Importance of salinity on regulating the environmental fate and bioaccumulation of lithium in the Yangtze River Estuary.

The demand of lithium (Li) has increased rapidly in recent decades under carbon neutrality strategies, but the environmental fate and potential risks of Li in aquatic ecosystem are barely known. This study conducted a comprehensive field survey in the Yangtze River Estuary (YRE) and the adjacent East China Sea (ECS), to investigate the spatial distribution of dissolved Li and bioaccumulation of Li in the coastal food web. The dissolved Li increased with salinity (from 7.39 to 189 µg L-1), controlled by the conservative mixing of Li-enriched seawater and Li-poor riverine water. Negative correlation was observed between Li content and stable nitrogen isotope in the coastal biota, indicating bio-diminish of Li in the food web. Furthermore, the Li contents in muscle tissues were significantly higher in bivalves (as filter-feeders; mean: 0.75 ± 0.41 µg g-1) than in fish (as predator; mean: 0.10 ± 0.05 µg g-1) and other biota species, indicating that dissolved uptake might be the major exposure pathway for Li. Importantly, it was noticed that the bioaccumulation factors (BAFs) in fish muscle varied greatly (from 0.17 to 5.82), showing lower BAFs for fish inhabiting in marine and benthic regions (with higher salinity and higher dissolved Li concentration). Such inhibition effects of salinity on Li bioaccumulation could not be explained by the modulation of salinity on Li speciation, but highly attributed to the inhibition of high salinity on the dissolved uptake of Li, which was associated with the co-transportation of Li and Na. Our results illuminate the importance of salinity on regulation the spatial variations of dissolved Li and Li bioaccumulation in the YRE and the adjacent ECS, which could help the understanding of Li biogeochemical cycling and potential risks in estuarine and coastal regions.

Quantifying the combined impacts of anthropogenic CO2 emissions and watershed alteration on estuary acidification at biologically-relevant time scales: a case study from Tillamook Bay, OR, USA.

The impacts of ocean acidification (OA) on coastal water quality have been subject to intensive research in the past decade, but how emissions-driven OA combines with human modifications of coastal river inputs to affect estuarine acidification dynamics is less well understood. This study presents a methodology for quantifying the synergistic water quality impacts of OA and riverine acidification on biologically-relevant timescales through a case study from a small, temperate estuary influenced by coastal upwelling and watershed development. We characterized the dynamics and drivers of carbonate chemistry in Tillamook Bay, OR (USA), along with its coastal ocean and riverine end-members, through a series of synoptic samplings and continuous water quality monitoring from July 2017 to July 2018. Synoptic river sampling showed acidification and increased CO 2 content in areas with higher proportions of watershed anthropogenic land use. We propagated the impacts of 1). the observed riverine acidification, and 2). modeled OA changes to incoming coastal ocean waters across the full estuarine salinity spectrum and quantified changes in estuarine carbonate chemistry at a 15-minute temporal resolution. The largest magnitude of acidification (-1.4 pH ⊤ units) was found in oligo- and mesohaline portions of the estuary due to the poor buffering characteristics of these waters, and was primarily driven by acidified riverine inputs. Despite this, emissions-driven OA is responsible for over 94% of anthropogenic carbon loading to Tillamook Bay and the dominant driver of acidification across most of the estuary due to its large tidal prism and relatively small river discharges. This dominance of ocean-sourced anthropogenic carbon challenges the efficacy of local management actions to ameliorate estuarine acidification impacts. Despite the relatively large acidification effects experienced in Tillamook Bay (-0.16 to -0.23 p H ⊤ units) as compared with typical open ocean change (approximately -0.1 pH ⊤ units), observations of estuarine pH ⊤ would meet existing state standards for pH ⊤ . Our analytical framework addresses pressing needs for water quality assessment and coastal resilience strategies to differentiate the impacts of anthropogenic acidification from natural variability in dynamic estuarine systems.

Thermally Primed Zostera muelleri Seeds Exhibit Higher Germination Rates Than Those From Ambient Conditions.

