Turbidity Maximum Research Articles

Retrieval and Validation of Water Turbidity at Metre-Scale Using Pléiades Satellite Data: A Case Study in the Gironde Estuary

This study investigated the use of frequent metre-scale resolution Pléiades satellite imagery to monitor water quality parameters in the highly turbid Gironde Estuary (GE, SW France). Pléiades satellite data were processed and analyzed in two representative test sites of the GE: 1) the maximum turbidity zone and 2) the mouth of the estuary. The main objectives of this study were to: (i) validate the Dark Spectrum Fitting (DSF) atmospheric correction developed by Vanhellemont and Ruddick (2018) applied to Pléiades satellite data recorded over the GE; (ii) highlight the benefits of frequent metre-scale Pléiades observations in highly turbid estuaries by comparing them to previously validated satellite observations made at medium (250/300 m for MODIS, MERIS, OLCI data) and high (20/30 m for SPOT, OLI and MSI data) spatial resolutions. The results show that the DSF allows for an accurate retrieval of water turbidity by inversion of the water reflectance in the near-infrared (NIR) and red wavebands. The difference between Pléiades-derived turbidity and field measurements was proven to be in the order of 10%. To evaluate the spatial variability of water turbidity at metre scale, Pléiades data at 2 m resolution were resampled to 20 m and 250 m to simulate typical coarser resolution sensors. On average, the derived spatial variability in the GE is lower than or equal to 10% and 26%, respectively, in 20-m and 250-m aggregated pixels. Pléiades products not only show, in great detail, the turbidity features in the estuary and river plume, they also allow to map the turbidity inside ports and capture the complex spatial variations of turbidity along the shores of the estuary. Furthermore, the daily acquisition capabilities may provide additional advantages over other satellite constellations when monitoring highly dynamic estuarine systems.

Habitat use by early life stages of the re-established striped bass and conspecific fish species along the St. Lawrence estuary

The St. Lawrence estuary (SLE) supports important nursery habitats for many fish species, including the newly re-established population of striped bass (Morone saxatilis).The present study aims to describe pelagic (5 m isobaths) and littoral (<1.5 m-deep) nearshore habitats of the SLE and to document their use by fish larvae and age-0 juveniles, especially for striped bass. From June to September 2014, 162 pelagic and 188 littoral stations were characterized in terms of biophysical conditions (turbidity, salinity, temperature, dissolved oxygen and chlorophyll-a concentration), abundance of main preys and fish assemblages. Striped bass larvae identification was confirmed with genetic analyses. In the pelagic and littoral zones, four estuarine habitats were defined based on turbidity and salinity: an upstream freshwater section (UP), an oligohaline (O-ETM) and a mesohaline (M-ETM) estuarine turbidity maximum zone, and a downstream polyhaline section (DOWN). Our results emphasized the spatial partitioning of fish early life stages among contrasting habitats, potentially to minimize interspecific competition. In the SLE, striped bass is using the wide spectrum of habitats available. At the onset of striped bass development, the UP and the O-ETM were the main nursery habitats, which is consistent with the known location of spawning grounds. Results suggest that the O-ETM provided the most suitable conditions for the species with optimal physical properties and abundant food resources. Later in the season, the M-ETM provided a favourable trade-off habitat offering a less optimal physical environment although potential for new prey.

Phytoplankton, dissolved oxygen and nutrient patterns along a eutrophic river-estuary continuum: Observation and modeling

