The effect of Zostera noltei recolonization on the sediment mercury vertical profiles of a recovering coastal lagoon.

Mercury's extreme toxicity and persistence in the environment justifies a thorough evaluation of its dynamics in ecosystems. Aveiro Lagoon (Portugal) was for decades subject to mercury effluent discharges. A Nature-based Solution (NbS) involving Zostera noltei re-colonization is being tested as an active ecosystem restoration measure. To study the effect of Zostera noltei on the sediment contaminant biogeochemistry, seasonal (summer/winter) sediment, interstitial water and labile mercury vertical profiles were made in vegetated (Transplanted and Natural seagrass meadows) and non-vegetated sites (Bare-bottom area). While no significant differences (p > 0.05) were observed in the sedimentary phase, Zostera noltei presence reduced the reactive/labile mercury concentrations in the top sediment layers by up to 40% when compared to non-vegetated sediment, regardless of season. No differences were found between vegetated meadows, highlighting the fast recovery of the contaminant regulation ecosystem function provided by the plants after re-colonization and its potential for the rehabilitation of historically contaminated ecosystems.

Zostera noltei response to transplantation into historically Hg-contaminated sediments (A mesocosm experiment): Growth, bioaccumulation and photosynthetic performance.

The present study aimed to evaluate the effectiveness of seagrass recolonization as a nature-based solution for the recovery of a coastal area historically contaminated by mercury (Laranjo Bay, Ria de Aveiro, Portugal). A mesocosm approach was employed to assess the resistance of Zostera noltei to transplantation into contaminated sediments collected in-situ (0.5-20 mg kg-1 Hg). At each sampling time (15, 30, 60, 120 and 210 days), the resistance of transplanted Z. noltei was evaluated through growth parameters (biomass and coverage area), photosynthetic performance and elemental composition. Although some significant differences (p ≤ 0.05) were observed between treatments, essentially associated with the elemental composition of plant tissues, the most relevant variations were associated with seasonality. Overall, plants were found to not be affected by sediment contamination, under the tested concentrations, suggesting that recolonization with Z. noltei can be an effective restoration strategy for historically contaminated coastal areas.

Metal(oid)s accumulation (Hg and As) and their biochemical effects in Halimione portulacoides (Ria de Aveiro, Portugal).

For decades, mercury (Hg) and arsenic (As) were discharged into the Aveiro Lagoon. This study was designed to assess the natural attenuation process evolution by: (1) evaluating Hg and As concentration in sediments and in Halimione portulacoides, and assess the long-term evolution of a historically contaminated salt-marsh system; (2) evaluating the stress levels imposed by Hg and As, and the mechanisms triggered to squash oxidative damage. Concentrations of Hg and As in sediments varied considerably between sampling locations. H. portulacoides did not bioconcentrate Hg and As, restricted translocation to aerial biomass and immobilized contaminants in cell walls. The ions that reached the cytosol induced oxidative stress, restrained by antioxidant mechanisms, especially SOD and CAT activity. Results show that after 25 years of natural attenuation, contamination decreased in the sediment, but remains above non-contaminated systems and continues to induce toxicity in the saltmarsh halophyte community, evidencing the persistent effect of Hg and As contamination.

Mercury bioaccessibility in fish and seafood: Effect of method, cooking and trophic level on consumption risk assessment.

The bioaccessible Hg fraction in fish and seafood commonly present in the Mediterranean diet was assessed through three distinct in vitro extraction schemes. Each extraction method provided different results, highlighting the lack of a universal methodology to estimate mercury (Hg) bioaccessibility in those matrices. Bioaccessible Hg fractions ranged from 10 to nearly 90% of total mercury (T-Hg) and increased in predator species (Swordfish - Xiphias gladius, Blue Shark - Prionace glauca and Tuna - Thunnus sp.). Among the three extractions tested, the Unified Bioaccessibility Method (UBM) provided the highest estimation of Hg bioaccessibility for consumers. The tested cooking procedures (frying, grilling and steaming) considerably reduced the bioaccessible fraction. Results indicate that bioaccessible Hg found in ingested fish and seafood is far below the levels set by the current safety risk assessment legislation. These findings highlight the importance of integrating bioaccessibility measurements in food safety legislation.

