runoff leading to gradual increases in salinity. Salinity enhances the acute toxicity of several agricultural chemicals in anadromous fish through augmented biochemical activation catalyzed by enzymes that are induced during hypersaline acclimation. This review discusses the mechanisms of the enhanced toxicity, the enzymes involved and the regulation of the enzymes by hypersaline conditions. Given the rapid changes taking place in the world''s waterways, environmental modification of toxicological pathways should be a significant focus of the research community as the toxicity of multiple xenobiotics may be enhanced. [Copyright &y& Elsevier]

Abstract: Suspended particulate matter (SPM) collected at regular stations from the Mandovi and Zuari estuaries indicates that the peaks of high SPM coincide with peaks of high rainfall and low salinity and also with peaks of moderate/low rainfall coupled with high salinity during the monsoon. The estuarine turbidity maximum (ETM) is a characteristic feature, it occurs in the channel accompanying spring tide during the monsoon and pre-monsoon, and shifts to the bay on neap tide during post-monsoon. ETM remains at the same position in the Mandovi River, both during the monsoon and pre-monsoon, whereas in Zuari it stretched upstream during monsoon and migrates seaward of the channel during pre-monsoon. The ETM coincides with the freshwater–seawater interface during the monsoon and is formed by the interaction between tidal currents and river flows. The ETM during pre-monsoon is associated with high salinities and is generated by tidal and wind-induced currents. The turbidity maximum on neap tide during post-monsoon may be due to the erosion and resuspension of sediments from the emergent tidal flats and transport of these turbid waters into the bay. Funneling effect of the narrowing bay in the Zuari estuary and associated physical processes effectively enhance the magnitude of the currents and transports sediments to the channel. SPM retention percentage indicates that the estuarine channel is prone to siltation. [ABSTRACT FROM AUTHOR]

Abstract: Over this one-year study, the variations of inorganic As species were examined monthly along the salinity gradient of the Penzé estuary (NW France) in relation with different biogeochemical parameters. In most cases, dissolved As exhibited a non-conservative behaviour which resulted from the competition between two major processes. In the upstream section of the estuary, a strong input of both total inorganic As and As(III) occurred. Then, the removal of the same species, under precipitation of iron oxides/oxyhydroxides, was observed in the low-salinity range (S < 10). Using our experimental data, the fluxes of the various As species were estimated for the first time in estuarine waters. Inputs from the river were mainly constituted of particulate As (∼70%). Conversely, dissolved species were predominant in the net fluxes (∼65%) and As(III) accounted for ∼15% of the dissolved net flux. [ABSTRACT FROM AUTHOR]

The hydrodynamics of an idealized estuarine plume along a straight coast are studied using a three-dimensional hydrodynamical module of the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas. The sensitivity of the solution with respect to river discharge, bottom roughness length, and the presence or absence of tides are studied. Assuming constant water depth and no wind forcing, modeled results for the surface layer are obtained, including: (1) variation in the horizontal shape of the plume; (2) horizontal distribution of the current vector; and (3) horizontal distribution of the salinity field within the plume over one tidal cycle. Effects of three different river discharges, four selected different bottom roughness lengths, and tides on its development are examined. Results suggest: (1) the plume length naturally increases through time under tidal forcing or as well as without tidal forcing, but its width shows approximately periodic variation; (2) both the plume width and length increase with increasing river discharge; (3) both the plume width and length decrease with increasing four selected bottom roughness lengths; and (4) both the bulge and coastal current of the plume are more evident without the presence of tidal effects. The results show their potential implications for contaminant fate and transport and novel methods of pollution prevention and control in estuarine and coastal waters. [ABSTRACT FROM AUTHOR]