Seagrasses provide critical ecosystem services such as carbon sequestration, sediment stabilisation and nursery habitat for juvenile fish. Zostera muelleri is ubiquitous within Australian and New Zealand estuaries, however, as a species is relatively understudied. We sourced seeds from a thermally affected east Australian estuary and investigated whether germination rates differed between ambient and thermally affected seeds over a variety of temperatures (16°C-28°C) to determine how seagrass systems might react in a warming climate. Germination for the experiment was low and totalled 5% of all seeds; however, similar numbers are typical in seed germination studies. Germination was highest at 16°C and was enhanced through the simulation of a 48-h freshwater pulse. Thermally affected sites germinated faster and had greater mean maximum germination when compared to control sites regardless of experimental temperature. These findings indicate that Z. muelleri in this system may be exhibiting transgenerational plasticity due to the thermal stress the parent experiences. This result provides an alternate viewpoint to the current literature by suggesting that unknown transgenerational effects may provide Z. muelleri with greater germination plasticity against temperatures expected under predicted climate change scenarios than previously expected.

Methylation of mercury and tin by estuarine microbial mats.

After tin and mercury salts were added to estuarine microbial mats increasing amounts of methyltin and methylmercury, respectively, were formed over a 30 to 100 hour time period. Inhibition of the methylation by molybdate, a metabolic inhibitor of sulfate reduction, stimulation by pyruvate addition and lack of methylation by sterilized mats, were evidence that sulfate reducing bacteria within the mats were responsible for the tin and mercury methylation. Methyl mercury was formed from mercuric chloride and mercuric cysteine, but not from mercuric sulfide. We suggest that mercury bound to organic complexes in anoxic sediments is likely methylated by microbial mats. Since estuarine meiofauna and macrofauna fed on microbial mats, the methylmercury and methyltin formed by microbial mats could be an important avenue for the entrance of these compounds into the marine food web.

Characterization of sediment organic matter in the outer Yangtze River Estuary using stable isotopes, optical techniques, and FT-ICR-MS: Implications for the carbon burial mechanism.

The burial of sediment organic matter (SOM) in the estuary and shelf plays an important role in the global carbon cycle. However, it is challenging to determine the source, composition, and burial of SOM in the coastal sea, especially at the molecular level. This was explored in the coastal area outside the largest Yangtze River of China with multiple techniques including elemental and stable isotopic analysis, absorption spectroscopy, fluorescence excitation-emission matrices coupled with parallel factor analysis (EEMs-PARAFAC), and ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The end-member mixing analysis based on δ13C and δ15N showed a dominance of marine contribution (up to 70%) at most stations while the terrestrial contribution increased to >55% nearshore in summer at a high fluvial sediment flux. This was consistent with the offshore decreasing humic-like C1 and C2, humification index (HIX), %lignin-like compounds, and %CHO but increasing tryptophan-like C3, biological index (BIX), %protein-like compounds, and %CHOS from EEMs-PARAFAC and FT-ICR-MS analysis. The %clay correlated positively with SOM content, HIX, %lignin-like compounds, O/C, and modified aromaticity index (AImod) but correlated negatively with %C3, H/C, and the relative abundance of labile formulas (MLBL), while %silt showed contrasting correlations. These results indicated the fine clay sediments adsorbed more humified, aromatic, oxygenated, and terrestrial compounds that were probably more resistant to biodegradation and thus had a higher burial efficiency than those on the silty sediments. Principal component analysis based on SOM indices further revealed different characteristics of SOM in the nearshore, northern offshore, and southern offshore regions, which were probably dependent on the delivery by local current systems. Overall, these findings contributed to unraveling the source and molecular composition of SOM associated with different grain size sediments and local current delivery, which are fundamental for understanding the factors underlying carbon burial in the complex coastal environment.

Microplastic contamination in three environmental compartments of a coastal lagoon in the southern Gulf of Mexico.

The Sontecomapan lagoon (Mexico) is a Ramsar site within the Los Tuxtlas Biosphere Reserve, facing the Gulf of Mexico. Although the site has a protected area status, it is vulnerable to microplastic contamination, whose long-term effects are uncertain. This study gives the first approach to the degree of contamination by microplastics in surface waters, zooplankton, and sediments in the lagoon. The samples in these three environmental compartments were collected in June 2018 and analyzed in the laboratory to extract and quantify the microplastics. The microplastics sampled were classified into fibers, fragments, and foams and identified as polyester, acrylic, and rayon, among others. In the surface waters, the mean concentration of microplastics was 7.5 ± 5.3 items/L, which is higher than the values registered in other protected coastal systems, perhaps because of differences in the methods used. Zooplankton, represented by copepods, luciferids, and chaetognaths, showed concentrations of 0.002 ± 0.005, 0.011 ± 0.011, and 0.019 ± 0.016 items/individual, respectively. These values were low compared to systems with high anthropic influence, and the differences between the three kinds of organisms were attributed to their feeding habits. In the sediments, the mean concentration was 8.5 ± 12.5 items/kg, lower than the values registered in sites of high human impact; the maximum value here found (43 items/kg) was recorded in the internal part of a lagoon arm of almost stagnant water. In general, the degree of contamination by microplastics in the lagoon was low; however, their presence indicates a potential risk to the biota.

