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PFAS, known as "forever" compounds, are prevalent in various environments, including soils and aquatic systems, due to extensive usage. Surface waters in several European countries, especially marinas and ports with high boat traffic, require further study as potential contamination sources. Reliable methods for the extraction and quantification of these emergent compounds are essential. This study aimed to improve an existent solid phase extraction method to analyse marinas and ports' surface waters with variable salinities (2, 9 and 17 PSU). The objectives were to: 1) optimise the solid phase extraction method, considering matrix salinity effects and cross-contaminations, 2) validate the extraction and quantification method of 18 EPA 537.1 PFAS in estuarine surface waters, using the Ultra-High Performance Liquid Chromatography - Quadrupole Time - Of - Flight - Tandem Mass spectrometry, and 3) apply the optimised method for PFAS quantification in three Portuguese marinas. All ICH criteria were successfully validated considering 9 PSU. Limits of quantification ranged from 117.80 ng/L to 385 ng/L, except for PFHpA (645.85 ng/L). PFAS levels (PFOA, HFPO-DA, PFBS, PFHxS and PFOS) were relatively low, reaching a maximum of 0.32 ng/L only for the PFOA. In Freixo marina, total average concentrations were slightly higher (∑PFAS = 1.02 ng/L) when compared to the ones found in Cais da Ribeira Port (∑PFAS = 0.94 ng/L) and Afurada marina (∑PFAS = 0.81 ng/L). PFOS concentrations are below the limit values set by the Environmental Quality Standards (36000 ng/L of PFOS for inland surface water, respectively), similar to other Portuguese river studies. This study enabled the development of a precise and reliable extraction and quantification method to quantify PFAS in estuarine surface waters, particularly from marinas. This method can be readily applied to analyse PFAS in other estuarine samples.

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In Great Bay Estuary, New Hampshire, USA, Haplosporidium nelsoni and Perkinsus marinus are 2 active pathogens of the eastern oyster Crassostrea virginica (Gmelin), that cause MSX (multinucleated sphere with unknown affinity 'X') and dermo mortalities, respectively. Whereas studies have quantified infection intensities in oyster populations and determined whether these parasites exist in certain planktonic organisms, no studies thus far have examined both infectious agents simultaneously in water associated with areas that do and do not have oyster populations. As in other estuaries, both organisms are present in estuarine waters throughout the Bay, especially during June through November, when oysters are most active. Waters associated with oyster habitats had higher, more variable DNA concentrations from these pathogenic organisms than waters at a non-oyster site. This finding allows for enhanced understanding of disease-causing organisms in New England estuaries, where oyster restoration is a priority.

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This study aimed to develop a multidisciplinary approach to assess the ecological status of six moderate-sized French estuaries. For each estuary, we gathered geographical information, hydrobiological data, chemistry of pollutants and fish biology, including integration of proteomics and transcriptomics data. This integrative study covered the entire hydrological system studied, from the watershed to the estuary, and considered all the anthropogenic factors that can impact this environment. To reach this goal, European flounder (Platichthys flesus) were collected from six estuaries in September, which ensures a minimum residence time of five months within an estuary. Geographical metrics are used to characterize land use in each watershed. The concentrations of nitrite, nitrate, organic pollutants, and trace elements were measured in water, sediments and biota. All of these environmental parameters allowed to set up a typology of estuaries. Classical fish biomarkers, coupled with molecular data from transcriptomics and shotgun proteomics, highlighted the flounder's responses to stressors in its environment. We analysed the protein abundances and gene expression levels in the liver of fish from the different estuaries. We showed clear positive deregulation of proteins associated with xenobiotic detoxification in a system characterized by a large population density and industrial activity, as well as in a predominantly agricultural catchment area (mostly cultures of vegetables and pig breeding) mainly impacted by pesticides. Fish from the latter estuary also displayed strong deregulation of the urea cycle, most probably related to high nitrogen load. Proteomic and transcriptomic data also revealed a deregulation of proteins and genes related to the response to hypoxia, and a probable endocrine disruption in some estuaries. Coupling these data allowed the precise identification of the main stressors interacting within each hydrosystem.

