Chaine trophique experimentale en milieu limnique: Contaminations directe et trophique d'un consommateur de 3ème ordre (Salmo gairdneri) par le methylmercure

Within the context of ecotoxicological research on an experimental trophic chain, the analysis of tissue spread of Hg with Salmo gairdneri, after global contamination by methylmercury, brings out a clear preponderance of bioaccumulated metal of a trophic origin. This is all the more visible as the length of time for the experiment and the temperature of the environment are higher (18°C and 30 days). After global contamination, various kinds of organs can be distinguished according to Hg concentration and to the effect of abiotic factors on accumulation processes: Brain and muscles: their ecotoxicological behavior is the same as that of the whole fish; Hg tenors are fairly low but their increase is quite significant when the temperature of the environment and contamination time are higher. Posterior intestine: the high Hg concentrations and the high retention capacity make it an accumulation organ. After 30 days' experimenting the trophic contribution represents over 95 % of the Hg in that organ. Gills: contrasting with the intestine gills are characteristic of direct contamination. Their metal concentration are fairly independent of environmental temperature and length of time. Figured elements and liver: they present the highest Hg concentrations; as with gills, the influence of experimenting time is slight.     

Chaine trophique experimentale en milieu limnique: Contamination directe d'un consommateur de troisieme ordre (Salmo gairdneri) par le methylmercure

Within the context of ecotoxicological research on an experimental trophic chain, in freshwater environment, we have studied the spread of Hg in the tissues of the final consumer — Salmo gairdneri —, after direct contamination by the CH₃HgCl (1 ppb in water). The experiment carried out at 10, 18 and 26°C for a 10-day and a 30-day exposure, has made it possible to analyse the various transfers between the outer environment and the main organs of the fish. A comparative study of tissue concentrations of the metal, according to abiotic conditions, shows proof of the transit and accumulative functions of the studied organs in a ‘chronic’ contamination by Hg through direct absorption.     

Conservation implications of the genetic and ecological distinction of Tursiops truncatus ecotypes in the Gulf of California

The demand for live bottlenose dolphins for commercial use is growing in Mexico, making the need for stock assessment and management ever more essential given their protected status. Tursiops truncatus is known to exhibit high levels of phenotypic polymorphisms. In the Gulf of California (GC), coastal and offshore ecotypes have been identified based on morphological, behavioral and ecological evidence, including different prey and habitat preferences. However, the extent to which this ecological and phenotypic variation is genetically correlated is unknown. Here we assess this correlation in GC bottlenose dolphins classified as coastal or offshore based on habitat, morphological and trophic evidence. Significant (p < 0.0001) haplotype heterogeneity (exact test) and genetic differentiation (F_ST = 0.069) were found in the mitochondrial control region, indicating some reproductive isolation between ecotypes. As elsewhere, coastal dolphins were less diverse than offshore. Phylogenetic analyses revealed paraphyletic coastal and offshore lineages and no evidence of lineage sorting, possibly due to recent isolation or gene flow. This is the first time that genetic, morphological and stable isotope evidence has been used to recognize ecotypes as different stocks for management purposes in bottlenose dolphins. Our results indicate that diversifying forces are shaping their phenotypic and genetic variation in the GC. Management and conservation efforts in this strategic region should aim to preserve these forces.     

Driftnet fishing and biodiversity conservation: the case study of the large-scale Moroccan driftnet fleet operating in the Alboran Sea (SW Mediterranean)