Advances on assessing nanotoxicity in marine fish - the pros and cons of combining an ex vivo approach and histopathological analysis in gills.

The need to overcome logistic and ethical limitations of in vivo nanotoxicity evaluation in marine organisms is essential, mostly when dealing with fish. It is well established that medium/solvent conditions affect dispersion and agglomeration of nanoparticles (NPs), which represents a constraint towards a solid and realistic toxicity appraisal. In this way the pros and cons of an ex vivo approach, using a simplified exposure medium (seawater) and addressing gills histopathology, were explored. The nanotoxic potential of environmentally realistic concentrations of titanium dioxide NPs (TiO2 NPs) was also assessed, disclosing the morpho-functional effects on the gills and the possible uptake/elimination processes. Excised gills of the Senegalese sole (Solea senegalensis) were directly exposed in artificial seawater to 20 and 200 µg L-1 TiO2 NPs, for 2 h and 4 h. Semi-quantitative and quantitative histological analyses were applied. The normal morphology of the gill's epithelia was only slightly altered in the control, reflecting protective mechanisms against the artificiality of the experimental conditions, which, together with the absence of differences in the global histopathological index (Ih), corroborated that the gill's morpho-functional features were not compromised, thereby validating the proposed ex vivo approach. TiO2 NPs induced moderate severity and dissemination of histopathological lesions. After 2 h, a series of compensatory mechanisms occurred in NP treatments, implying an efficient response of the innate defense system (increasing number of goblet cells) and effective osmoregulatory ability (chloride cells proliferation). After 4 h, gills revealed signs of recovery (normalization of the number of chloride and goblet cells; similar Ih), highlighting the tissue viability and effective elimination and/or neutralization of NPs. The uptake of the TiO2 NPs seemed to be favored by the higher particle sizes. Overall, the proposed approach emerged as a high-throughput, reliable, accurate and ethically commendable methodology for nanotoxicity assessment in marine fish.

Multiple regression analysis to assess the spatial distribution and speciation of mercury in surface sediments of a contaminated lagoon.

The concentrations of inorganic mercury (IHg) and methylmercury (MeHg) in surface sediments from the contaminated Aveiro Lagoon (Portugal) were determined by species-specific isotope dilution analysis. Different behaviour of IHg and MeHg was observed based upon multiple regression analysis, including geochemical characteristics of the surface sediments (fine fraction, concentrations of organic matter and metals) and non-Euclidean distances between sampling points. This data treatment method was valid over the entire concentration range of IHg and MeHg, allowing robust quantitative evaluation with respect to extrapolation. For IHg, there was statistical separation of the dispersion away from the contamination source and of Al concentration in the sediments. The MeHg concentrations followed those of IHg at high concentrations. The geochemical variables, such as concentrations of Ca (marine influence proxy), Mn and organic matter, were necessary to describe the behaviour of MeHg across the whole concentration range. The models for MeHg demonstrated that, close to the mouth of the lagoon, net production of MeHg was higher. In future, multiple regression analysis could be applied to separate and to evaluate quantitatively the effects of geochemistry and dispersion away from the contamination source in sediments contaminated with other substances.

Mobility versus retention of mercury in bare and salt marsh sediments of a recovering coastal lagoon (Ria de Aveiro, Portugal).