Transport and fate of phytoplankton blooms and excessive nutrients along salinity and turbidity gradients of a river-estuary continuum could determine when and where impaired water quality occurs. However, the general spatiotemporal patterns, underlying mechanisms and their implication for water quality management are not well understood. This study reveals typical seasonal variations and longitudinal patterns of phytoplankton, dissolved oxygen (DO) and nutrients (C, N, and P) in the lower St. Johns River estuary in Florida based on 23 years of data and a model which spans 3 years. Evident declines in freshwater phytoplankton and DO concentrations were observed in the freshwater-saltwater transition zone and the estuarine turbidity maxima along the river-estuary continuum. Observations show that most cyanobacteria blooms originating from upstream lakes collapsed in the freshwater-saltwater transition zone where salinity was greater than 1 ppt, but data analysis and model simulation both indicate salinity stress was not the sole reason, other factors such as changes in hydrodynamics and river morphology also contributed to the bloom crashes. Inorganic nutrients (ammonium, nitrate, and phosphate) exhibited inverse longitudinal patterns with phytoplankton. Due to algal uptake, summer concentrations of inorganic nutrients were low in the freshwater, but substantially elevated in the marine reach as a result of large point source inputs and nutrient regeneration from organic detritus. However, because of strong river-ocean mixing, the dramatic increase in nutrients did not promote a phytoplankton bloom in the local marine reach. The nutrients were eventually transported into coastal waters or oceans where they fueled phytoplankton blooms. Our findings highlight that strategies for nutrient reduction and phytoplankton bloom management should be developed beyond local reaches and across a river-estuary-ocean continuum, exploring the possibility that freshwater phytoplankton blooms and excessive nutrients may be transported to downstream estuaries, coastal waters and even oceans that are vulnerable to poor water quality.

Dynamic Response of the Fluid Mud to a Tropical Storm

AbstractFluid mud (FM) is a unique sedimentary feature in high‐turbidity estuaries, where it can make a rapid contribution to morphodynamics. Insufficient field measurements and fixed‐point monitoring lead to deficient understandings of the formation, transport, and breakdown of the FM under extreme weather conditions. A field survey was conducted in the Changjiang Estuary during the period of turbidity maximum, just after Typhoon Haikui. The measurements captured the formation of the FM beneath the suspended layers, particularly around the lower reach of the North Passage. The thickness of the observed FM gradually decreased landward along the channel, with the maximum value reaching ~0.9 m. The major features of the observed storm‐induced FM were simulated using the Finite‐Volume Community Ocean Model. The results indicated that the initial appearance of the FM was the result of a typhoon‐intensified, salinity‐induced stratification in the outlet region. The subsequent landward propagation of the FM was driven by the combined effects of the FM‐induced mud surface pressure gradient force and saltwater intrusion near the bottom. Weak mixing during the subsequent neap tidal period sustained the FM as it rapidly extended into the middle region of the North Passage. This produced a large velocity shear at the interface of the FM and upper suspension layer, increasing the entrainment from the FM to the upper suspension layer. As a result of the increased tidal mixing, the FM weakened and then finally broke down in the subsequent spring tidal period.

Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968–1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1–10 km downwards from the upstream estuary entry, during the dry season, to a location 40–70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.

Association of Branched Dextrin from Nägeli Amylodextrin in Water for Screening of Additives Affecting Starch Gel Properties

AbstractA novel model system for characterization of double helix in starch is developed using branched dextrin (BD) purified from Nägeli amylodextrin by repeated precipitation in 10% ethanol. The solution of one‐time precipitated BD (BD1, 3% in water) starts to develop turbidity in 8 h of incubation at 4 °C. Volume particle size distribution during the association of four‐time precipitated BD sample (BD4) is analyzed using dynamic light scattering. The mean diameter of the particles jumps from 3.8 to 170 nm between 2 and 2.5 h of incubation, suggesting a prerequisite lag time for maturation of intermolecular association of BD4. Flakes of BD4 in water exhibit a B‐type crystal by X‐ray diffraction analysis. A screening system of additives affecting turbidity development of BD1 in water is applied for 82 compounds, and sodium tetraborate (borax) significantly retards the development. Borax affects the rate of turbidity development and maximum turbidity at the plateau stage in a concentration‐dependent manner. Borax is also added to rice starch gel, which reduces the increase of hardness of the gel during retrogradation. This novel assay system with BD can be used for screening and evaluation of additives affecting starch gelation and retrogradation.