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Coastal upland forests are facing widespread mortality as sea-level rise accelerates and precipitation and storm regimes change. The loss of coastal forests has significant implications for the coastal carbon cycle; yet, predicting mortality likelihood is difficult due to our limited understanding of disturbance impacts on coastal forests. The manipulative, ecosystem-scale Terrestrial Ecosystem Manipulation to Probe the Effects of Storm Treatments (TEMPEST) experiment addresses the potential for freshwater and estuarine-water disturbance events to alter tree function, species composition, and ecosystem processes in a deciduous coastal forest in MD, USA. The experiment uses a large-unit (2000 m2), un-replicated experimental design, with three 50 m × 40 m plots serving as control, freshwater, and estuarine-water treatments. Transient saturation (5 h) of the entire soil rooting zone (0-30 cm) across a 2000 m2 coastal forest was attained by delivering 300 m3 of water through a spatially distributed irrigation network at a rate just above the soil infiltration rate. Our water delivery approach also elevated the water table (typically ~ 2 m belowground) and achieved extensive, low-level inundation (~ 8 cm standing water). A TEMPEST simulation approximated a 15-cm rainfall event and based on historic records, was of comparable intensity to a 10-year storm for the area. This characterization was supported by showing that Hurricane Ida's (~ 5 cm rainfall) hydrologic impacts were shorter (40% lower duration) and less expansive (80% less coverage) than those generated through experimental manipulation. Future work will apply TEMPEST treatments to evaluate coastal forest resilience to changing hydrologic disturbance regimes and identify conditions that initiate ecosystem state transitions.

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Exposure of aquatic environments to emerging contaminants is a global issue, special relevant in many estuaries due to impacts from anthropogenic activity. The aim of this work was to evaluate thirty-seven endocrine disruptor chemicals (EDCs) from four different classes (pesticides, bisphenols, polycyclic musks and UV-filters) in water and sediment samples collected during one-year in the estuaries of Tagus and Douro Rivers located into the NE Atlantic Ocean coast. EDCs analysis was achieved afterward validation of a gas-chromatography mass spectrometry (GC-MS) method using Dispersive Liquid-Liquid Microextraction (DLLME) as extraction procedure for water samples, and Quick, Easy, Cheap, Efficient, Rugged and Safe (QuEChERS) combined with DLLME for sediments. Tagus estuary presented higher levels of contamination with pesticide residues and bisphenols (BPs) than the Douro estuary in both water and sediment samples. Contrariwise, levels and frequency of polycyclic musks (PCMs) and UV-filters (UVF) were slightly higher in Douro estuary. Levels of pesticide residues in both sediment and water samples, and levels of PCMs and UVF in water samples were higher in warmer seasons (summer and spring) than in colder ones (winter and autumn). The opposite was found in what respect levels of BPs in water and sediment samples, and PCMs and UVF levels in sediment samples. Although the levels found for each contaminant are low, usually in the order of a few ng/mL(g), the presence of a high number of toxic compounds is a source of concern and requires constant monitoring.

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The Defense Coastal/Estuarine Research Program (DCERP) was a 10-year multi-investigator project funded by the Department of Defense to improve understanding of ecosystem processes and their interactions with natural and anthropogenic stressors at the Marine Corps Base Camp Lejeune (MCBCL) located in coastal North Carolina. The project was aimed at facilitating ecosystem-based management (EBM) at the MCBCL and other coastal military installations. Because of its scope, interdisciplinary character, and duration, DCERP embodied many of the opportunities and challenges associated with EBM, including the need for explicit goals, system models, long-term perspectives, systems complexity, change inevitability, consideration of humans as ecosystem components, and program adaptability and accountability. We describe key elements of this program, its contributions to coastal EBM, and its relevance as an exemplar of EBM.

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It is important to study the fate and transport of antibiotics in aquatic environments to reveal their pollution status. The premise behind fate and transport studies is to evaluate the reaction processes of the target antibiotics. However, available research on the environmental behaviors of antibiotics in certain natural waters, such as estuarine water, is scarce. In this study, single reactions such as sorption, biodegradation, and photolysis and multiple degradation reactions of sulfamethoxazole (SMX), trimethoprim (TMP), and ciprofloxacin (CIP) in the estuarine water were studied. The sorption rates of the target antibiotics in the estuarine water-sediment system were very fast, and the sorption amounts varied among sediments and antibiotics. Hydrolysis did not contribute to the degradation of the target antibiotics. Biodegradation had a low contribution to the degradation of the target antibiotics in the estuarine water. In comparison, photolysis was the dominant degradation process for SMX, TMP, and CIP. The rates of photolysis of the tested antibiotics in the estuarine water were greater than those in pure water; thereby, indicating photolysis of these antibiotics was more prone to occur in the estuarine water. In the multiple degradation experiments, it was found that there may be synergistic effects between the single degradation processes. Thus, the aqueous concentrations of antibiotics decreased rapidly by sorption after entering the estuarine water and then decreased relatively slowly by photolysis and biodegradation. This study provides information for evaluating the environmental behaviors of antibiotics in estuarine environments.