During decades, mercury (Hg) was discharged into the Aveiro Lagoon. Twenty-five years after the cessation of discharges, sediment cores were collected at two areas to assess the evolution of the mercury contamination status. Vertical Hg profiles differed considerably between bare and vegetated sediments. Bare sediments contained significantly less Hg (<8 mg kg-1) than historical data of 1995 (up to 40 mg kg-1), probably resulting from erosion. Salt marsh sediments were marked by a Hg sub-surface enrichment, reaching 44 mg kg-1 in the site closer to the industrial discharge point. High Hg concentrations in 2007, 2011 and 2016 were found at similar sediment layers. These results emphasise the role of halophyte plants in the cycling and retention of Hg in sediments. The persistency of high Hg in the upper marsh layers highlights the complexity in the recovery of historically contaminated marshes and the vulnerability to modifications in hydrology associated with climate changes.

Addressing the impact of mercury estuarine contamination in the European eel (Anguilla anguilla L., 1758) - An early diagnosis in glass eel stage based on erythrocytic nuclear morphology.

The decline of the European eel (Anguilla anguilla L., 1758) population throughout Europe has been partially attributed to pollution. As glass eel estuarine migration may represent a considerable threat, the impact of mercury (Hg) contamination at this stage was evaluated through an in situ experiment (7days). Total Hg (tHg) bioaccumulation was evaluated concomitantly with erythrocytic nuclear morphology alterations: erythrocytic nuclear abnormalities assay (ENA), frequency of immature erythrocytes (IE) and the erythrocytic maturity index (EMI). The ENA results suggested a genotoxic pressure at the most contaminated sites, in line with the tHg increase. The EMI data, together with IE frequency, showed that fish exposed to high levels of Hg exhibited alterations of haematological dynamics, translated into an erythropoiesis increment. Despite the presence of these compensatory mechanisms, the present findings suggest a harmful impact of Hg on genome integrity at this early development stage, potentially affecting eels' condition and ultimately the population sustainability.

Estimation of mercury background values in sediment and biota of the Bijagós archipelago, Guinea-Bissau.

This work evaluates the mercury (Hg) contamination status (sediments and biota) of the Bijagós archipelago, off the coast of Guinea-Bissau. Sediments exhibited very low concentrations (<1-12ngg(-1)), pointing to negligible sources of anthropogenic Hg in the region. Nevertheless, Hg is well correlated to the fine fraction, aluminium, and loss on ignition, indicating the effect of grain size and organic matter content on the presence of Hg in sediments. Mercury in the bivalves Tagelus adansoni and Senilia senilis did not vary considerably among sites, ranging within narrow intervals (0.09-0.12 and 0.12-0.14µgg(-1) (dry weight), respectively). Divergent substrate preferences/feeding tactics may justify slight differences between species. The value 11ngg(-1) is proposed as the sediment background concentration for this West-African coastal region, and concentrations within the interval 8-10ngg(-1) (wet weight) may be considered as reference range for S. senilis and T. adansoni in future monitoring studies.

Field transplantation of the bivalve Scrobicularia plana along a mercury gradient in Ria de Aveiro (Portugal): uptake and depuration kinetics.

The bioaccumulation and depuration capabilities of mercury by the edible bivalve Scrobicularia plana was studied in a coastal lagoon (Ria de Aveiro, Portugal) through a transplantation experiment. Little information on this topic is available in the literature, especially concerning different tissues' responses to contaminant exposure, but the present study is one of the few works that can surpass this knowledge gap. Organisms from a reference area were transplanted to two different contaminated areas in the Ria de Aveiro. In both areas, the bivalves (i.e., entire organism, digestive gland and the rest of the organism) presented a similar saturation model of mercury accumulation, the digestive gland being the tissue that reached the highest concentrations after 25 days of exposure to the contaminant. During this short uptake period, the transplanted organisms reached 20-30% of the concentrations observed in resident contaminated organisms. After the exposure period, the organisms were transplanted to a clean area for more than 25 days of depuration. At the end of the transplantation period, organisms lost approximately 50% of their mercury body burden (60%: the entire organism and digestive gland; 35%: gills and 40%: the rest of the organism) and the ones from the least contaminated site almost reached the concentrations recorded in the reference area. So, the results suggest that S. plana is a promising biomonitoring species, since it accumulates the contaminant in a considerable extent quite rapidly and at the same time it has a low metal retention capacity (low biological half-life) when exposed to clean sediments.