Illegal, large-scale driftnets are still used in several Mediterranean areas. According to international official sources, Morocco harbors the bulk of this fleet in the Mediterranean. To ascertain its biodiversity impact, 369 fishing operations (worth 4140 km of driftnets set) made by the driftnet fleet targeting swordfish (Xiphias gladius) based in Al Hoceima (Alboran Sea) were monitored between December 2002 and September 2003. Parallel surveys were made in the main Mediterranean ports and in that of Tangiers, in the Gibraltar Straits, to estimate the total fishing effort. Results showed an active driftnet fleet conservatively estimated at 177 units. Estimated average net length ranges from 6.5 to 7.1 km, depending on the port, though actual figures are suspected to be much higher (12-14 km). Most boats perform driftnet fishing all year round, resulting in very high annual effort levels. A total of 237 dolphins (short-beaked common dolphin, Delphinus delphis, and striped dolphin, Stenella coeruleoalba), 498 blue sharks (Prionace glauca), 542 shortfin makos (Isurus oxyrinchus) and 464 thresher sharks (Alopias vulpinus) were killed by the boats monitored during the sampling period, during the peak of the swordfish fishery, along with 2990 swordfish. Loggerhead turtle (Caretta caretta) was also caught (46 individuals). Estimates for a 12-month period by the whole driftnet fleet yielded 3110-4184 dolphins (both species) and 20,262-25,610 pelagic sharks distributed in roughly equal proportions for P. glauca, I. oxyrinchus and A. vulpinus, in the Alboran Sea alone; further 11,589-15,127 dolphins and 62,393-92,601 sharks would be killed annually around the Straits of Gibraltar. Dolphins suffer from annual take rates exceeding 10% of their population sizes in the Alboran Sea; this unsustainable impact is particularly worrying for D. delphis, because its last remnant healthy population in the Mediterranean occurs in this area. Average catch rate for swordfish, the main target species, amounted to only 0.8 individuals/km net set. Pelagic sharks are actively targeted by a part of the fleet.     

The influence of trophic level as measured by δ15N on mercury concentrations in freshwater organisms

The relationship between mercury (Hg) concentrations in freshwater biota and trophic position, as defined by stable nitrogen isotope ratios (δ¹⁵N), was examined in 6 lakes in northwestern Ontario. The heavier isotope of nitrogen (¹⁵N) increases an average of 3 parts per thousand (‰) from prey to predator and is used as a measure of an organism's trophic position. Dorsal muscle from lake trout, burbot, walleye, northern pike, white sucker, lake cisco, lake whitefish, and yellow perch was analyzed for Hg and δ¹⁵N using flameless atomic absorption and mass spectrometry respectively. Within each lake, log Hg was significantly related to δ¹⁵N (r ² ranged from 0.47 to 0.91,P<0.01). For four species, yellow perch, northern pike, lake cisco, and lake trout, log Hg was positively related to δ¹⁵N (r ² ranged from 0.37 to 0.47,P≤0.09) across all lakes. We also used δ¹⁵N measurements (assuming a 3‰ shift between an organism and its diet) and the developed within-lake regression equations to calculate a prey Hg for each individual fish. These food Hg values were then used to predict predator Hg using Norstromet al's bioenergetics model. Predicted results were strongly correlated to measured Hg concentrations (r=0.91,P<0.001), indicating that δ¹⁵N has potential to be used in modeling.     

Behavioral responses of bottlenose dolphins, Tursiops truncatus, to gillnets and acoustic alarms

Along the east coast of the United States, by-catches of bottlenose dolphins, Tursiops truncatus, in gillnet fisheries exceed removal levels set under the US Marine Mammal Protection Act. One measure proposed to reduce this mortality is the use of acoustic alarms, or pingers, which have proven effective in reducing by-catches of other small cetaceans, but have not been tested with bottlenose dolphins. We examined the responses of bottlenose dolphins to a commercial gillnet equipped with functional (active) and non-functional (control) Dukane NetMark® 1000 alarms near Fort Macon, NC. Between 5 April and 10 May 2001 we used a theodolite to track 59 groups of dolphins around the net. Choice of treatment was random each day and the two shore-based observers were unaware of whether alarms were active (13 days) or controls (9 days). There were no significant differences in the number of groups observed (P=0.315; 1−β=0.835) or in the closest observed approach to the net (P=0.307; 1−β=0.828) between treatments. However, dolphins entered a circular buffer approximately 100 m around the net more frequently with control than active alarms (P=0.015). We conclude that some dolphins responded to the alarms by avoiding the net, but caution that the potential efficacy of alarms is confounded by dolphin behavior. Most dolphins were aware of the net, regardless of the status of alarms, and some dolphins fed on fish in the net or discarded by the fishing vessel. We believe that it would be unwise to use pingers in these fisheries because of the limited behavioral responses we observed in our experiment. Furthermore, the responses we observed are likely to diminish or change over time as dolphins habituate or sensitize to these alarms. Further research is required to understand the behavior responsible for entanglement.     