Copepods (Acartia tonsa) were exposed (48 h) to waterborne, diet-borne (non-Cu-equilibrated and Cu-equilibrated food), and waterborne plus diet-borne Cu in either the absence or the presence of food (diatom Thalassiosira weissflogii). Toxicity tests were run in different salinities (5, 15, and 30 ppt) together with measurements of physicochemical parameters and total and dissolved Cu concentrations in the experimental media. Results show that most of the toxic Cu fraction was in the dissolved phase. In general, Cu toxicity was higher in low (5 ppt) than in high salinity (30 ppt), regardless of the pathway of Cu exposure tested. In the absence of food, data clearly indicate that differences in waterborne Cu toxicity can be explained by changes in water chemistry. However, addition of food (either non-Cu-equilibrated or Cu-equilibrated) to the experimental media protected against acute Cu toxicity in salinities 5 and 15 ppt, suggesting that A. tonsa requires extra energy to cope with the stressful condition imposed by Cu exposure associated with the ionoregulatory requirements in low salinities. For diet-borne exposure, a very high Cu concentration was necessary to precontaminate the diatoms to a level resulting in copepod mortality. Therefore, availability of food exerted a more important positive impact in protecting against acute Cu toxicity than its potential negative impact via contamination resulting in toxicity. Findings indicate the need for incorporation of both salinity and food in a future biotic ligand model (BLM) version for Cu in estuarine and marine waters. In this context, the euryhaline copepod A. tonsa would be a suitable model species with which to perform experiments to validate and calibrate any future saltwater BLM. Environ. Toxicol. Chem. 2010; 29:1834–1840. © 2010 SETAC [ABSTRACT FROM AUTHOR]

Abstract: Non-vegetated creek edges were investigated to explore spatial nekton use patterns in a low salinity intertidal salt marsh creek network of the Yangtze River estuary along a stream-order gradient with four creek orders. Non-vegetated creek edges were arbitrarily defined as the approximately 3 m extending from the creek bank (the marsh–creek interface) into open water. Nekton was sampled using seine nets during daytime high slack water during spring tides for two or three days each in May through July 2008. Twenty-three nekton species (16 fishes and 7 crustaceans) were caught during the study. Fishes were dominated by gobies (Mugilogobius abei, Periophthalmus magnuspinnatus, Periophthalmus modestus, Synechogobius ommaturus), mullets (Chelon haematocheilus, Liza affinis) and Chinese sea bass (Lateolabrax maculatus). Crustaceans were dominated by mud crab (Helice tientsinensis) and white prawn (Exopalaemon carinicauda). Rank abundance curves revealed higher evenness of nekton assemblages in lower-order creeks compared to higher-order creeks. Fish abundance tended to increase with increasing creek order. Crustacean abundance was higher in the first–third order creeks than in the fourth-order creek. Dominant nekton species displayed various trends in abundance and length–frequency distributions along the stream-order gradient. The spatial separation of nekton assemblages between the first–third order creeks and the fourth-order creek could be attributed to geomorphological factors (distance to mouth and cross-sectional area). These findings indicate that both lower- and higher-order creek edges play important yet different roles for nekton species and life history stages in salt marshes. [Copyright &y& Elsevier]

Abstract: The spatial and temporal patterns in the hyperbenthic community structure (>500 μm) in the warm temperate, permanently open Kariega Estuary situated along the south-eastern coastline of South Africa was investigated monthly over a period of twelve months. Data were collected using a modified hyperbenthic sledge at six stations along the length of the estuary. Physico-chemical data indicate the presence of a constant reverse salinity gradient, with highest salinities measured in the upper reaches and lowest at the mouth of the estuary. Strong seasonal patterns in temperature, dissolved oxygen and total chlorophyll-a (chl-a) concentration were evident. Total average hyperbenthic densities ranged between 0.4 and 166 ind.m−3 in the lower net and between 0.2 and 225 ind.m−3 in the upper net. Hyperbenthic biomass values ranged between 0.02 and 11.9 mg.dry weight.m−3 in the lower net and between 0.02 and 17.4 mg.dry weight.m−3 in the upper net. Both the lower and upper nets were numerically dominated by decapods (mainly brachyuran crab zoea) with the exception of June and July 2008 when mysids (mainly Mesopodopsis wooldridgei) dominated, comprising up to 72.4 ± 58.14% of the total abundance in the lower net. A redundancy analysis (RDA) indicated that 99.2% of the variance in the hyperbenthic community structure could be explained by the first two canonical axes. Axis one, which accounted for 96.8% of the total variation detected in the ordination plot was highly correlated with sedimentary organic content and to a lesser extent the chl-a concentration within the Kariega Estuary. The correlations with the second canonical axis (2.4%) were less obvious, however, salinity and seston concentration were weakly correlated with this axis. [Copyright &y& Elsevier]