Kinetics of the PO4-P adsorption onto soils and sediments from the Mondego estuary (Portugal).

In order to assess the status of a North Atlantic Ocean Ecoregion (Mondego system, Portugal) after the implementation of a management programme, a study on PO4-P sorption was carried out. Considering that sorption velocity markedly determines the systems promptness to phosphorus external loading, the kinetics of the sorption of PO4-P onto estuarine sediments and the upstream agricultural soils was studied. The pseudo-second order kinetic equation gave an appropriate description of PO4-P sorption onto these soils and sediments, allowing for a quantitative comparison. For soils, the kinetic constants (k2*, µg(-1) g h(-1)) ranged between 0.007 and 0.017 µg(-1) g h(-1) while, in the case of sediments these ranged between 0.008 and 0.012 µg(-1) g h(-1). Results showed that the reduction of water residence time after the management action was relevant for the system status, affecting its reaction capacity to phosphorus loading.

The effects of mercury on the dynamics of the peracarida community in a temperate coastal lagoon (Ria de Aveiro, Portugal).

This study focuses on the impact of mercury on the dynamics and bioaccumulation patterns of suprabenthic peracarida community along a mercury gradient. Suprabenthic peracarida was negatively affected in terms of biomass, diversity and productivity. Mysids including Mesopodopsis slabberi dominated the community, presenting a regular distribution along the Hg gradient, so are considered tolerant species to mercury. Then, isopods, the second most abundant group, dominated in the most contaminated areas being considered also a tolerant group. Contrarily, amphipods were negatively correlated with higher Hg concentrations, dominating in the intermediate areas, thus are considered more sensitive species. In addition, isopods and amphipods were the taxa with higher mercury body burden, followed by mysids. This paper highlights the importance of suprabenthic peracarida as a vehicle of mercury transference through the estuarine trophic web since it is a main link between benthic and pelagic levels and an essential food resource for numerous fish species.

Mercury biomagnification in a contaminated estuary food web: effects of age and trophic position using stable isotope analyses.

The main aim of this study was to ascertain the biomagnification processes in a mercury-contaminated estuary, by clarifying the trophic web structure through stable isotope ratios. For this purpose, primary producers (seagrasses and macroalgae), invertebrates (detritivores and benthic predators) and fish were analysed for total and organic mercury and for stable carbon and nitrogen isotopic signatures. Trophic structure was accurately described by δ(15)N, while δ(13)C reflected the carbon source for each species. An increase of mercury levels was observed with trophic level, particularly for organic mercury. Results confirm mercury biomagnification to occur in this estuarine food web, especially in the organic form, both in absolute concentrations and fraction of total mercury load. Age can be considered an important variable in mercury biomagnification studies, and data adjustments to account for the different exposure periods may be necessary for a correct assessment of trophic magnification rates and ecological risk.

Trace elements in two marine fish species during estuarine residency: non-essential versus essential.

Trace element levels in fish are of particular interest, owing the potential risk to human health. In accordance, juveniles of Dicentrarchus labrax and of Liza aurata were sampled and arsenic, cadmium, chromium, selenium and zinc were determined in the muscle. The levels of trace elements in muscle demonstrated to be similar for both species and sites, with the exception of selenium levels at reference, which seemed to be higher in D. labrax. Moreover, apart from arsenic levels in muscle, all elements were in conformity with the existent regulatory guidelines for fish consumption. The dietary intake of each element was also calculated, with arsenic and selenium showing intakes above the recommended dietary allowances. Nevertheless, no arsenic speciation was carried out and thus no accurate risk evaluation could be established. Additionally, selenium levels never exceeded the dietary allowances more than five times, which are considered safe.