Fundamental roles of biological barriers in mercury accumulation and transfer in freshwater ecosystems (analysis at organism,organ, cell and molecular levels)

In the framework of an ecotoxicological approach to the processes of bioaccumulation and transfer of Hg in freshwater systems, we present a synthesis of our experimental studies concerning the interactions between inorganic Hg and MeHg and biological barriers

- at organism and organ levels : three biological models are selected: fish (Salmo gairdneri), burrowing mayfly nymphs (Hexagenia rigida) and rooted macrophytes (Elodea densa, Ludwigia natans). Results show strong specificities of the biological barriers (gills, intestine, roots, ...) towards metal fixation and absorption, closely related to the chemical form of the metal, the contamination sources (water, sediments or food) and the physico-chemical characteristics of the medium ;

- at cell and molecular levels : biophysical study of Hg fixation on membrane reveals a new binding site on the phospholipids, the primary amine group on serine and ethanolamine polar heads, jointly with the SH groups of proteins ; Hg(II) induces a strong rigidification of the phospholipidic bilayers. Inorganic Hg and MeHg transports through model membranes (BLM) are essentially due to diffusion of neutral chloride species. These interactions between Hg compounds and membranes are strongly dependent on Hg chemical speciation (pH and pCl effects).

     

Titanium Dioxide Nanoparticle Circulation in an Aquatic Ecosystem

Nanotechnology is a dynamically developing field of scientific and industrial interest across the entire world, and the commercialization of nanoparticles (NPs) is rapidly expanding. Incorporation of nanotechnologies into a range of manufactured goods results in increasing concern regarding the subsequent release of engineered NPs into the environment. One of the biggest threats of using NPs is the transfer and magnification of these particles in the trophic chain. The aim of the studies was the evaluation of the distribution of TiO₂ NP contamination in the aquatic ecosystem under laboratory conditions. Bioaccumulation of TiO₂ NPs by plants (Elodea canadensis) and fish (Danio rerio) in the source of contamination was investigated. The studies were focused on the consequences of short-term water contamination with TiO₂ NPs and the secondary contamination of the components of the investigated model ecosystem (plants, sediments). It was found that in the fish and the plants exposed to NP contamination, the amount of Ti was higher than in the control, indicating an effective bioaccumulation of NPs or ions originating from NPs. It was clearly shown that the NPs present in the sediments are available to plants and fish. Additionally, the aquatic plants, an important trophic level in the food chain, can accumulate NPs and be a source of NPs for higher organisms. It was concluded that even an incidental contamination of water by NPs may result in long-term consequences induced by the release of NPs.     

Mercury contamination of the harbour porpoise Phocoena phocoena and other cetaceans from the North Sea and the kattegat

Total Hg levels were determined in 17 harbour porpoises Phocoena phocoena collected in the North Sea and the Kattegat (muscle, liver and kidney), as well as in a sperm whale Physeter macrocephalus, a common dolphin Delphinus delphis and two bottle-nose dolphins Tursiops truncatus collected in Belgium. Juvenile porpoises (total body length less than 140 cm) were characterized by total Hg concentrations increasing with length (t.i. with age) with median values of 3.1 μg g^?1 dw in muscle, 6.2 in liver and 5.7 in kidney, and liver to muscle resp. liver to kidney ratios varying between 1 and 2. In adults, liver concentrations were higher (up to 500 μg g^?1 dw) with liver to muscle and to kidney ratios up to 20. Median McHg concentrations in 5 of the porpoises were 4.0 μg g^?1 dw in muscle and 6.9 in liver. These values did not increase as strongly with length (t.i. with age, at least for juveniles) as total Hg did; as a consequence, the relative McHg levels decreased from 100% in juveniles to 2 or 3 % in the liver of adults. Such results seem to reflect the existence of a slow mineralization phenomenon and storage of inorganic Hg, mainly in the liver.     

Facts and fallacies concerning mercury uptake by fish in acid stressed lakes

Concerns about Hg contamination of fish have changed in emphasis from Hg-contaminated systems to more remote and apparently unpolluted systems. For remote lakes, a negative relationship between lake pH and Hg in fish has been demonstrated, implying an effect on Hg uptake from lake acidification. Though this relationship was discussed, and hypotheses put forward concerning the possible mechanisms 8 yr ago, the factors regulating Hg uptake by aquatic biota are still poorly understood. Several recent studies have prompted workers to observe that frequently cited concepts about processes affecting Hg accumulation and cycling are in fact over-simplifications. This review attempts to synthesize and clarify the present state of knowledge. We critically evaluate evidence for a number of controlling factors in the context of the concentrations, the chemical species and the biological uptake processes for Hg. The factors include: trophic status and organic content of water, food chain transfer and biomagnification of Hg, organism age and size effects, feeding strategies, biological and chemical methylation, concentration of cations including H⁺ and Ca²⁺ and the immediate source of Hg.     

