Mercury speciation and exchanges at the air-water interface of a tropical artificial reservoir, French Guiana

The distribution and speciation of mercury (Hg) in air, rain, and surface waters from the artificial tropical lake of Petit-Saut in French Guiana were investigated during the 2003/04 period. In the air, total gaseous mercury (TGM) at the dam station averaged 12+/-2 pmol m(-3) of which >98% was gaseous elemental mercury (GEM). GEM distribution depicted a day-night cycling with high concentrations (up to 15 pmol m(-3)) at dawn and low concentrations (down to 5 pmol m(-3)) at nightfall. Reactive gaseous mercury (RGM) represented <1% of the GEM with a mean concentration of 4+/-3 fmol m(-3). Diel RGM variations were negatively related to GEM. In the rain, the sum of all Hg species in the unfiltered (HgT(UNF)) averaged 16+/-12 pmol L(-1). Temporal distribution of HgT(UNF) exhibited a pattern of high concentrations during the late dry seasons (up to 57.5 pmol L(-1)) and low concentrations (down to 2.7 pmol L(-1)) in the course of the wet seasons. Unfiltered reactive (HgR(UNF)), dissolved gaseous (DGM) and monomethyl (MMHg(UNF)) Hg constituted 20, 5 and 5% of HgT(UNF), respectively. All measured Hg species were positively related and displayed negative relationships with the pH of the rain. In the reservoir surface waters, dissolved total mercury (HgT(D)) averaged 3.4+/-1.2 pmol L(-1) of which 10% consisted of DGM. DGM showed a trend of high concentrations during the dry seasons (480+/-270 fmol L(-1)) and lower (230+/-130 fmol L(-1)) in the course of the wet seasons. Diel variations included diurnal photo-induced DGM production (of about 60 fmol L(-1) h(-1)) coupled to minute to hour oxidation/reduction cycles (of >100 fmol L(-1) amplitude). Finally, calculated atmospheric Hg inputs to the Petit-Saut reservoir represented 14 mol yr(-1) whereas DGM evasion reached 23 mol yr(-1). Apportionment among forms of Hg deposition indicated that up to 75% of the total Hg invasive flux follows the rainfall pathway.     

Dissolved gaseous mercury formation under UV irradiation of unamended tropical waters from French Guyana

Filtered and non-filtered natural waters from French Guyana were irradiated with lamps emitting within the wavelength range 300-450 nm for 4 days with and without oxygen. Dissolved gaseous mercury (DGM) evolution was observed and quantified in the course of the irradiation. Measurements of total mercury in waters prior to and after the irradiations were also performed. The mass balance in the various mercury species (Hg(total), Hg(reactive) and DGM) proves the capability of the light to extract the mercury linked to the organic matter. DGM evolvement was greater in N2- than in air-saturated solutions, and the formation of volatile oxidized species can account for the inhibiting effect of oxygen. Filtration did not affect the mercury photoreduction, but reduced the formation of DGM in the dark. Great care has to be taken with regard to the following artifact: it was found that DGM originated not only from the natural waters, but also from the experimental device itself when exposed to the light. These non-expected DGM entries were quantitatively evaluated. This stresses the difficulty in measuring mercury at environmental concentrations.     

Mercury emission from a temperate lake during autumn turnover

Lakes in temperate regions stratify during summer and winter months, creating distinct layers of water differentiated by their physical and chemical characteristics. When lakes mix in autumn and spring, mercury cycling may be affected by the chemical changes that occur during mixing. Sampling was conducted in Lake Lacawac, Eastern Pennsylvania, USA, throughout the autumn of 2007 to characterize changes in emission of gaseous elemental mercury (Hg⁰) from the lake surface and dissolved mercury profiles in the water column during mixing. Water chemistry and weather parameters were also measured, including dissolved organic carbon (DOC), iron, and solar radiation which have been shown to interact with mercury species. Results indicate that emission of Hg⁰ from the lake to the atmosphere during turnover was controlled both by solar radiation and by surface water mercury concentration. As autumn turnover progressed through the months of October and November, higher mercury concentration water from the hypolimnion mixed with epilimnetic water, increasing mercury concentration in epilimnetic waters. Dissolved absorbance was significantly correlated with mercury concentrations and with iron, but DOC concentrations were essentially constant throughout the study period and did not exhibit a relationship with either dissolved mercury concentrations or emission rates. Positive correlations between dissolved mercury and iron and manganese also suggest a role for these elements in mercury transport within the lake, but iron and manganese did not demonstrate a relationship with emission rates. This research indicates that consideration of seasonal processes in lakes is important when evaluating mercury cycling in aquatic systems.     