Observation of the sediment trapping during flood season in the deep-water navigational channel of the Changjiang Estuary, China

After the construction of the Deep-water Navigational Channel (DNC) in the North Passage of the Changjiang Estuary, siltation in the channel became severe during the flood season in the navigation channel. To investigate the causes, field observations of tidal current, salinity and suspended sediment concentration (SSC) were conducted by 16 anchored boats in the navigation channel of North Passage during spring tide in the flood season. The across-channel differences of salinity, SSC, and near bottom residual sediment transport at the high siltation area of the DNC were observed and analyzed. The results show that the near bottom SSC presented a south-to-north decreasing trend during the flood tide at the lower reach of the North Passage. During the ebb tide, a suspended sediment cloud of extremely high concentration was detected within the channel. The observed longitudinal sediment trapping was not only shown as the turbidity maximum (TM) trapped within the moving salinity front but also as the longitudinal convergence of the residual sediment transport at the middle reach. The mechanism for the longitudinal sediment trapping was due to the tidal asymmetry in mixing. The observed lateral sediment trapping was prominent at the most severe siltation area of the DNC in the section #2. The lateral sediment trapping at the across-channel section #2 was not only shown as the lateral transport of the sediments from the channel flanks during the flood slack but also as the lateral convergence of the residual sediment transport during the entire spring tidal cycle. The mechanism for the lateral sediment trapping was dominated by the advective transport. The lateral impact on the channel sediment trapping was not negligible in view of the depth-mean lateral tidal straining on the water stratification. The observed convergences of both the longitudinal and the lateral residual sediment transports were responsible for the severe channel siltation in the North Passage.

Sediment quality of a Ramsar site assessed by chemical and ecotoxicological approaches

The sediment quality of the Protected Cananéia-Iguape-Peruíbe Area (APA-CIP), listed as one of the Wetlands of International Importance (Ramsar Site), was assessed through geochemical and ecotoxicological analyses, in order to determine if organic and inorganic contaminants retained in the sediments were bioavailable to benthic organisms and could trigger negative effects. Lethal (Tiburonella viscana) and sublethal (Lytechinus variegatus and Nitokra sp.) ecotoxicological endpoints were assessed, as well as sediment texture and organic and inorganic contaminants in sediments obtained at six stations distributed along the APA-CIP. Sites under the influence of the maximum turbidity zone and close to the main local urban center (Cananéia city) presented the worst environmental conditions, indicated by sediment toxicities and high contaminants levels. These conditions may be associated to fine-grained sediments. Metal (Pb, Co, Ni and Zn) concentrations exceeded the regional Sediment Quality Guidelines (SQGs) at stations PT and PM, while Pb also exceeded the Canadian Interim Marine Sediment Quality Guidelines (ISQGs). Total Aliphatic Hydrocarbons (AH) were close to threshold sediment contamination values at PT (maximum turbidity zone). These findings indicate that metals from former mining activities reach APA-CIP, in addition to contaminants from local sources (urban centers, docks, fishing wharfs). Contaminants accumulate in low energy regions, where fine particle and organic matter contents are high. Toxicities appeared to be associated to contaminants, suggesting that the multiple local anthropogenic sources can produce ecological risks, indicating the need for multiple efforts to control external and internal contamination sources in this protected estuarine area.

Numerical modelling of suspended-sediment transport in a geographically complex microtidal estuary: Sydney Harbour Estuary, NSW