Abstract: We examined and compared tidal currents and water column structure between a near-shore station (12 km from the coast) and an offshore station (32 km from the coast) adjacent to the Caeté River, Pará Region, Brazil. Although the coastal system of Pará is largely influenced by local tides and wind, we found substantial differences in the dominant forcing agents between stations. Water column dynamics at the near-shore station were largely affected by local tidal processes, while differences between surface and bottom layer flows also indicated the importance of gravitational circulation at this station and a substantial influence of the adjacent Caeté River discharge. In comparison, at the offshore station, water column structure was largely influenced by a semi-diurnal tidal flow, an along-coastal current flow (mainly associated with the North Brazil Current) and the dynamics of local wind flow. The near-shore station at low tide showed a high level of stratification; at high tide such stratification was reduced. In comparison, stratification was only apparent within the upper 6 m at the offshore station, the rest of the water column was relatively well-mixed. The stratification within Station 1 at low tide was a result of the bi-directional movement of water discharged from the Caeté River, with lower salinity surface water and high salinity bottom water resulting in an estuarine-like circulation environment. The spatial variability and lack of correlation in current flow and water column structure between the near-shore and offshore stations suggest that a flow field resulting from differences in local circulation, tidal variability and wind persistence separate areas. We argue that this separation may indicate that the offshore station is located in a transition region between the Caeté River waters and the local coastal area. [Copyright &y& Elsevier]

Abstract: The majority of the definitions of estuaries have been based on the characteristics of estuaries in temperate regions of the northern hemisphere. As previously pointed out (), such definitions do not take into account such features as periodic closure of their mouths and hypersaline conditions during dry periods, which characterise many estuaries in southern Africa and south-western Australia. There is also ambiguity as to whether an estuary sensu stricto must be fed by a river. The following definition was developed to encompass the main characteristics of all estuaries: An estuary is a partially enclosed coastal body of water that is either permanently or periodically open to the sea and which receives at least periodic discharge from a river(s), and thus, while its salinity is typically less than that of natural sea water and varies temporally and along its length, it can become hypersaline in regions when evaporative water loss is high and freshwater and tidal inputs are negligible. Estuaries are thus regarded as unique ecosystems, which, in the case of fishes, for example, are occupied by species that collectively represent a particular suite of guilds. [Copyright &y& Elsevier]

Abstract: The distribution of methane in the western Pearl River Estuary (PRE) and northern South China Sea (SCS) was investigated during an oceanic cruise in September, 2006. Results show that the methane concentration in surface water ranged from 6.9 to 173.7nmolL−1 at the 27 stations sampled in the western PRE. The methane saturation ratio ranged from 329 to 7896%, and the average sea-to-air methane flux was about 63.5±32.2μmolm−2 d−1. The methane concentration in surface water was high in the western margin of the upper estuary, near marshes and shoals. The methane concentration decreased rapidly from the estuarine margin to the open coastal ocean, resulting from salinity (sulfate) increase in the overlying water, increasing water depth, diffusion across the air–sea interface and dilution of seawater during estuarine mixing. The major methane source in the PRE is thought to be methanogenic degradation in organic-enriched sediments. In the northern SCS, the concentration of methane varied from 2.4 to 5.9nmolL−1 in surface water at the 55 testing stations. The methane saturation ratio was between 134 and 297 %, and the sea-to-air methane flux was estimated to be 15.6±8.0μmolm−2 d−1. Two stations located at E7 (15° 59.405″; 17° 59.165″) and E6 (115° 0.065″; 17° 0.032″) showed abnormal methane concentrations of 9.5 and 13.8nmolL−1 in surface water, respectively, and were probably affected by pollution from allochthonous oil. A subsurface methane maximum (4–5nmolL−1) occurred in water that was 150–200m deep within this area. This maximum may be due to accumulation of plankton productive particulates and limited sea–air exchange in the sea surface layer. Abnormally high methane values of 4.3 and 8.7nmolL−1 were measured in the bottom waters of stations B8 and E4, respectively. These can be interpreted as being due to methane seeps from seafloor sediments. [Copyright &y& Elsevier]