Photo-induced formation of dissolved gaseous mercury in coastal and offshore seawater of the Mediterranean basin

The first measurements on the daily trend of dissolved gaseous mercury (DGM) concentration determined in coastal and offshore waters of the Mediterranean basin are reported. Marked daily behaviour tracking solar radiation has been observed at the coastal sampling station with DGM values ranging from 11.0 to 38.9 pg/l. Contrary to these observations the DGM values in offshore water samples (11.9-20.0 pg/l) were independent of the sampling time, thus identifying the absence of higher levels during the hours of maximum insolation. The availability of Hg2+ substrate necessary for the photo-reaction processes of DGM formation has been evaluated by measuring the reactive mercury concentration. In offshore waters the lower DGM concentrations are attributable to the substrate as a limiting factor. The highest concentration of DOC measured in coastal seawater with respect to the offshore one could moreover enhance the reaction rates of DGM production through the formation of inorganic mercury complexes and weaker organic associations.     

Changes in the non-protein thiol pool and production of dissolved gaseous mercury in the marine diatom Thalassiosira weissflogii under mercury exposure

Two detoxification mechanisms working in the marine diatom Thalassiosira weissflogii to cope with mercury toxicity were investigated. Initially, the effect of mercury on the intracellular pool of non-protein thiols was studied in exponentially growing cultures exposed to sub-toxic HgCl₂ concentrations. T. weissflogii cells responded by synthesizing metal-binding peptides, named phytochelatins (PCs), besides increasing the intracellular pool of glutathione and γ-glutamylcysteine (γ-EC). Intracellular Hg and PC concentrations increased with the Hg concentration in the culture medium, exhibiting a distinct dose-response relationship. However, considerations of the PCs-SH:Hg molar ratio suggest that glutathione could also be involved in the intracellular mercury sequestration. The time course of the non-protein thiol pool and Hg intracellular concentration shows that PCs, glutathione and γ-EC represent a rapid cellular response to mercury, although their role in Hg detoxification seems to lose importance at longer incubation times. The occurrence of a process of reduction of Hg(II) to Hg° and subsequent production of dissolved gaseous mercury (DGM) was also investigated at lower Hg concentrations, at which the PC synthesis doesn't seem to be involved. The significant (P < 0.01) correlation between the cellular density in solution and the production of DGM suggests that this diatom is capable of directly producing DGM, both in light and dark conditions. This finding has been confirmed by the absence of DGM production in the culture media containing formaldehyde-killed cells. Finally, the relationship between these two different pathways of Hg detoxification is discussed.     

Mercury methylation rates of biofilm and plankton microorganisms from a hydroelectric reservoir in French Guiana

The Petit-Saut ecosystem is a hydroelectric reservoir covering 365 km² of flooded tropical forest. This reservoir and the Sinnamary Estuary downstream of the dam are subject to significant mercury methylation. The mercury methylation potential of plankton and biofilm microorganisms/components from different depths in the anoxic reservoir water column and from two different sites along the estuary was assessed. For this, reservoir water and samples of epiphytic biofilms from the trunk of a submerged tree in the anoxic water column and from submerged branches in the estuary were batch-incubated from 1 h to 3 months with a nominal 1000 ng/L spike of Hg(II) chloride enriched in ¹⁹⁹Hg. Methylation rates were determined for different reservoir and estuarine communities under natural nutrient (reservoir water, estuary freshwater) and artificial nutrient (culture medium) conditions. Methylation rates in reservoir water incubations were the highest with plankton microorganisms sampled at − 9.5 m depth (0.5%/d) without addition of biofilm components. Mercury methylation rates of incubated biofilm components were strongly enhanced by nutrient addition. The results suggested that plankton microorganisms strongly contribute to the total Hg methylation in the Petit-Saut reservoir and in the Sinnamary Estuary. Moreover, specific methylation efficiencies (%Me¹⁹⁹Hg_net/cell) suggested that plankton microorganisms could be more efficient methylating actors than biofilm consortia and that their methylation efficiency may be reduced in the presence of biofilm components. Extrapolation to the reservoir scale of the experimentally determined preliminary methylation efficiencies suggested that plankton microorganisms in the anoxic water column could produce up to 27 mol MeHg/year. Taking into account that (i) demethylation probably occurs in the reservoir and (ii) that the presence of biofilm components may limit the methylation efficiency of plankton microorganisms, this result is highly consistent with the annual net MeHg production estimated from mass balances (8.1 mol MeHg/year, Muresan et al., 2008a).     