A numerical study was conducted to investigate the sediment dynamics in a geographically complex estuary, the Sydney Harbour Estuary (SHE). The SHE is a good example of a microtidal estuary, with irregular shorelines and a complex bathymetry, characterized by many headlands and islands forming a meandering main channel. Horizontal sediment transport showed a local estuarine turbidity maximum (ETM) as a result of complex topography, independent of salinity fields and river flows during dry weather. The along-estuary advection of sediment was mainly driven by the mean advection, with a minor contribution by tidal pumping. Mean advection associated with barotropic forcing drives sediment flux seaward in the upper estuary and landward in the middle estuary, leading to a longitudinal convergence of sediment transport, without upstream or downstream migration of ETM during high river flows. The interactions between tidal currents, complex topography and asymmetric vertical mixing led to spring-neap and flood-ebb variations in sediment distribution. The Singular Spectrum Analysis (SSA) method was used to calculate the relative contributions of the identified environmental forcing frequencies (tidal range, tidal frequency, river discharges, wind stress) to the variability in suspended-sediment concentration. Tidal frequency and river discharges were the major contributors to this variability. Tidal range made the highest contribution in the middle estuary, where the ETM was located, driving the spring-neap cycle of the ETM.

Selective Complex Coacervation of Pea Whey Proteins with Chitosan To Purify Main 2S Albumins.

Proteins of pea whey were separated by 1-D electrophoresis and 2-D electrophoresis and identified by MALDI-TOF/TOF-MS. In addition to lectin, pea albumin 2 (PA2) and pea albumin 1a (PA1a) were identified as the main 2S albumins. The complex behavior of pea whey proteins with chitosan as a function of pH and protein to polysaccharide ratio was studied by turbidimetric titration, zeta potential, and Tricine-SDS-PAGE. During pH titration, the zeta potential reveals that at maximum turbidity (pHmax), charge neutrality was fulfilled. The maximal protein recovery was obtained at a mass ratio of 1:1. After coacervation with chitosan, lectin was not involved in the formation of complexes and PA2 transferred into complex preferentially as compared to PA1a. The weak binding affinity and high hydrophilicity of PA1a made it selectively dissolve out from the PA2/PA1a complex at acidic pH conditions. After removal of chitosan and small molar weight peptides, high-purity PA2 and PA1a (>90% by SEC-HPLC) could be obtained. This work provides a novel strategy for the purification of proteins from a multiprotein pea whey system.

Impaired Fibrinolysis Predicts Adverse Outcome in Acute Coronary Syndrome Patients with Diabetes: A PLATO Sub-Study.

Hypofibrinolysis is a key abnormality in diabetes but the role of impaired clot lysis in predicting vascular events and mortality in this population is yet to be determined. We aimed to investigate the relationship between fibrin clot properties and clinical outcomes in patients with diabetes and recent acute coronary syndrome (ACS). Plasma samples were collected at hospital discharge from 974 ACS patients with diabetes randomised to clopidogrel or ticagrelor in the PLATO trial. A validated turbidimetric assay was employed to study fibrin clot lysis and maximum turbidity. One-year rates of cardiovascular (CV) death, spontaneous myocardial infarction (MI) and PLATO-defined major bleeding events were assessed after sample collection. Hazard ratios (HRs) were determined using Cox proportional analysis. After adjusting for CV risk factors, each 50% increase in lysis time was associated with increased risk of CV death/MI (HR 1.21; 95% confidence interval [CI] 1.02–1.44; p = 0.026) and CV death alone (HR 1.38; 1.08–1.76; p = 0.01). Similarly, each 50% increase in maximum turbidity was associated with increased risk of CV death/MI (HR 1.25; 1.02–1.53; p = 0.031) and CV death alone (HR 1.49; 1.08–2.04; p = 0.014). The relationship between lysis time and the combined outcome of CV death and MI remained significant after adjusting for multiple prognostic vascular biomarkers ( p = 0.034). Neither lysis time nor maximum turbidity was associated with major bleeding events. Impaired fibrin clot lysis predicts 1-year CV death and MI in diabetes patients following ACS. Clinical Trial Registration URL: http://www.clinicaltrials.gov . Unique identifier NCT00391872.