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Distribution and ecological risks of Phthalic acid esters (PAEs) are poorly studied in estuarine environments in India. An attempt is made to chart the sources and assess the ecological risk of six PAE congeners (∑6PAEs), present in dissolved and particulate forms in a tropical ecosystem (Cochin Estuary, India). Terrestrial input, as attested by a clear seasonality with substantial enrichment during monsoon (2-28 µg/L and 31-1203 µg/g; dissolved and particulate PAEs respectively) and post-monsoon (1-7 µg/L and 7-321 µg/g; dissolved and particulate PAEs respectively), was identified as the primary source. DnBP (di-n-butyl phthalate) and DEHP (diethylhexyl phthalate) were found to be the dominant species except for dissolved PAEs at pre-monsoon season. Statistical analysis identified two major clusters, in the ∑6PAEs, composed of medium to high molecular weight PAEs (derived from plastic products) and low molecular weight PAEs (derived from cosmetic products). Calculated Risk Quotient (RQ) indicated values indicated moderate to high ecological risk for DnBP and DEHP congeners is a grim pointer to their detrimental effects on human health through consumption of contaminated organisms. Although substantial enrichment of suspended matter gets flushed out of the estuary during monsoon, there is a net PAE accumulation in the estuary during post-monsoon following an increased sedimentary restitution. Data of PAEs generated herein raises a challenge for immediate enactment of statutory legislation to curb and regulate hazardous contamination of estuaries by phthalic acid esters.

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The Mediterranean aquaculture has been developed mostly in brackish environment in inactive coastal salt production areas. This study aims to utilise Technosols made with aquaculture sediments for Limonium algarvense Erben cultivation. This species that has nutraceutical potential thrives in halophilic environments in the southwest of the Iberian Peninsula and in Morocco. A microcosm assay was set up with plants grown in bottom sediments (C+), commercial substrate (C-), and Technosols with amendments mixture application at 180 g/kg (Tec180) or at 360 g/kg (Tec360). These plants were irrigated with saline (assay 1) and/or with deionised water (assay 2). The bottom pond sediments, coffee wastes and the estuarine water were evaluated for diverse physicochemical parameters. Plant growth was characterised through a combined methodology using morphometric, SEM and physiological analysis. The Technosols were constructed with bottom sediments and a mixture of organic wastes used as amendments. Results revealed that the bottom sediments had low pH 3.2, Corg and extractable P and K contents, and high electroconductivity (EC) and N-NH4 concentration. The estuarine water had a neutral pH, high EC and high Cl-, HCO3-, Na+, Mg2+ and Ca2+ but low N-NO3- content. The Technosols showed a significant increase of pH, Corg, K and P and a decrease in N-NH4 and EC in comparison with sediments. Principal component analysis separated the different experiments in three groups: C-, A1 and A2 assays. The C- was highly correlated with Corg, P, K, N-NO3 parameters and total ascorbate. The A1 assay showed a strong association with Na, Ca and EC parameters, whereas the A2 assay presented a strongly correlation with plant growth. Plants from Technosols had greater development when irrigated with deionised water than under salty irrigation as opposed to plants cultivated in unamend sediments. In conclusion, these results support that highly saline sediments could be valorised through Technosols construction to cultivate plants with saline water, with potential application in the agro-food and pharmaceutical industry.

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Many studies have examined the acute toxicity of nanoparticles (NPs) towards model bacteria. In this study, we report the time-dependent effects of ZnO NPs on native, selected Zn-resistant and dominant bacteria in estuarine waters. An initial inhibition of bacterial growth followed by a recovery at 24 h was observed, and this rebound phenomenon was particularly notable when the raw water samples were treated with relatively high ZnO NP concentrations (1 and 10 mg/L).By comparing the groups treated with Zn2+, Zn2+ was shown to largely explain the acute cytotoxic effect of ZnO NPs on bacteria in raw waters. Furthermore, similar to the native bacteria, especially the dominant bacteria, the viability of Escherichia coli (E. coli) decreased with the increasing treatments time and the concentrations of ZnO NPs in water with different salinities. Moreover, the expression of Zn-resistance genes including zntA and zntR in E. coli suggested that the Zn-resistance system in E. coli can be activated to defend against the stress of Zn2+ released from ZnO NPs, and salinity may promote this process in estuarine aquatic systems. Thus, the effect of ZnO NPs on bacteria in estuarine water bodies is likely determined by the synergistic effect of environmental salinity and dissolved Zn ions. As such, our findings are of high relevance and importance for understanding the ecological disturbances caused by anthropogenic NPs in estuarine environments.