Deciphering the natural and anthropogenic drivers on the fate and risk of antibiotics and antibiotic resistance genes (ARGs) in a typical river-estuary system, China.

This study conducts an in-depth assessment of the spatial distribution, ecological risks, and correlations among 12 antibiotics, antibiotic resistance genes (ARGs), and dominant microorganisms in a representative river-estuary system, classified by land use and hydrodynamic conditions. Sulfonamides and quinolones were identified as the major contaminants in surface waters, with aquaculture and healthcare wastewater responsible for over 80 % of the antibiotic load. Contrasting seasonal patterns were observed between freshwater (wet season: 215 ng/L, dry season: 99.9 ng/L) and tidal estuaries (wet season: 45.9 ng/L, dry season: 121 ng/L), attributed to antibiotic transport from terrestrial sources or coastal aquaculture areas. The estimated annual antibiotic influx into Jiaozhou Bay was 70.4 kg/year, posing a considerable threat to aquatic algae and disrupting the stability of aquatic food chain. BugBase predictions suggested that antibiotics in the environment suppressed bacteria characterized by biofilm formation (FB) and the presence of mobile elements (CME). However, ARG transmission was likely to drive the spread of CME, FB, and stress-tolerant (OST) bacteria within microbial communities. The significant positive correlations observed between sulfamethoxazole and 63 microbial genera indicate a broad distribution of microbial resistance, which exacerbates the potential for ARG accumulation and dissemination across both the bay and the Yellow Sea.

Spring resting egg production of the calanoid copepod, Eurytemora affinis, in a freshet-dominated estuary.

Seasonal peaks in river discharge, such as snowmelt-dominated freshets, are predictable events that can have a large effect on flushing rates and salinity in estuaries. Resting eggs, which many coastal and estuarine copepods produce for overwintering or aestivation, could also serve to bridge predictable peaks in river discharge. We assessed the timing of resting egg production of the egg-carrying estuarine copepod, Eurytemora affinis (Poppe), in relation to river discharge in the Fraser River Estuary, Canada. Approximately 30 field-collected females were individually incubated on 12 occasions over the period February 2015-May 2016. Eurytemora affinis abundance and population structure were investigated from vertical net tow samples collected twice monthly to monthly. Resting eggs occurred primarily in May 2015 and May 2016 (6.5 and 9.2 eggs day-1, respectively), a month prior to peak flows, and the proportion of offspring that were resting eggs increased with river discharge. Eurytemora affinis reached a minimum abundance in July 2015, when the population was dominated by adults (86%). Resting egg production in E. affinis is typically considered an overwintering mechanism but we suggest that the ultimate driver of resting egg production in this population is avoidance of flushing and/or low salinities.

Plastic pollution on beaches in an Antillean manatee conservation area, Brazil.

The presence of plastic debris along shorelines is a widespread problem, and plastic debris can interact directly with marine wildlife. In northeast Brazil, the Antillean manatees are the most common marine mammals and use estuarine waters mainly as their home range. We aimed to investigate the characteristics of plastic debris on the estuarine-oceanic continuum and the accumulation density of plastic on beaches. The density of plastic debris on beaches located in transitional areas was 7-10-fold greater than that on beaches in estuarine and oceanic areas. Fragments were considered the most abundant form of plastic debris on all the beaches. Our findings could lead to improved pollution management in this marine protected area, principally due to the risks of remobilizing small plastic particles through tides and wind effects that scatter them in resting and feeding manatees areas.

Assessment of metal-associated health risk in different trophic levels in tropical estuary on the southeast coast of India.