Development and validation of a simple thermo-desorption technique for mercury speciation in soils and sediments.

An innovative technique for rapid identification and quantification of mercury (Hg) species in soils and sediments was developed using a direct mercury analyser. Speciation was performed by the continuous thermal-desorption of mercury species (temperature range 76-770 °C), in combination with atomic absorption spectrophotometry detection. Standard materials HgCl(2), Hg bound to humic acids and HgS were characterized; thermo-desorption curves of each material showed a well-resolved peak at specific temperature intervals: 125-225 °C, 100-250 °C and 225-325 °C, respectively. Certified reference materials (CRM) BCR(®) 142R, RTC(®) CRM 021, NRC(®) MESS-3 and PACS-2 were tested. Although the CRM were not certified for Hg species, the sum of Hg species obtained was compared to the certified value for total Hg; recoveries were 92%, 100%, 97%, and 95%, respectively. One sediment and three soil samples from mercury contaminated areas (total Hg concentrations 0.067-126 mg kg(-1)) were analysed as well. It was possible to compare peaks of thermo-desorption curves from the samples with those from standard materials and thereby distinguish different Hg species in solid samples. Generally, Hg was present as bound to chloride or humic substances. The precision was satisfactory, as reflected by the relative standard deviations determined for standards and certified reference materials (<11%; n=10).

Role of non-enzymatic antioxidants on the bivalves' adaptation to environmental mercury: Organ-specificities and age effect in Scrobicularia plana inhabiting a contaminated lagoon.

This study aimed to investigate the role of non-enzymatic antioxidants on adaptive skills over time in the bivalve Scrobicularia plana environmentally exposed to mercury. Inter-age (2(+), 3(+), 4(+), 5(+) year old) and organ-specific (gills, digestive gland) approaches were applied in bivalves collected from moderately and highly contaminated sites at Ria de Aveiro (Portugal). S. plana's adaptive skills were dependent on the contamination extent; under moderate contamination scenario, the intervention of the different antioxidants took place harmoniously, evidencing an adjustment capacity increasing with the age. Under higher contamination degree, S. plana failed to cope with mercury threat, showing an age-dependent deterioration of the defense abilities. In organ-specific approach, the differences were particularly evident for thiol-compounds, since only gills displayed the potential to respond to moderate levels by increasing non-protein thiols and total glutathione. Under high contamination degree, both organs were unable to increase thiol-compounds, which were compensated by the ascorbic acid elevation.

Evaluation of species-specific dissimilarities in two marine fish species: mercury accumulation as a function of metal levels in consumed prey.

The aim of this research was to compare mercury (Hg) accumulation (total and organic) and tissue distribution in two marine fish species with contrasting feeding tactics. Thus, juvenile specimens of European sea bass and Golden grey mullet were surveyed in an estuary historically affected by Hg discharges. Total Hg was preferentially accumulated in intestine, muscle, and liver, whereas gills and brain presented the lowest Hg levels observed in both species. Significant differences between species were only verified for muscle, with D. labrax's levels being greater than L. aurata's. Muscle accounted for >87% of the Hg relative tissue burden, whereas liver did not exceed 11%. Organic Hg accumulation occurred mainly in liver and muscle, with D. labrax evidencing significantly greater loads. Moreover, organic Hg in consumed prey items was also significantly greater in D. labrax. Accumulation of organic Hg in liver, intestine, and muscle seemed to vary as a function of the consumed prey items contamination, suggesting fish feeding strategies as the dominant factor determining metal accumulation. For both fish species, a stable ratio was observed between Hg increments from the reference to the contaminated site, possibly indicating that the organic Hg content of diet may regulate the internal levels of this contaminant. Thus, this ratio might prove to be a useful contamination predictor tool in early life stages of fish.

Mercury bioaccumulation in the spotted dogfish (Scyliorhinus canicula) from the Atlantic Ocean.