Site fidelity and along-shore range in Hector's dolphin, an endangered marine dolphin from New Zealand

To document site fidelity and the alongshore range of individual Hector's dolphins we analysed sightings of 32 photographically identified dolphins, each seen ?10 times at Banks Peninsula, New Zealand, between 1985 and 1997. The furthest two sightings of an individual were 106 km apart. All other individuals ranged over less than 60 km (x?=31.0 km, SE=2.43) of coastline. Gender did not significantly influence alongshore range (female x?=30.4 km, SE =3.21, n=18; male x?=27.4 km, SE=5.68, n=5). Site fidelity was high: for example, on average, individuals were seen in Akaroa Harbour for about two thirds of the years they were known to be alive. These data suggest that impacts on Hector's dolphins are most appropriately managed on a small spatial scale.     

Trophic Structure and Bioaccumulation of Mercury in Fish of Three Natural Lakes of the Brazilian Amazon

The bioaccumulation of mercury by fish was studied in three natural lakes lining the Tapajós River, Brazilian Amazon. The Hg content variations are also reported between the rainy and the rising water seasons. Position of fish in the food chain and the source of carbon at the base of the food chain were determined using nitrogen and carbon stable isotopes ratios respectively. During the two seasons, the effect of the Hg biomagnification between the non-piscivorous and piscivorous fish of the three lakes is observed. The correlation between the mercury concentrations and the nitrogen isotopic ratios suggests that the fish mercury content variability is connected with the trophic position and size. Aside from classic observations on mercury biomagnification by certain species of fish (i.e. contents increase with size or trophic status), novel results were revealed as well, such as some species decrease of mercury contents with size of fish. Aquatic organisms in the Amazon are adapted to the complexity of trophic webs coupled with changes in environmental variables, depending on the location and/or the season, which results in a wide range of mercurial contamination among the Tapajós ichthyofauna.     

Quality Evaluation of Sediments from 24 Tributaries of the Po River, Italy

Sediment samples from 24 tributaries of the Po River (Italy) were screened for selected trace elements (Cd, Cu, Hg, Pb, and Zn) and extractable organic compounds; a proxy for contamination by organic microcontaminants. The toxicity of sediment extracts was evaluated using a battery of biotests (Dugesia gonocephala, Paracentrotus lividus, and Tamnocephalus platyurus). Contamination by trace elements (including very high Hg pollution – 4 to 16ppm total Hg – in one sub-basin) reached potentially harmful levels only in the sediments of four tributaries; while contamination by organic microcontaminants was present in most sub-basins. Sediments from most study sites did actually show signs of anthropogenic stress and were able to elicit a toxic response. A more detailed evaluation of sediment quality in the Po River tributaries seems to be urgently needed for developing the necessary remediation strategies. Research priorities should include more thorough testing of sediment toxicity, determination of metal background levels in the various sub-basins and a more detailed identification of the organic micropollutants of possible concern.     

Heavy Metal Content of Plant Species along Nilüfer Stream in Industrialized Bursa City, Turkey

The Paraíba do Sul river is located in one of the most developed part of Brazil and receives many organic and industrial effluents directly affecting the ichthyofauna. Concentration of four heavy metals (Cu, Cr, Zn and Pb) were determined in two tissues (muscle and gonads) of three abundant fish species from different trophic levels (Oligosarcus hepsetus—carnivore, Geophagus brasiliensis—omnivore and Hypostomus luetkeni—detritivore) between November 2002 and April 2003. The aim was to test the hypothesis that the trophic level and the proximity from impacted areas influence levels of contamination and to assess if these species are indicators of large-scale habitat quality. Levels of heavy metals were detected by Total Reflection X-ray Fluorescence with Synchrotron Radiation (SR-TXRF) at the Brazilian National Synchrotron Radiation Laboratory (LNLS). Generally, gonads showed higher metal concentration than muscles, except for Cr. All examined metals, but Cu, exceeded the maximum permitted concentration (mpc) by the Brazilian legislation for human consumption in at least one tissue. O. hepsetus (carnivore) showed the highest contamination levels, followed by G. brasiliensis (omnivore) and H. luetkeni (detritivore). The middle-upper segment, which encompasses large urban areas, showed the highest levels of metal contamination in most cases. O. hepsetus showed the highest levels of contamination in muscles for Pb in the middle-upper river segment (7.98?±?3.73; mpc?=?2.0 μg g^?1) and for Cr in the upper (5.53?±?0.05; mpc?=?0.10 μg g^?1) and middle-upper (4.20?±?0.85; mpc?=?0.10 μg g^?1) segments, which indicates that human population should avoid to consume these fishes species from these segments of the Paraíba do Sul river.     