Metal contamination in coastal and marine ecosystems has become a significant global concern due to its hazardous characteristics, environmental persistence, and ability to bioaccumulate in aquatic ecosystems. This poses a serious threat to the environment and the health of humans worldwide. To address these concerns, this study estimated the concentrations of metals in various trophic levels, including phytoplankton, zooplankton, bivalve, and fish. Monthly sampling was carried out in Pondicherry Fishing Harbor (PFH) and Pondicherry Open Sea (POS) between January 2017 and December 2018. The value of Cd, Pb, Cr, Co, Ni, Cu, Zn, and Mn in sediment at the PFH was considerably higher compared to the POS, indicating the impact of human activity there. The contamination factor (CF) for other metals was <0.5, suggesting minor contamination in the Pondicherry coastal sediment, the CF value for Cd was higher at PFH. Comparably, the risk index (RI) at the PFH was likewise greater because of Cd, leading to an overall risk grade of "considerable" at the PFH whereas it was "low" at the POS. The marine pollution index (MPI) showed minimal values in fish regardless of the collection sites, which was calculated based on the values of all metals. The estimated daily intake, target hazard quotient, and hazard index assessed for potential human health risk suggest that the values were within acceptable thresholds for adults and children for fish consumption from POS. However, the direct consumption of bivalve for the long term poses significant non-carcinogenic health risks in both age groups, particularly in children, who are 1.31 times more susceptible than adults. These findings highlight the need to evaluate the presence of metals in the food chain to determine their transfer to the different trophic levels, which can help mitigate the associated risks for sustainable coastal ecosystem management.

Distribution and control mechanism of pCO2 and water-air CO2 efflux in the Pearl River Estuary.

Estuaries are generally considered to be important sources of atmospheric CO2. However, the differences between estuaries, and inadequate observations of partial pressure of CO2 in estuarine water (pCO2water) hamper global estuarine CO2 budgeting. In this study, the longitudinal distribution of CO2 in the waters of Modaomen (MSE) and Lingdingyang (LSE), two sub-estuaries of the Pearl River Estuary (PRE), and its influencing mechanism are studied. The change in the distribution of pCO2water along the distance from the upstream estuary to the ocean between LSE and MSE was significantly different. pCO2water at the LSE ranges from 238 to 7267 µatm, whereas the MSE ranges from 406 to 3078 µatm. Stronger microbial respiration and relatively long water retention times were the main influences that led to higher pCO2water at LSE than at MSE. Seasonally, the increase of soil CO2 into the water in the upstream basin caused by precipitation is the potential influencing factor that the water pCO2water in the flood season is higher than in the dry season. PRE was a net source of atmospheric CO2 with an average annual water-air flux of 41.2 ± 33.3 mmol m-2 day-1. Our results suggest that the differences in longitudinal gradients of pCO2water between estuaries in the same region and the effects of different gas transport velocity models on CO2 emission estimates need to be considered in estuarine CO2 emission budgeting.

Microplastics pollution in sediments of the Thames and Medway estuaries, UK: Organic matter associations and predominance of polyethylene.

Microplastics at 10 sites along a 77 km transect of the river Thames estuary (UK) and 5 sites along 29 km of the Medway estuary were separated from sediment and analysed by ATR-FTIR spectroscopy. Microplastics were observed at all sites. Highest Thames concentrations were in urban London between Chelsea and West Thurrock (average 170.80 particles kg-1 ± 46.64, 3.36 mg kg-1 ± 1.79 by mass), mid-outer estuary sites were two to three times lower. Microplastics were slightly dominated by particles (54 %) over fibres (45 %), including polymer types ranked: polyethylene > PET > polypropylene > polyamide. Medway microplastics decreased seaward, with one urban-municipal site impacted by a combined-sewer-overflow containing a high proportion of fibres (Rochester, 484 particles kg-1, 7.39 mg kg-1 by mass). Microplastic abundance was correlated to organic carbon (TOC %) (R2 of 0.71 Thames and 0.96 Medway), but not sediment particle size. Sedimentary microplastics accumulation in the Thames was controlled by urbanisation-distance, and site hydrodynamics.

Interactions of multiple stressors on the Bombay-duck Harpadon nehereus population in a complex estuarine ecosystem.