Preliminary investigation of the mercury saturation in the Baltic Sea winter surface water

A new experimental measuring system for the total gaseous mercury saturation in the sea surface water has been developed. The technique applied to the system is based on equilibrating water-air mercury partial pressures at the air-sea interface. First data using this system were collected in the Baltic Sea on 13-22 January 2000. Measurements were completed from the research vessel 'Alexander von Humboldt' on the way from the Gotland Sea to the Mecklenburg Bight. In general the results showed domination of supersaturated winter sea surface waters with respect to gaseous mercury. Two areas with distinctly elevated mercury saturation were found: one in the east-west shipping route, south of Bornholm, where the equilibrated gaseous Hg content of water was approximately 9.2-fold higher, and another over the ammunition disposal region south-east of Hoburgs Bank, where the Hg content of water was approximately 3.8-fold higher than that in air. In general, the data indicate that the south-western Baltic Sea, and in particular both maritime areas identified with elevated mercury saturation, can emit gaseous mercury from surface waters into the atmosphere and contribute to long-range atmospheric transport.     

Continuous analysis of dissolved gaseous mercury in freshwater lakes

The concentration of dissolved gaseous mercury (DGM) in freshwaters changes more quickly than the 40-min processing time of current analytical methods. A new method for continuous field analysis of DGM was developed using a Tekran 2537A to achieve a DGM analysis time of 5 min. Samples were concurrently analyzed for temperature, oxygen, conductivity, pH, and oxidation-reduction potential using a Hydrolab. The detection limit for DGM ranged between 5 and 20 fmolL(-1) with 99% removal efficiency. Control experiments showed that there was no interference due to methyl mercury, which is present in similar concentrations to DGM. Controlled experiments comparing continuous DGM analysis with discrete DGM analysis showed that the results are not significantly affected by typical variations in water temperature (4-30 degrees C), oxidation-reduction potential (135-355 mV), dissolved organic carbon (4.5-10.5 mgL(-1)), or pH (3.5-7.8). The continuous analysis was within 4.5% of the discrete analysis when compared across 12 samples analyzed in triplicate. The field performance of this method was tested over two 48-h periods in two lakes in Kejimkujik Park, Nova Scotia where over 1000 data points were collected.     

Diurnal variation of dissolved gaseous mercury (DGM) levels in a southern reservoir lake (Tennessee, USA) in relation to solar radiation

Variations of dissolved gaseous mercury (DGM) concentrations in a southern reservoir lake (Cane Creek Lake, Cookeville, TN, USA) in relation to solar radiation were investigated consecutively from June 2003 to May 2004. The daytime DGM levels in the lake exhibited a two-phase diurnal trend; the DGM concentrations rose in the morning, peaked around noontime and then fell in the afternoon through the evening; these trends followed the general pattern of diurnal solar radiation variations. The morning and afternoon phases appeared to be asymmetrical with the former relatively steep and the latter gradual. A variety of daytime DGM level variations other than the typical two-phase diurnal patterns were also observed. For the time spans studied, the daytime mean DGM concentrations of the lake ranged from 12 to 68 pg L(-1) (60-340 fM). The daytime mean DGM levels in the summertime (June, July, August) showed values above 30 pg L(-1) (150 fM) in most cases and a large number of peak DGM concentrations above 50 pg L(-1) (250 fM). The summer DGM levels in the lake appear to be comparable to those observed in the large northern lakes for the summertime. The daytime DGM levels in the lake were found to correlate with solar radiation to various degrees (cases of r values above 0.8: approximately 12% and approximately 18% of the total sampling days for correlation with global solar radiation and UVA radiation, respectively). Correlating trends are recognizable between the daytime mean DGM concentration and the corresponding mean global solar radiation (r = 0.66, p < 0.0005) and between the daytime mean DGM concentration and the corresponding mean UVA radiation (r = 0.62, p < 0.0005).     