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This study was performed to improve the ability to predict the concentrations of Escherichia coli in oyster meat and estuarine waters by using environmental parameters, and microbiological and heavy metal contamination from shellfish growing area in southern Thailand. Oyster meat (n = 144) and estuarine waters (n = 96) were tested for microbiological and heavy metal contamination from March 2016 to February 2017. Prevalence and mean concentrations of E. coli were 93.1% and 4.6 × 103 most probable number (MPN)/g in oyster meat, and 78.1% and 2.2 × 102 MPN/100 mL in estuarine water. Average 7-day precipitation, ambient air temperature, and the presence of Salmonella were associated with the concentrations of E. coli in oyster meat (p < 0.05). Raw data (MPN/g of oyster meat and MPN/100 mL of estuarine water) and log-transformed data (logMPN/g of oyster meat and logMPN/100 mL of estuarine water) of E. coli concentrations were examined within two contrasting regression models. However, the more valid predictions were conducted using non-log transformed values. These findings indicate that non-log transformed data can be used for building more accurate statistical models in microbiological food safety, and that significant environmental parameters can be used as a part of a rapid warning system to predict levels of E. coli before harvesting oysters.

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Emerging pollutants known as endocrine-disrupting compounds (EDCs) are a contemporary global issue, especially in aquatic ecosystems. As aquaculture production through mariculture activities in Malaysia supports food production, the concentration and distribution of EDCs in estuarine water ecosystems may have changed. Therefore, this current study aims to prepare a suitable and reliable method for application on environmental samples. Besides, this study also presented the occurrence of EDCs pollutant in Pulau Kukup, Johor, where the biggest and most active mariculture site in Malaysia takes place. Analytical methods based on a combination of solid-phase extraction with liquid chromatography tandem mass spectrometry (Solid-phase extraction (SPE)-LC-MS/MS) have been modified and optimised to examine the level of targeted EDCs contaminant. In the current study, this method displays high extraction recovery for targeted EDCs, ranging from 92.02% to 132.32%. The highest concentration detected is diclofenac (<0.47-79.89 ng/L) followed by 17ß-estradiol (E2) (<5.28-31.43 ng/L) and 17α-ethynylestradiol (EE2) (<0.30-7.67 ng/L). The highest percentage distribution for the targeted EDCs in the current study is diclofenac, followed by EE2 and dexamethasone with the percentages of 99.44%, 89.53% and 73.23%, respectively. This current study can be a baseline assessment to understand the pollution profile of EDCs and their distribution in the estuarine water of the mariculture site throughout the world, especially in Malaysia. Owing to the significant concentration of targeted EDCs detected in water samples, the need for further monitoring in the future is required.

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Automated, colorimetric analysis of nutrients in samples with high and variable salinity can be time consuming due to the need to matrix match calibration and reference solution matrices with those of samples-particularly when using flow-based analyzers that are prone to detector artifacts caused by optical inhomogeneities, "schlieren," that form at interfaces between samples and deionized water carrier or wash solutions. Such detector artifacts do not occur in discrete analyzers. Here we report spike recoveries when nitrite plus nitrate, nitrite, ammonia, orthophosphate, and silica were determined in estuarine waters, seawater, and hypersaline surface water samples without calibrator matrix matching using an automated discrete analyzer set up with standard colorimetric methods. Salinities of these samples varied from 0 to 22%. Spike recoveries and precision were excellent for nitrite and nitrite plus nitrate analyses in samples with salinities up to 15%, for ammonia in samples with salinities up to about 1.7%, and for orthophosphate in samples with salinities up to about 3.5%. Orthophosphate spike recoveries were high biased in hypersaline matrices (salinity >3.5%), likely due to an unidentified sample matrix interference. After applying linear correction factors to account for chemistry-related salt effects, spike recoveries were acceptable for ammonia analyses in samples with salinities in the range of 1.7-3.5%, and for silica analyses in samples with salinities in the range of 0-3.5%.