In an era marked by unprecedented anthropogenic change, marine systems are increasingly subjected to interconnected and dynamic external stressors, which profoundly reshape the behavior and resilience of marine ecological components. Nevertheless, despite widespread recognition of the significance of stressor interactions, there persist notable knowledge deficits in quantifying their interactions and the specific biological consequences that result. To bridge this crucial gap, this research detected and examined the causal relationships between five key exogenous stressors in a complex estuarine ecosystem. Furthermore, a Bayesian Hierarchical Spatio-temporal modeling framework was proposed to quantitatively evaluate the distinct, interactive, and globally sensitive effects of multiple stressors on the population dynamics of a crucial fish species: Harpadon nehereus. The results showed that interactions were detected between fisheries pressure (FP), the Pacific Decadal Oscillation index (PDO), runoff volume (RV), and sediment load (SL), with five of these interactions producing significant synergistic effects on H. nehereus biomass. The SL*PDO and RV*PDO interactions had positive synergistic effects, albeit through differing processes. The former interaction amplified the individual effects of each stressor, while the latter reversed the direction of the original impact. Indeed overall, the synergistic effect of multiple stressors was not favorable, with FP in particular posing the greatest threat to H. nehereus population. This threat was more pronounced at high SL or negative PDO phases. Therefore, local management efforts aimed at addressing multiple stressors and protecting resources should consider the findings. Additionally, although the velocity of climate change (VoCC) failed to produce significant interactions, changes in this stressor had the most sensitive impacts on the response of H. nehereus population. This research strives to enhance the dimensionality, generalizability, and flexibility of the quantification framework for marine multi-stressor interactions, aiming to foster broader research collaboration and jointly tackle the intricate pressures facing marine ecosystems.

Heavy Metal Accumulation in Oysters from an Aquaculture Area in the Luoyangjiang River Estuary.

Oysters are a group of economically important bivalves in China, with estuaries serving as one of their primary cultivation areas. However, heavy metal pollution in these estuarine environments poses a potential threat to aquaculture by leading to the accumulation of heavy metals in farmed oysters, which could impact their safety and marketability. This study was conducted in the aquaculture area of the Luoyangjiang River estuary, where eight sampling sites were selected. Water, sediment, and oysters categorized by shell length were collected from each site. The concentrations of heavy metals (Ag, As, Cd, Cr, Cu, Ni, Pb, and Zn) were determined in both the environmental samples and oyster tissues. Additionally, multiplex species-specific PCR was used to identify oyster species. The results showed significant variations in dissolved-phase and suspended particulate matter (SPM) metal concentrations across different sampling sites, while sediment metal concentrations were more consistent but similar to those in SPM. The large oysters were comprised of 50% Magallana angulata and 50% Magallana gigas, while small oysters were identified as Magallana sikamea. The Cd, Cu, Pb, and Zn levels in both size groups of oysters exceeded data from previous studies, indicating contamination in the estuary. The observed differences in heavy metal concentrations between large and small oysters primarily reflect species-specific variability in metal accumulation, which may also be influenced by factors such as growth and exposure duration. Furthermore, the lack of significant correlation between metal concentrations in environmental media and oysters suggests that oysters may be exposed to multiple sources of metal contamination.

Occurrence, virulence, and AMR profile of Vibrio parahaemolyticus isolated from shellfish growing areas located along the south-west coast of India.

Vibrio parahaemolyticus is a leading cause of human gastroenteritis associated with seafood consumption. The present study aimed to investigate the occurrence and risk assessment of V. parahaemolyticus isolated from live Indian black clams, sediment, and water samples collected from shellfish harvesting areas located along the south-west coast of India. Out of the total 72 samples collected, 55.6% revealed the presence of V. parahaemolyticus; the highest occurrence was observed in shellfish samples. The presence of tdh and trh virulence genes was screened by multiplex PCR. Virulence genes could be detected in 25.8% of the strains; 19.35% of them were trh positive and 3.2% were tdh positive, while 3.2% of strains exhibited the coexistence of both virulence genes. Antimicrobial resistance (AMR) determined by the disk diffusion method revealed that 87% of the strains were multiple drug resistant and exhibited 21 diverse resistance patterns. The overall multiple antibiotic resistance (MAR) index values ranged from 0 to 0.8. To the best of our knowledge, this is the first report to document the presence of pathogenic and multidrug-resistant V. parahaemolyticus in shellfish harvesting areas of the Indian sub-continent. The study reveals possible health hazards associated with consuming shellfish harvested from the study area.

Distribution of habitat suitability for Suaeda salsa in the Liaohe River Estuary and its relationship with carbon storage.