Dissolved gaseous mercury production in the dark: Evidence for the fundamental role of bacteria in different types of Mediterranean water bodies

It is well established that the dissolved gaseous mercury (DGM) production in waters is mainly driven by photochemical processes. The present paper provides evidence for a significant bacteria-mediated DGM production, occurring also under dark conditions in environmentally different types of coastal water bodies of the Mediterranean basin. The DGM production was laboratory determined in sea, lagoon-brackish and lake water samples, comparing the efficiency of the DGM production processes in darkness and in the light. This latter condition was established by exposing samples at solar radiation intensity in the Photosyntetical Active Radiation region (PAR) of 200 W m^− 2. Mercury reduction rate in the dark was of the order of 2-4% of the DGM production in lightness, depending on the total mercury concentration in the water, rather than the bacterial abundance in it. Support for the active bacterial role in mercury reduction rate under dark conditions was provided by: 1) absence of significant DGM production in sterilized water samples (following filtration treatment or autoclaving), 2) restored DGM production efficiency, following re-inoculation into the same water samples of representatives of their bacterial community, previously isolated and separately cultured. Notwithstanding the low bacteria-mediated vs. the high photo-induced DGM production, whatever natural water body was considered, it is worth stressing the significant contribution of this organismal-mediated process to oceanic mercury evasion, since it occurs continuously along the entire water column throughout the 24 h of the day.     

Mercury methylation and bacterial activity associated to tropical phytoplankton

The methylated form of mercury (Hg), methylmercury (MeHg), is one of the most toxic pollutants. Biotic and/or abiotic methylation, often associated to sulfate-reducing bacteria metabolism, occurs in aquatic environments and in many tropical areas, mostly in the periphyton associated to floating macrophyte roots. Data about mercury methylation by phytoplankton are scarce and the aim of this study was to verify the biotic influence in the methylation process in Microcystis aeruginosa and Sineccocystis sp. laboratory strains and in natural populations of phytoplankton from two different aquatic systems, the mesotrophic Ribeir?o das Lajes reservoir and hypereutrophic oligohaline Jacarepaguá lagoon, Rio de Janeiro state, Brazil. Adapted radiochemical techniques were used to measure sulfate-reduction, mercury methylation and bacterial activity in phytoplankton samples. Methyl-(203)Hg formation from added inorganic (203)Hg and (3)H-Leucine uptake were measured by liquid scintillation as well as sulfate-reduction, estimated as H(2)(35)S produced from added Na(2)(35)SO(4). There was no significant difference in low methylation potentials (0.37%) among the two cyanobacterium species studied in laboratory conditions. At Ribeir?o das Lajes reservoir, there was no significant difference in methylation, bacterial activity and sulfate-reduction of surface sediment between the sampling points. Methylation in sediments (3-4%) was higher than in phytoplankton (1.5%), the opposite being true for bacterial activity (sediment mean 6.6 against 150.3 nmol gdw(-1) h(-1) for phytoplankton samples). At Jacarepaguá lagoon, an expressive bacterial activity (477.1 x 10(3) nmol gdw(-1) h(-1) at a concentration of 1000 nM leucine) and sulfate-reduction ( approximately 21% H(2)(35)S trapped) associated to phytoplankton (mostly cyanobacteria M. aeruginosa) was observed, but mercury methylation was not detected.     