Two-dimensional modeling of fine sediment transport with mixed sediment and consolidation: Application to the Gironde Estuary, France

In order to optimize ship navigation in the macrotidal Gironde Estuary, a recent project funded by the port of Bordeaux aims at better understand and forecast hydrodynamic and fine sediment transport within the estuary. In the framework of this project, a two-dimensional hydro-sedimentary model is built. The model includes hydrodynamic forcings, mixed-sediment transport, and consolidation processes. The harmonic analysis of the astronomical tides reveals a strong distortion of the tidal wave inducing the growth of overtide constituents and the non-significant effect of tide-surge interactions in annual-scale prediction. Depending on hydrological conditions, river discharge can considerably alter the model accuracy due to the migration of the turbidity maximum zone modifying the bottom roughness. Comparison with measurements shows the ability of the model to reproduce suspended-sediment concentrations in the central Estuary. Sensitivity of the model to sediment features has also been discussed in regard of suspended-sediment concentrations and fluid mud deposits. The model will be further coupled with ship squat predictions and a morphodynamic model.

Optimization of water use in a rapid filtration system: A case study

The objective of this research was to reduce water use during the operation of anthracite-sand filters used for treating drinking water. The variables that affect the saturation of the filter bed were evaluated, using procedures proposed by the Pan American Center for Sanitary Engineering and Environmental Sciences (CEPIS). The following were evaluated: filtration velocity, initial filtration quality, filtration runs duration, filter bed expansion, duration of the washing process, washing velocity, granulometry, and mud balls. The results obtained were compared with the design parameters recommended by CEPIS. Maximum turbidity of 0.5 NTU was obtained. Filtration runs were in the lower limit of the range indicated by CEPIS. The filter medium expansion in five of the eight filters was lower than recommended by CEPIS. The optimal washing time was 18 min. The washing velocity was low producing little filter bed expansion. The granulometric result indicated that the effective size and the anthracite uniformity coefficient were different between filters. The results also presented significant improvements in the filtration process and the amount of water saved once the filter washing process was optimized. The results also demonstrated that measuring filters performance could improve the filtration system efficiency and ensure the drinking water quality.

Links between size fractionation, chemical speciation of dissolved copper and chemical speciation of dissolved organic matter in the Loire estuary

Environmental contextThe toxicity of copper to aquatic life is highly dependent on its chemical form. In the vineyards of the Loire valley, mixtures of copper compounds are increasingly being used as fungicides. Our study investigating the origin and behaviour of dissolved copper on the land-sea continuum of the Loire advances our understanding of the chemical forms and fate of copper in estuarine systems. AbstractWe present data on the organic speciation of dissolved copper (dCu) in the Loire estuary for the soluble (&amp;lt;0.02µm) and dissolved (&amp;lt;0.45µm) fractions. These results were interpreted according to the distribution of natural organic matter along the estuary. We observed a high concentration of dCu (80nmolkg−1) upstream of the tidal front (S=0.1, where S is the salinity), possibly induced by drainage from vineyards located on the watershed of the estuary, and a concentration typical of coastal seawater at the end of the salinity gradient (3–4nmolkg−1). dCu showed a non-conservative distribution along the estuary with a notably strong decrease in concentration at the very first stage of mixing (S ~ 0.5) that increased again at low salinity (S=4.7). The latter increase in the concentration of dCu was related to the supply of colloidal copper induced by particle desorption in the maximum turbidity zone. The organic speciation of dCu revealed that the complexing capacity of copper ligands (LCu, up to 147 nanomoles of Cu per kilogram of water) exceeded dCu in both the soluble and dissolved fractions, which kept the activity of cupric ions below toxic levels for most unicellular phytoplankton. Humic substances comprised up to 95% of LCu in the continental shelf sample, but their complexing capacity did not account for all of the LCu in the inner estuary. We conducted pseudopolarographic experiments and found two other additional ligand classes: thiol-like and amino-acids/proteins. While humic substances are the dominant component of dissolved organic matter (DOM), the study of DOM suggested the release of colloidal DOM from a wastewater treatment plant. This structure could be a major source of proteinaceous LCu in the system that changes the dCu speciation in the middle of the estuary.