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Estuaries are productive environments used by many fish as nursery grounds. The initial stages of fishes are highly vulnerable to (a)biotic factors, and anthropogenic pressures, influencing fish larvae assemblages along the estuary. Microplastics (MPs < 5 mm) are particularly dangerous to early life stages of fishes because their ingestion can induce gut blockage, limiting food intake or exposing organisms to contamination due to MPs capacity to absorb pollutants. Present work aimed to investigate the contamination of an urban impacted estuary (Douro estuary, NW Portugal) by MPs, and study the abundance and distribution of MPs and fish larvae in this estuary. Monthly sampling surveys were performed from December 2016 to December 2017, in nine stations along the estuary. Sub-surface planktonic horizontal trawls were performed to collect fish larvae and MPs. Planktonic samples were sorted, and fish larvae identified. MPs density was determined using a protocol optimized in our laboratory. A total of 1498 fish larvae belonging to 32 taxa were collected, with a mean density of 11.66 fish larvae 100 m-3. During the spring-summer period, it was observed the typical increase in the density and diversity of the larval assemblage. Diversity was generally low, with the high dominance of very few taxa, namely the common goby, Pomatoschistus microps. Different types of MPs were found, namely fibers, soft/hard plastic, colorful/transparent plastic, in a total of 2152 particles, with a mean density of 17.06 MPs 100 m-3. Hard MPs and fibers were the most predominant types, representing 83% of the total MPs collected. In some months the number of MPs surpassed the number of fish larvae, with an average ratio of 1.0 fish larvae:1.5 MPs. Such results are concerning, highlighting that a higher availability of MPs may facilitate their ingestion by fish and therefore increase possible impacts in these communities.

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Estuaries provide significant cultural ecosystem services, including recreation and tourism. Disruptions of estuarine biogeochemical processes resulting from environmental degradation could interrupt the flow of these services, reducing benefits and diminishing the welfare of local communities. This study focused on recreational shellfishing in Buzzards Bay, Massachusetts (41.55°N, 70.80°W). Relationships among measures of recreational shellfishing, estuarine water quality, and local socioeconomic conditions were tested to understand how the benefits of cultural ecosystem services to local communities might be affected by declining water quality. Transferring estimated economic benefits from an analysis of nearby municipalities, the study finds that increases in Chl a during the 24-year period were associated with losses in recreational shellfishing benefits of $0.08-0.67 million per decade. The approach presented here suggests a more broadly applicable framework for assessing the impacts of changes in coastal ecosystem water quality on the welfare of local communities.

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The goal of this study was to investigate the involvement of a tropical Indian estuary in the emergence of antibiotic resistance genes (ARGs)-harboring hypervirulent E. coli of global significance. A total of 300 E. coli isolates was tested for antibiotic susceptibility to ß-lactams, aminoglycosides, chloramphenicol, quinolones, sulphonamides, tetracyclines, and trimethoprim. The E. coli isolates were screened for the presence of antibiotic resistance genes (blaTEM, blaCTX-M, tetA, tetB, sul1, sul2, strA, aphA2, catI, dhfr1, and dhfr7), integrase (int1, int2, and int3), Shiga toxin genes (stx1 and stx2) and extraintestinal virulence genes (papAH, papC, sfa/focDE, kpsMT II, and iutA). The highest prevalence of antibiotic resistance was observed for ampicillin, followed by tetracycline, and nalidixic acid. Among E. coli isolates, 64% were resistant to at least one of the 15 antibiotics tested, and approximately 40% were multiple antibiotic-resistant (MAR). More than 40% (n = 122) of E. coli isolates had ARGs. Integrase 1 (int1) was found in 7.6% of E. coli isolates. Among E. coli isolates, 16.3% (n = 49) were extraintestinal pathogenic E. coli (ExPEC), and approximately 34.6% (n = 17) of ExPEC had ARGs. A hypervirulent ARGs-harboring STEC was isolated. The prevalence of Shiga toxin-producing E. coli (STEC) was low (n = 1). The prevalence of ARGs-harboring pathogenic E. coli isolates was higher in stations close to the City (urban area), than that of other stations. ERIC-PCR (enterobacterial repetitive intergenic consensus sequence polymerase chain reaction) analysis revealed a high degree of genetic diversity among the ARGs-harboring E. coli isolates. The results demonstrate a high prevalence of ARGs-harboring E. coli in estuarine water and confirm the need for a better wastewater treatment facility and proper control measures to reduce the discharge of sewage and wastewater into the aquatic environments.