Suaeda salsa acts as a natural barrier between land and sea in estuarine wetlands while also serving as a significant source of carbon storage. Understanding the synergistic relationship between the spatial distribution of Suaeda salsa habitat suitability and its carbon storage capabilities is essential for guiding ecological restoration and bolstering the carbon sequestration potential of wetlands. Drawing on field survey data from the Liaohe River Estuary wetlands collected in 2021 and 2022, we applied the Maxent model to delineate the spatial distribution of Suaeda salsa habitat suitability. The findings indicate that the principal environmental determinants for the Suaeda salsa community are elevation, soil phosphorus, and sand and soil salt content. Habitats can be divided from into four classes with increasing suitability index: I, II, III, and IV. As the most favorable habitat, class IV spanned 33.07 km2 and constituted 22.37 % of the area. By integrating remote sensing and ground survey data, a carbon storage evaluation model for Suaeda salsa was developed. This model revealed that the carbon storage within the Liaohe River Estuary reached 8238.18 and 16,194.08 tons in 2021 and 2022, respectively. Finally, the spatial overlay analysis of habitat suitability distribution and carbon storage revealed that an increase in habitat suitability led to an increase in carbon storage in Suaeda salsa, which was also influenced by the surrounding land use types. This work demonstrates a significant positive correlation between habitat suitability and carbon sequestration capacity. In the future, we suggest that ecological restoration projects in the Liaohe Estuary region focus on areas with higher habitat suitability. This study supports the sustainable management of the Suaeda salsa community and provides a theoretical basis for enhancing the carbon sequestration capacity of estuarine wetlands.

Distribution and ecological risk assessment of perfluoroalkyl carboxylic acids (PFCAs) in the coastal river, China.

Perfluoroalkyl carboxylic acids (PFCAs) enter surface waters through wastewater discharge and atmospheric deposition, accumulating within aquatic ecosystems and organisms, causing dysfunction or mortality. This study investigates the migration, sources and ecological risks of six PFCAs in the coastal Wulong River basin and the estuary along the eastern coast of China. The six PFCAs detected in the Wulong River included, at concentrations ranging from 17.36 to 57.92 ng/L. The distribution of PFCAs concentrations throughout the estuary were as follows: Northeast China (414.07 ng/L) > North China (325.97 ng/L) > East China (249.53 ng/L) > South China (63.61 ng/L), with perfluorooctanoic acid (PFOA) being the dominant PFCA. Toxicity data was collected for all detected PFCAs and the species sensitivity distribution (SSD) method was used to derive the predicted no effect concentrations (PNECs) for all six PFCAs. The risk quotient (RQ) method showed that the risk posed to aquatic organisms in the river basin from all six PFCAs was relatively low (RQ < 1). However, joint probability curve (JPC) analysis revealed that the probability of perfluoropentanoic acid (PFPeA) causing harm to 5% of aquatic species reached 41.13%. Considering the frequency of occurrence and persistence of PFCAs in aquatic environments, these findings indicate they have the potential to induce serious toxic effects on aquatic organisms.

Heavy metals and metalloid contamination and risk evaluation in the surface sediment of the Bakkhali River estuary in Bangladesh.

Current state of contamination and subsequent risk of contaminated sediment of a tidal river of Bangladesh was evaluated in the present study. Sediment samples were collected from five locations in the tributary of Bakkhali River estuary during summer (April) and winter (December) season, 2020. Collected samples were processed using standard protocol and the content of heavy metals (Cd, Cr, Cu, Pb and Zn) and metalloid (As), were analyzed by the Flame Atomic Absorption Spectrometer. Sediment contamination was evaluated by pollution load index (PLI), contamination factor (CF), degree of contamination (Cd), potential ecological risk index (PERI), non-carcinogenic and carcinogenic risk (CR) due to the dermal contact of the sediment. Multivariate statistical analysis such as principal component analysis (PCA) and cluster analysis (CA) were also applied to find out the possible sources of the contaminant in the sediment. Results showed the average concentration of As, Cd, Cr, Pb, Cu and Zn was 9.74 ± 3.57, 2.00 ± 0.85, 48.75 ± 8.92, 29.78 ± 8.39, 5.44 ± 2.03 and 56.94 ± 8.57 mg/kg, respectively. Concentration of Cu, Pb and Zn were within the recommended level whereas the concentration of As, Cd and Cr were suppressed the recommended level of WHO and FAO/WHO standards. PLI, CF and Cd revealed considerably low degree of contamination of the sediment. Geo-accumulation index indicated uncontaminated to moderately contaminated condition of the sediment. Although the values of enrichment factor revealed no potential enrichment for most of the metals, Cd showed a minor enrichment during the winter season. Based on the ecological risk assessment, the sediment from all of the sample locations was found to be of moderate to low risk. PCA and CA analysis revealed the origin of contaminants mainly from anthropogenic sources. Although different metals showed non-carcinogenic risk to the inhabitants, cancer risk values for dermal contact (CRderm) were much lower than 10-6 indicating no cancer risk for adult and child. However, the findings also revealed that children were more susceptible to CRderm compared to adults. The present study concluded that long term dermal contact of the sediment of Bakkhali River estuary will be contagious to the people. Therefore, regular monitoring of the estuarine environment is necessary so that contamination does not get worse.