Evaluation and optimization of the coagulation-flocculation process using conventional rice starch in potable water treatment

In this study, the application of conventional rice starch as coagulant in potable water treatment has been assessed. Rice starch was found to gelatinize at 83 °C for effective flocculation. Besides, respond surface method (RSM) was used to identify the significant factors and to optimize the turbidity reduction in potable water treatment. pH, dosage of starch and settling time were identified as significant factors. To achieve the maximum turbidity reduction (89%), the optimum conditions of the factors are with 9.64 mg/L dosage of starch, 20 min of settling time and pH 3. Interestingly, increase of dosage of starch can reduce up to 20 times of settling time which enhance the feasibility of starch to use in the potable water treatment industry. Validation test was conducted to validate the predicted data from designed model.

Multi-response optimization of the coagulation process of real textile wastewater using a natural coagulant

The present study is an attempt to investigate the applicability of a natural coagulant Moringa Oleifera (MO) as a potential impending alternative for the treatment of textile wastewater. Face Central Composite Design in Response Surface Methodology was used to optimize the performance of the coagulation process in terms of turbidity and colour removal efficiency. Three independent variables which are pH (2-4), coagulant dosage (4-8 ml) and settling time (10-30 min) were investigated. Quadratic models were developed for each response and their statistical fitness was evaluated using ANOVA analysis. All models were proved to be statistically adequate with R2 higher than 96%. Of the investigated variables, pH was found to have significant effect on both responses (P-value <0.001), whereas, coagulant dosage affected the colour removal efficiency significantly. Multi-response optimization was applied to optimize the process variables for both responses simultaneously using the RSM desirability function approach. Maximum turbidity reduction by more than 81% and colour removal efficiencies by more than 77% were reached at the optimum conditions of pH 2, coagulant dosage 4 mL and settling time 21 min. These results prove that MO could be used as an alternative to the conventional coagulants for real textile wastewater treatment.

Dual Treatment of Milk Processing Industry Wastewater Using Electro Fenton Process Followed by Anaerobic Treatment

Abstract Milk processing industry produced in large amounts wastewater with high pH, temperature and toxic matters. These effluents can exhibit serious environmental problems and public health concerns if improperly disposed. In this present study, an attempt has been made to investigate the efficiency of combined treatment for milk processing industry wastewater. Advanced oxidation process used as primary treatment where as biological process is applied as post treatment to degrade the toxic matters. Parameters affecting the combined process on the percentage removal of turbidity and chemical oxygen (COD) demand were studied in detail. Under the optimum conditions, 93 % of turbidity and 97 % of COD were removed. These results indicated that the proposed technique could be used to degrade the maximum turbidity and chemical oxygen demand from milk processing industry wastewater.

Using ichnological relationships to interpret heterolithic fabrics in fluvio‐tidal settings

AbstractIn fluvio‐tidal settings, the sediment is dominantly derived from the river systems. However, the importance of landward tidal transport of sediment in tidally influenced sedimentary environments is difficult to assess, particularly in the rock record. This problem is addressed using two intervals within the Lower Cretaceous McMurray Formation, each representing a distinct inclined heterolithic stratification motif. The ichnological variation between the heterolithic intervals is analyzed to determine which lithosomes are associated with brackish‐water (tidally influenced) colonization windows. From this, the relative fluvial influence responsible for the deposition of the fine and coarse members can be determined. Both of the inclined heterolithic stratification fabrics studied record the deposition of fluvio‐tidal point bars wherein the heterolithic bedding represents variations in river discharge. The first fabric comprises inclined heterolithic stratification in which bioturbation only occurs in mudstone beds. This fabric indicates that deposition occurred in more proximal positions within a fluvio‐tidal system (i.e. the outermost inner to middle estuary or distributary channels). In this example sand deposition is interpreted to represent high‐energy, freshwater dune migration within a fluvial‐dominated setting, whereas mud beds reflect brackish‐water suspension deposition during times of low river discharge. The second fabric, which is interpreted to have developed in more distal depositional positions (i.e. the middle estuary or seaward of the turbidity maximum in deltas), consists of inclined heterolithic stratification with laminated mudstone and bioturbated sandstone. In these inclined heterolithic stratification successions the mudstone beds were deposited under the influence of freshwater and heightened sedimentation rates, whereas bioturbated sandstone was colonized under brackish‐water conditions and in the presence of tidally facilitated sediment transport. In both examples, the bioturbated lithosomes are related to colonization windows that indicate the predominance at that time of marine or tidally influenced processes over fluvial processes.