Abstract: Spatial and temporal variations in particle-bound pesticide contamination, natural environmental variables, and benthic abundance were measured during the dry summer season within a temporarily open estuary (Lourens River). This study focused on the effect of particle-associated pesticides on the dynamics of the benthic community (including epi-benthic, hyper-benthic, and demersal organisms) by comparing two runoff events, differing in their change in pesticide concentration and environmental variables. The two chosen sites were situated within the upper and middle reaches of the estuary and differ significantly in salinity (p =0.001), flow (p =0.5), temperature (p <0.001) and particulate organic carbon in the sediment (p <0.001). Generally higher particle-bound pesticides were found in the upper reaches. The first runoff event was characterised by an increase in pesticides (chlorpyrifos, endosulfan and cypermethrin) and hardly any change in natural environmental variables, whereas the second runoff event was characterised by no increase in pesticide but a significant change in natural environmental variables like salinity, temperature and flow. The most evident spatial difference in community structure was shown by the use of Principal Response Curve after the first runoff event, whereas no response was shown after the second runoff event. The variables which explained most of the spatial differences are Total Organic Carbon, salinity, chlorpyrifos and endosulfan concentrations. The species contributing most to the spatial differences are the estuarine harpacticoid Mesochra and Canthocamptus (lower abundance at the upper reaches) and the freshwater species Dunhevedia and Thermocyclops (higher abundance within upper reaches). Within the spatial variability (between upper and middle reaches) the authors were able to detect a link between endosulfan, chlorpyrifos exposure, TOC and salinity and community change by comparing the two runoff events. [Copyright &y& Elsevier]

Concentrations of volatile hydrogen sulfide (H2S), carbonyl sulfide (OCS), methane thiol (MeSH), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS) in the Seine estuary (France) were investigated in spring 2005 using samples collected from the first meter beneath the surface. Levels of dissolved metals (Ag, Cd, Cu, Zn, Ni, Co, Pb), suspended particulate matter, and particular organic carbon were also assessed. Maximum concentrations were 0.80 nM for H2S, 0.64 nM for OCS, 3.06 nM for MeSH, 11.06 nM for DMS, and 1.18 nM for DMDS, and different features were observed for the five volatile reduced sulfur compounds (VRSCs). Experiments were conducted to determine silver- VRSC conditional stability constants. Major interactions were observed between H2S and Ag, which may partially account for the absence of volatile H2S at the lowest salinities. OCS, MeSH, and DMS did not interact with Ag, as revealed by their insignificant K′ values. Variations in concentrations along the salinity gradient may be explained by the influence of phytoplanktonic compounds. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional, time-dependent hydrodynamic model, SELFE, was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundary and freshwater flows from the main stem and tributaries in the Danshuei River system. The model was calibrated and verified with measured water surface elevation, tidal current, residual current, and salinity in 2000 and 2001, respectively. The overall model simulation results are in qualitative agreement with the available field data. The validated model was then used to probe the salinity response to the pulse of high freshwater discharge and residence time in the Danshuei River estuary. The modeling results show that salinity recovers to normal conditions in about 14 days at the Kuan-Du Bridge, 22 days at the Taipei Bridge, and 30 days at both Hwa-Jian Bridge and Pa-Ling Bridge. Regression analysis of model results reveals that an exponential regression equation can be used to correlate the estuarine residence time to change of freshwater input. A short residence time exhibits in the Danshuei River estuary. The simulated results provide useful information for understanding the transport processes and can be employed to estimate dissimilative capacity in the estuary. [Copyright &y& Elsevier]