Intertidal microphytobenthic primary production and net metabolism of a tropical estuary.

Tidal flats are inhabited by benthic microalgae (microphytobenthos, MPB) supporting important ecosystem functions and services. Studies on MPB have been conducted mainly in temperate systems, despite that the majority of tidal flats on Earth are found in the tropical zone (∼55%). To fill this gap of knowledge and evaluate the contribution of tidal flat MPB in one of the most productive estuaries worldwide, sediment cores were collected from 14 stations along the inner Gulf of Nicoya (Costa Rica) at different tidal heights or sea levels (SL) from October 2013 to April 2014. MPB abundance, using chlorophyll a (Chla) biomass as a proxy, and net primary production (PN) and dark respiration (RD), using O2 microsensors, were measured together with other sediment biogeochemical variables in muddy and sandy sediments. Landsat-8 satellite images were used to map the extension of tidal flats and the abundance of MPB using Normalized Difference Vegetation Index (NDVI) as a proxy. Chla ranged from 0.45 to 7.45 µg cm-2, with higher concentrations observed closer to the river mouth. There was no significant correlation between Chla and SL nor with any other sediment physicochemical variable. MPB abundance estimated by remote sensing displayed considerable spatial heterogeneity, both within and among tidal flats, and clear seasonal differences with higher abundance during the rainy season. PN ranged between 0.8 and 8.6 mmol O2 m-2 h-1, being positively correlated to SL and to the accumulated rain during 30 days prior to the sampling date and inversely to the mean irradiance at noon during the previous month. Daily net community metabolism estimated from PN and RD data and converted to carbon fixation rates indicates that the unvegetated tidal flats of the gulf contribute as much as the total input of allochthonous C from the Tempisque River. However, the results presented here have to be confirmed with direct measurements of C transfer, including the contribution of the adjacent mangrove system. Such studies are crucial to assess the local, regional and global importance of production and other ecosystem services by MPB in tropical areas.

Temperature differentially regulates estuarine microbial N2O production along a salinity gradient.

Nitrous oxide (N2O) is atmospheric trace gas that contributes to climate change and affects stratospheric and ground-level ozone concentrations. Ammonia oxidizers and denitrifiers contribute to N2O emissions in estuarine waters. However, as an important climate factor, how temperature regulates microbial N2O production in estuarine water remains unclear. Here, we have employed stable isotope labeling techniques to demonstrate that the N2O production in estuarine waters exhibited differential thermal response patterns between nearshore and offshore regions. The optimal temperatures (Topt) for N2O production rates (N2OR) were higher at nearshore than offshore sites. 15N-labeled nitrite (15NO2-) experiments revealed that at the nearshore sites dominated by ammonia-oxidizing bacteria (AOB), the thermal tolerance of 15N-N2OR increases with increasing salinity, suggesting that N2O production by AOB-driven nitrifier denitrification may be co-regulated by temperature and salinity. Metatranscriptomic and metagenomic analyses of enriched water samples revealed that the denitrification pathway of AOB is the primary source of N2O, while clade II N2O-reducers dominated N2O consumption. Temperature regulated the expression patterns of nitrite reductase (nirK) and nitrous oxide reductase (nosZ) genes from different sources, thereby influencing N2O emissions in the system. Our findings contribute to understanding the sources of N2O in estuarine waters and their response to global warming.