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There is a substantial need for tools that effectively predict spatial and temporal fecal pollution patterns in estuarine waters. In this study, statistical models of exceedances of shellfish fecal coliform (FC) water quality criteria were developed using a 10-year dataset of FC levels and environmental data. Performance (sensitivity, specificity, and predictive capacity) of five different types of models was tested (MLR regression, Tobit (censored) regression, Firth's binary logistic regression (BLR), classification trees, and mixed-effects regression) for each of three conditionally managed shellfish-harvesting areas in Tillamook Bay, Oregon (USA). The most influential variables were related to precipitation and river stage height in the wet season and wind and tidal-stage in the dry season. Classification tree and Firth's BLR approaches better explained exceedances of shellfish water quality standards than the current closure thresholds. Findings demonstrate the utility of statistical modeling approaches for improved management of shellfish harvesting waters.

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This study characterized microbiological and chemical contamination of oyster meat and estuarine water in Phang Nga, Thailand. Pooled oyster meats (n = 144), estuarine waters (n = 96) and environmental parameters were collected from March, 2016 to February, 2017, and assessed for levels of total coliforms (TC), fecal coliforms (FC), Escherichia coli (EC), and Vibrio parahaemolyticus (VP), presence of Salmonella and Shigella and levels of heavy metals (Mn, Pb and Cd). The prevalence of TC, FC and EC were in 99.3%, 94.4% and 93.1% of oyster meat and 94.8%, 79.2%, and 78.1% of water, respectively. The average VP levels was 8.5 × 107 most probable number (MPN)/g oyster. Prevalence of Shigella and Salmonella in the pooled oysters were 7.6% and 30.6%, respectively. The dominant Salmonella serovars were Paratyphi B followed by Seremban, and Kentucky. In contrast, the prevalence of Shigella were 27.1%, but Salmonella was not detected in estuarine water. Factors statistically associated with EC accumulation in oyster were level of FC, 7-day average precipitation, temperature, relative humidity, and presence of Salmonella in the sample. The optimal cutoff value of EC to predict Salmonella in oyster was 420 MPN/g. Results indicate this area has relatively safe levels of heavy metals, whereas bacterial contamination was very high for oysters.

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Magnetic nanoparticles (MNPs), based on iron oxide (magnetite) and ferrogel of gelatin and MNPs, were employed as efficient tools for the removal of heavy metals and nutrients from water samples from Bahia Blanca estuarine (BBE). An exhaustive adsorption performance of Cu, NO3-, and NO2- was conducted in batch using model solutions aiming to adjust the adsorption conditions. Adsorption studies using water simulating the real samples were also performed. Both kinds of nanomaterials demonstrated an efficiency between 60 and 80%, and 85% for the removal of heavy metals and NO3- and NO2-, respectively. Similar adsorption assays were performed using BBE water samples, employing the experimental conditions explored with model and simulated water. The reached efficiency was 30 and 45% for heavy metal and nutrient removal, respectively, using nanoparticles; meanwhile, ferrogels displayed a removal capacity around 50-60%. The nanoparticles showed structural instability by the leaching of iron to the medium after the adsorption processes. Ferrogels remained almost inalterable in terms of their integrity during the adsorption time. These materials showed satisfactory perspectives regarding their reuse possibilities. They were used for almost five repeated cycles of adsorption without losing efficiency on the adsorption. The results of this study suggest that MNPs and FGs appear as versatile and promising materials that may be considered as valid alternatives to the actual tools for the remediation of real water samples.

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Zinc (Zn) has been classified as a "Specific Pollutant" under Annex VIII of the EU Water Framework Directive by two thirds of the EU member states. As a result, the UK Environmental Quality Standard (EQS) for Transitional and Coastal (TrAC) Waters has been reduced from 612 nM to 121 nM total dissolved Zn. It is widely accepted that the free metal ion ([Zn(2+)]) is the most bioavailable fraction, but there are few techniques available to determine its concentration in these waters. In this work, Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) has been applied, for the first time, to determine [Zn(2+)] in estuarine waters. The AGNES method had a mean RSD of ±18%, a (deposition time dependent) limit of detection of 0.73 nM and a [Zn(2+)] recovery of 112 ± 19% from a certified reference material (BCR-505; Estuarine Water). AGNES results for 13 estuarine samples (salinity 0.1-31.9) compared well (P = 0.02) with Competitive Ligand Exchange Cathodic Stripping Voltammetry (CLE-AdCSV) except for one sample. AGNES requires minimal sample manipulation, is unaffected by adsorption of interfering species at the electrode surface and allows direct determination of free zinc ion concentrations. Therefore AGNES results can be used in conjunction with ecotoxicological studies and speciation modelling to set and test compliance with water quality standards.