Assessment of Mercury in Water, Sediments and Biota of a Southern European Estuary (Sado Estuary, Portugal)

The aim if this study was to assess the distribution of mercury in water, suspended particulate matter, surface sediments and biota from the Sado estuary, which, for the most part, is classified as a natural reserve, so as to evaluate its environmental quality status in terms of mercury contamination. Besides the diffuse sources of mercury coming into the Sado estuary, there are also additional contributions from the northern industrial zone and from the urban areas within the system. Applying national and international guidelines to different environmental matrices, the results obtained show that the system does not seem to be under environmental risk as far as mercury contamination is concerned. These quality guidelines can be used to rank and prioritise sites of concern. Hence, the area at the northern industrialised area deserves particular attention. The concentration of mercury in sediments of this area (0.54?mg kg^?1) simultaneously succeeded the European Union Water Framework Directive (EU-WFD) reference value, the OSPAR Convention Ecotoxicological Assessment Criteria (EACs) and is classified as class 2 in degree of contamination by the National legislation, which implies some legal restrictive rules. Considering the stations close to urbanised areas, one exceeded the OSPAR EACs for dissolved mercury, whilst the other exceeded the EU-WFD reference value for mercury concentrations in sediments. No statistical significant relations were found between mercury concentrations in biota (Ulva sp, Hediste diversicolor, Scrobicularia plana, Cerstoderma edule and Carcinus maenas) and in the abiotic matrices (sediment and water column, including mercury in its dissolved form and associated to suspended particulate matter). This paper provides an overview of the guidelines for Hg proposed for a considerable number of coastal systems of the northern hemisphere and highlights the complex interactions of Hg in the different environmental compartments in low contaminated systems.     

Temperature,pH and photoperiod effects on mercury bioaccumulation by nymphs of the burrowing mayflyHexagenia rigida

Accumulation of HgCl₂ and CH₃HgCl byHexagenia rigida nymphs from contaminated sediment and water column was investigated experimentally, taking into account 3 abiotic factors (temperature, pH and photoperiod). When the contamination of the experimental units was based on sediment compartment, Hg concentrations at the whole organism level revealed very high bioaccumulation differences between the two chemical forms of Hg (ratio close to 20 in favour of MeHg). When Hg compounds were added to the water column, the highest Hg accumulation rates were observed for MeHg, but with a small difference between the 2 compounds (ratio close to 2.0–3.0). These bioaccumulation processes were very dependent on the 3 abiotic factors taken into account, especially temperature and water column pH.     

Major controlling factors and prediction models for mercury transfer from soil to carrot

Purpose

Soil-plant transfer models are needed to predict levels of mercury (Hg) in vegetables when evaluating food chain risks of Hg contamination in agricultural soils.

Materials and methods

A total of 21 soils covering a wide range of soil properties were spiked with HgCl₂ to investigate the transfer characteristics of Hg from soil to carrot in a greenhouse experiment. The major controlling factors and prediction models were identified and developed using path analysis and stepwise multiple linear regression analysis.

Results and discussion

Carrot Hg concentration was positively correlated with soil total Hg concentration (R ²?=?0.54, P?<?0.001), and the log-transformation greatly improved the correlation (R ²?=?0.76, P?<?0.001). Acidic soil exhibited the highest bioconcentration factor (BCF) (ratio of Hg concentration in carrot to that in soil), while calcareous soil showed the lowest BCF among the 21 soil types. The significant direct effects of soil total Hg (Hg_soil), pH, and free Al oxide (Al_OX) on the carrot Hg concentration (Hg_carrot) as revealed by path analysis were consistent with the result from stepwise multiple linear regression that yielded a three-term regression model: log [Hg_carrot]?=?0.52log [Hg_soil]???0.06pH???0.64log [Al_OX]???1.05 (R ²?=?0.81, P?<?0.001).