Abstract: The field observation of this study was carried out in the Changjiang Estuary from May 19 to 26, 2003, just a few days before the Three Gorges Dam began to store water. A total of 29 stations, including 2 anchor stations, were distributed through almost the whole salinity gradient. Based on the data gained from these stations, the biogeochemical characteristics of dissolved oxygen (DO) were examined. Spatial distribution of DO concentrations showed the pattern that it increased in a downriver direction. DO concentration generally varied within a narrow range of 7.33–8.10 mg l−1 in the freshwater region and the west part of the mixed water region, and after that it increased rapidly. In vertical direction, the differences in DO concentrations between surface and 2 m above the bottom were big at the stations with water depths exceeding 20 m; DO concentration up to 14.88 mg l−1 was recorded at the sea surface, while at 2 m above the bottom its concentration was only about 4 mg l−1. The fluctuation in DO concentrations was small during a period of 48 h in the mixed water region and 2 m above the bottom of the seawater region; while it was large during the same period in the seawater region for surface and 5 m below the surface layer, and a maximum variation from 8.77 to 12.66 mg l−1 in 4 h was recorded. Oxygen fluxes also showed a marked spatio-temporal variation. As a whole, the freshwater region and mixed water region were an oxygen sink, while the seawater region was a source. Relationships between dissolved oxygen and some biogeochemical parameters which could markedly influence its spatio-temporal distribution were discussed in this paper. [Copyright &y& Elsevier]

The diversity of aerobic anoxygenic phototrophic (AAP) bacteria has been examined in marine habitats, but the types of AAP bacteria in estuarine waters and distribution of ecotypes in any environment are not well known. The goal of this study was to determine the diversity of AAP bacteria in the Delaware estuary and to examine the distribution of select ecotypes using quantitative PCR (qPCR) assays for the pufM gene, which encodes a protein in the light reaction center of AAP bacteria. In PCR libraries from the Delaware River, pufM genes similar to those from Beta-(Rhodoferax-like) or Gammaproteobacteria comprised at least 50% of the clones, but the expressed pufM genes from the river were not dominated by these two groups in August 2002 (less than 31% of clones). In four transects, qPCR data indicated that the gammaproteobacterial type of pufM was abundant only near the mouth of the bay whereas Rhodoferax-like AAP bacteria were restricted to waters with a salinity of <5. In contrast, a Rhodobacter-like pufM gene was ubiquitous, but its distribution along the salinity gradient varied with the season. High fractions (12 to 24%) of all three pufM types were associated with particles. The data suggest that different groups of AAP bacteria are controlled by different environmental factors, which may explain current difficulties in predicting the distribution of total AAP bacteria in aquatic environments. [ABSTRACT FROM AUTHOR]

Relationships between environmental variables and diversity (Shannon-Weaver index) of the fish communities in the Tagus estuary and adjacent coastal areas were analyzed. The focus was on the linearity or nonlinearity of these abiotic/biotic characteristics, with the aim to obtain an accurate short–medium term time-scale diversity prediction from habitat variables alone. Multiple Linear Regressions (MLR) were used for the linear approach and Artificial Neural Networks (ANNs) for the nonlinear approach. MLR results in the external validation phase indicated a lack of model accuracy ( R2 = 0.0710; %SEP = 47.5868; E = −0.0217; ARV = 1.0217; N = 43). Results of the best of the Artificial Neural Networks used in this study (12-15-15-1 architecture) in the external validation phase (ANN: R2 = 0.9736; %SEP = 7.8499; E = 0.9722; ARV = 0.0278; N = 43) were more accurate than those obtained with MLR. This indicates a clear nonlinear relationship between variables. In the best ANN model, nitrate concentration, depth, dissolved oxygen and temperature were the most important predictors of fish diversity in the Tagus estuary. The sensibility analysis indicated that the remaining variables (silicate, nitrite, transparency, salinity, slope, phosphate, water particulate organic matter, and chlorophyll a) played lesser roles in the model. [ABSTRACT FROM AUTHOR]

Abstract: The effect of low salinity on the survival of recently hatched veliger of the gastropod Nassarius reticulatus (L.) was studied under laboratory conditions at 17 °C±1 °C. Significant mortality occurred for salinities ≤17 psu and the LS50 (salinity that causes 50% mortality) varied from 14.5 psu to 17.2 psu for exposures between 1 h and 48 h, respectively. The results were included in a mathematical model in order to estimate the mortality of N. reticulatus veliger in Ria de Aveiro under different salinity scenarios determined by numerical simulation. The model predicts a massive mortality of veliger immediately after their eclosion (1 h) in Ria de Aveiro during spring tide when the river input reaches its expected maximum — a situation that has been observed occasionally and may overlap a massive eclosion of veliger in the water. Except in this extreme situation, the mortality in Ria de Aveiro is generally restricted to the upstream limits of the channels. The results indicate that N. reticulatus recruitment in estuaries may strongly depend on the prevailing salinity regime. [Copyright &y& Elsevier]