The annual and life-cycle mercury bioaccumulation pattern in selected tissues of the economically relevant Elasmobranchii species Scyliorhinus canicula was studied, and the risks associated with its consumption evaluated. Preferential mercury bioaccumulation occurred in muscle tissue, and followed the order muscle>heart>liver>gills>pancreas. Total mercury in muscle tissue ranged from 0.13mgkg(-1) (wwt) in 1+year old males to 0.8mgkg(-1) (wwt) in 8+ year old mature females, with no significant differences found between genders, and no clear lifespan bioaccumulation pattern observed, except for mature females. Organic mercury in the muscle ranged from 0.05mgkg(-1) (wwt) to 0.52mgkg(-1) (wwt), corresponding to an average of 70% of total mercury content. In mature females, a significant correlation (R=0.99, P=0.01) was found between size and organic mercury fraction, suggesting reproduction as an important factor controlling organic mercury bioaccumulation in the spotted dogfish.

Mercury pollution in Ria de Aveiro (Portugal): a review of the system assessment.

The Ria de Aveiro (Portugal) is a coast al lagoon adjacent to the Atlantic Ocean and it has an inner bay (Laranjo bay) that received a highly contaminated effluent discharged by a mercury cell chlor-alkali plant from the 1950s until 1994. The aim of this study is to review in a holistic way several research studies that have been carried out in the Ria de Aveiro, in order to evaluate the remobilization of the mercury accumulated within the system and the recovery of the lagoon. The spatial distribution of the total mercury in the surrounding terrestrial environment has also been considered. Results indicate that the main mercury contamination problems in the Ria de Aveiro are confined to the Laranjo bay. Mercury export to the coastal waters and its impact on the nearshore compartments (water column, sediment and biota) are low. No direct effects of the mercury from nearby industrial activities were detected in Aveiro's urban soils, although historical mercury contamination is still affecting soil quality in the immediate vicinity of the chlor-alkali plant, located in Estarreja. Moreover, macrophyte harvesting for human direct or indirect use and the consumption of mussels, crabs and the sea bass from the Laranjo bay may constitute a health risk. Further studies focusing on developing skills for the restoration of the ecosystem are presently underway.

Contribution of primary producers to mercury trophic transfer in estuarine ecosystems: possible effects of eutrophication.

There is an ongoing eutrophication process in the Ria de Aveiro coastal lagoon (Portugal), with progressive replacement of rooted primary producers for macroalgae. Taking advantage of a well-defined environmental contamination gradient, we studied mercury accumulation and distribution in the aboveground and the belowground biomass of several salt marsh plants, including the seagrass species Zostera noltii and the dominant green macroalgal species Enteromorpha sp. The results of these experiments were then placed into the context of the estuarine mercury cycle and transport from the contaminated area. All salt marsh plants accumulated mercury in the root system, with Halimione portulacoides showing the highest levels, with up to 1.3 mg kg(-1) observed in the most contaminated area. Belowground/aboveground ratios were generally below 0.4, suggesting that salt marsh plants are efficient immobilizers and retainers of mercury agents. Moreover, due to their sediment accretion capacities, salt marsh plants seem to play an important role in the sequestration of mercury in estuarine sediments. Seagrasses, on the other hand, accumulated considerable amounts of mercury in the aboveground biomass with belowground/aboveground ratios reaching as high as 1.4. These results may be due to their different routes of uptake (roots and foliar uptake) which suggests that seagrass meadows can be an important agent in the export of mercury from contaminated areas, considering the high aboveground biomass replacement rates. Rooted macrophytes accumulate less mercury in their aboveground biomass than macroalgae. The change of primary producer dominance due to eutrophication can originate a 4- to 5-fold increase in primary producer associated mercury. This mercury would be available for export, making it bioavailable to estuarine food webs, which stresses the need to reverse the current eutrophic status of estuarine systems.