Conclusions

Soil Hg concentration, pH, and Al_OX content were the three most important variables associated with carrot Hg concentration. The extended Freundlich-type function could well describe Hg transfer from soil to carrot.     

Genetic isolation and morphological divergence of Black Sea bottlenose dolphins

The Black Sea is a semi-enclosed body of water that differs from the adjacent Mediterranean Sea in terms of its biodiversity, oceanographical and ecological characteristics. There is growing international concern about pollution in the Black Sea and other anthropogenic threats to its fauna. The bottlenose dolphin (Tursiops truncatus) is one of three species of cetaceans living in the Azov-Black Sea basin. Despite considerable research on bottlenose dolphins elsewhere, the extent of human impacts on the Black Sea populations is unknown. Previous attempts to award special conservation status to Black Sea cetaceans have failed specifically because policy makers have viewed their ecological and evolutionary uniqueness as equivocal. This study assessed divergence between Black Sea, Mediterranean Sea and Atlantic Ocean bottlenose dolphins for 26 cranial measurements (n = 75 adult bottlenose dolphin skulls) and mitochondrial DNA (n = 99 individuals). Black Sea bottlenose dolphins are smaller than those in the Mediterranean, and possess a uniquely shaped skull. As in a previous study, we found the Black Sea population to be genetically distinct, with relatively low levels of mtDNA diversity. Population genetic models suggest that Black Sea bottlenose dolphins have so little gene flow with the Mediterranean due to historical isolation that they should be managed separately.     

Distribution of mercury in the aquatic food web of Onondaga Lake,New York

Historical discharges of mercury (Hg) to Onondaga Lake, New York, have resulted in elevated Hg concentrations in lake fishes. In 1990, a remedial investigation and feasibility study (RI/FS) was initiated to evaluate problems related to Hg and other hazardous substances in the lake. As part of the RI/FS, the distribution of Hg in the aquatic food web was determined to provide input to a site-specific model of Hg cycling and to evaluate potential ecological risks of Hg in the lake. Mercury concentrations were measured in surficial sediments, sediment interstitial water, lake water, phytoplankton, Zooplankton, benthic macroinvertebrates, and fishes (including planktivores, benthivores, and piscivores). The percentage of total Hg accounted for by methyl-Hg (CH₃Hg) generally increased with higher trophic levels, confirming that CH₃Hg is more efficiently transferred to higher trophic levels than is inorganic Hg. Concentrations of total Hg in amphipods and chironomids were closely related to concentrations of total Hg in sediments, suggesting that sediments are a likely source of Hg for benthic macroinvertebrates. Mercury concentrations in edible muscle tissue (fillets) of lake fishes have declined substantially from values found in the early 1970s, reflecting the large reductions in Hg discharges to the lake that have occurred since that time. The CH₃Hg concentrations in fillets and whole bodies of fishes generally were similar, indicating that concentrations in fillets often can provide estimates of concentrations in whole bodies. Methyl-Hg concentrations and bioaccumulation factors increased with higher trophic levels in both the pelagic and benthic components of the lake food web.     

Environmental fate of mercury discharged into the upper Wisconsin river

We studied the distribution of Hg in sediments, fish, and crayfish in a 60 km reach of the Upper Wisconsin River that formerly received Hg in discharges from pulp and paper mills. The most heavily contaminated strata of sediments were deposited during the 1950s and early 1960s and buried under subsequent deposits; however, surficial sediments remained substantially enriched at certain sites in 1981. Median concentrations of Hg in surficial sediments, adjusted for grain size, were at least 10-fold greater at the main study area than at an upstream reference site. Total concentrations exceeded 1.0 μg g^?1 wet weight in axial muscle tissue in only 2 of 173 fish analyzed from the study area; however, historical comparisons revealed that Hg contamination of fish (common carp Cyprinus carpio and walleye Stizostedion vitreum vitreum) and crayfish (Orconectes) in the river had not decreased since the early 1970s. The availability of Hg to biota in this system may be enhanced by rapid methylation of the metal in surficial sediments, despite burial of the most heavily contaminated sediments. Management practices for this river should be designed to prevent conditions favoring mobilization and methylation of Hg in sediments.