Pharmaceuticals and selected major human metabolites are ubiquitous in Jamaica Bay, a wastewater-impacted estuary at concentrations in the low ng/L to low μg/L range. Concentrations throughout the bay are often consistent with conservative behavior during dry-weather conditions, as evidenced by nearly linear concentration-salinity relationships. Deviation from conservative behavior is noted for some pharmaceuticals and attributed to microbial degradation. Caffeine, cotinine, nicotine, and paraxanthine were detected with the greatest analytical signal, although evidence is presented for in situ removal, especially for nicotine and caffeine. There is little evidence for significant removal of carbamazepine and sulfamethoxazole, suggesting they are more conservative and useful wastewater tracers. Immediately following heavy precipitation, which induced a combined sewer overflow (CSO) event, the concentrations of all compounds but acetaminophen and nicotine decreased or disappeared. This observation is consistent with a simple model illustrating the effect of precipitation has on pharmaceutical concentration in the wastewater stream, given the balance between dilution from rain and the bypass of treatment. [ABSTRACT FROM AUTHOR]

Organisms tend to inhabit predictable portions of estuaries along salinity gradients between the ocean inlets (salinity> 35 psu) and the freshwater tributaries (salinity = 0). Previous studies have suggested that the continuous change in biological community structure along this gradient is relatively rapid at certain salinities. This is the basis for estuarine salinity zonation schemes similar to the classic Venice System (i.e., 0-0.5, 0.5-5, 5-18, 18-30, 30-40, > 40). An extensive database (n > 16,000 samples) of frequency of occurrence of nekton was used to assess evidence for estuarine salinity zones in two southwest Florida estuaries: Tampa Bay and Charlotte Harbor. Rapid change in nekton community structure occurred at each end of the estuarine salinity gradient, with comparatively slow (but steady) change in between. There was little strong evidence for estuarine salinity zones at anything other than low salinities (0.1-1). As previously suggested by other authors, estuaries may be regarded as ecoclines, because they form areas of relatively slow but progressive ecological change. The ends of the estuarine salinity gradient appear to be ecotones (areas of rapid change) at the interfaces with adjacent freshwater and marine habitats. This study highlights the rapid change that occurs in nekton community structure at low salinities, which is of relevance to those managing freshwater inflow to estuaries. [ABSTRACT FROM AUTHOR]

The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption of normal flow. flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m-2 yr-1. [ABSTRACT FROM AUTHOR]

Abstract: Although otolith Strontium (Sr)/calcium (Ca) ratios have been widely used to reconstruct the past salinity environmental history of anguillid eels, factors affecting the Sr/Ca ratios in otoliths are incompletely understood. Japanese Eel (Anguilla japonica) elvers (mean length 54.7±2.1 mm) were collected in the estuary during their upstream migration and reared at 5 different salinities (0, 5, 15, 25, and 35 psu) and 3 types of feeding conditions (formulated feed, tubifex, and starvation) for 30 days to evaluate the effects of salinity and diets on otolith Sr/Ca ratios. Ca and Sr concentrations in the ambient water significantly increased with salinity (SAL) as [Ca] water =15.50SAL−5.56, and [Sr] water =0.21SAL+0.03, respectively. Sr/Ca ratios in otoliths increased with salinity (SAL) of the rearing water as [(Sr/Ca)×1000] otolith =0.091SAL+3.790. In diets, Sr/Ca ratios were 4 times higher in tubifex than in formulated feed. However, in otoliths, ANOVA indicated that Sr/Ca ratios did not differ significantly between groups fed on tubifex or formulated feed (p =0.118). Otolith Sr/Ca ratios were negatively correlated with fish growth rates while the growth rates differed significantly among rearing conditions with different salinities and diets. Partition coefficients of the Sr/Ca ratios from ambient water to fish tissues and otoliths significantly increased with salinity. The Sr/Ca ratios of Japanese Eel otoliths thus were positively correlated with the ambient salinity and decreased with increasing fish growth rate, but was not affected by fish diet. [Copyright &y& Elsevier]