Comparing the efficiency of hypoxia mitigation strategies in an urban, turbid tidal river via a coupled hydro-sedimentary–biogeochemical model

Abstract. Coastal-water hypoxia is increasing globally due to global warming and urbanization, and the need to define management solutions to improve the water quality of coastal ecosystems has become important. The lower tidal Garonne River (TGR; southwestern France), characterized by the seasonal presence of a turbidity maximum zone (TMZ) and urban water discharge, is subject to episodic hypoxia events during low river flow periods in the summer. Future climatic conditions (higher temperature and summer droughts) and increasing urbanization could enhance hypoxia risks near the city of Bordeaux in the coming decades. A 3-D model of dissolved oxygen (DO) that couples hydrodynamics, sediment transport and biogeochemical processes was used to assess the efficiency of different management solutions for oxygenation of the TGR during summer low-discharge periods. We ran different scenarios of reductions in urban sewage overflows, displacement of urban discharges downstream from Bordeaux and/or temporary river flow support during the summer period. The model shows that each option mitigates hypoxia, but with variable efficiency over time and space. Sewage overflow reduction improves DO levels only locally near the city of Bordeaux. Downstream relocation of wastewater discharges allows for better oxygenation levels in the lower TGR. The support of low river flow limits the upstream TMZ propagation and dilutes the TGR water with well-oxygenated river water. Scenarios combining wastewater network management and low-water replenishment indicate an improvement in water quality over the entire TGR. These modelling outcomes constitute important tools for local water authorities to develop the most appropriate strategies to limit hypoxia in the TGR.Highlights. A 3-D model shows different efficiencies of management actions to limit hypoxia. Downstream relocation of wastewater discharge totally mitigates hypoxia. Sewage overflow reduction improves DO levels but only locally. Water replenishment improves DO in the upper estuary.

Observations and Simulations of Microplastic Debris in a Tide, Wind, and Freshwater-Driven Estuarine Environment: the Delaware Bay.

Microplastic (MP) in estuarine and coastal environments remains poorly characterized, despite the importance of these physically dynamic regions as a buffer between land, freshwater environments, and the open ocean where plastic debris accumulates. We sampled MP particles to determine concentration, size, and type in Delaware Bay and numerically simulated transport and distribution at a high spatiotemporal resolution of positively buoyant particles, representing common MP types. Baywide MP concentrations averaged between 0.19 and 1.24 pieces m-3 depending on size fraction (300-1000 and 1000-5000 μm) and sampling month (April and June 2017). Upper bay stations, which are located in or near the estuarine turbidity maximum, had higher MP concentrations than lower bay and New Jersey shore stations. Fragments were predominately polyethylene, and filaments predominately polypropylene. Model results suggest that buoyant particles quickly (i.e., within hours) organize in patchy, highly inhomogeneous distributions, creating "hot spots" of MP. In the presence of variable currents driven by buoyancy, wind, and tides, we predict high spatial and temporal variability of MP distributions in Delaware Bay; MP concentrations could vary by a factor of 1000 within a tidal cycle at our sample locations. Collectively, these observations and simulations provide a baseline of MP concentrations in Delaware Bay along with broader, contextual understanding for how measurements reflect MP concentrations in a dynamic estuarine system.