Mercury net methylation in five tropical flood plain regions of Brazil: high in the root zone of floating macrophyte mats but low in surface sediments and flooded soils

In aquatic systems, bottom sediments have often been considered as the main methylmercury (MeHg) production site. In tropical floodplain areas, however, floating meadows and flooded forests extend over large areas and can be important Hg methylating sites. We present here a cross-system comparison of the Hg net methylation capacity in surface sediments, flooded soils and roots of floating aquatic macrophytes, assayed by in situ incubation with 203Hg and extraction of formed Me203 Hg by acid leaching and toluene. The presence of mono-MeHg was confirmed by thin layer chromatography and other techniques. Study areas included floodplain lakes in the Amazon basin (Tapajós, Negro and Amazon rivers), the Pantanal floodplain (Paraguay river basin), freshwater coastal lagoons in Rio de Janeiro and oxbow lakes in the Mogi-Gua?ú river, S?o Paulo state. Different Hg levels were added in assays performed in 1994-1998, but great care was taken to standardise all other test parameters, to allow data comparisons. Net MeHg production was one order of magnitude higher (mean 13.8%, range 0.28-35) in the living or decomposing roots of floating or rooted macrophyte mats (Eichhornia azurea, E. crassipes, Paspalum sp., Eleocharis sellowiana, Salvinia sp., S. rotundifolia and Scirpus cubensis) than in the surface layer of underlying lake sediments (mean 0.6%, range 0.022-2.5). Methylation in flooded soils presented a wide range and was in some cases similar to the one found in macrophyte roots but usually much lower. In a Tapajós floodplain lake, natural concentrations of MeHg in soil and sediment cores taken along a lake-forest transect agreed well with data on net methylation potentials in the same samples. E. azurea, E. crassipes and Salvinia presented the highest methylation potentials, up to 113 times higher than in sediments. Methylation in E. azurea from six lakes of the Paraguay and Cuiabá rivers, high Pantanal, was determined in the 1998 dry and wet seasons and ranged from 1.8 to 35%. Methylation was lower in washed roots than in untreated roots of E. azurea and methylation in solids isolated from the roots, was higher than in sediments but lower than in untreated roots. This indicates that the methylation in roots zones occurs mainly in the root-associated solids. Floating meadows are sites of intense production of biomass and of highly bioavailable MeHg and appear to be an essential link of the MeHg cycle in tropical aquatic systems.     

Temporal and spatial influences of flooding on dissolved mercury in boreal reservoirs

Dissolved total mercury and methylmercury concentrations ([HgT]D and [MeHg]D, respectively) in natural lakes and reservoirs of varying age (2 months to 69 years) were monitored between 1993 and 1997, during different times of the year and at several sites within the same reservoir. Both HgT-D and MeHgD in flooded environments were significantly greater (Student's t-test, P < 0.001) than those determined for neighbouring lakes, with average concentrations of 2.35+/-0.85 ng(-1) (n = 79) vs. 1.51+/-0.43 ng l(-1) (n = 70) for HgT-D and 0.28+/-0.10 ng Hg l(-1) (n = 87) vs. 0.05+/-0.025 ng Hg l(-1) (n = 30) for MeHgD. Furthermore, the proportion of HgT-D in the methylated form was, on average, nearly four times greater in the reservoir sites as compared with those of the lakes (12% vs. 3.6%). Simple linear regressions used to compare [MeHg]D with nine water quality parameters suggest that factors controlling the presence of MeHgD are not the same in all flooded environments. Overall, our results demonstrate that [MeHg]D are higher in reservoirs as compared with natural lakes and that up to 18 years after reservoir construction [MeHg]D remain elevated as compared with background levels.     

Strategies to lower methyl mercury concentrations in hydroelectric reservoirs and lakes: A review

Mercury (Hg) concentrations in fish in lakes are elevated due to increased global cycling of Hg. A special case of elevated Hg concentrations in fish occurs in new hydroelectric reservoirs because of increased rates of converting Hg in the environment into methyl mercury (MeHg). People and wildlife that eat fish from hydroelectric reservoirs have an elevated risk of accumulating too much MeHg. Demand for electrical energy is leading to the creation of new reservoirs. In 2005, Canada derived 60% of its electricity from hydroelectric reservoirs. As a result, hydroelectric companies and governing agencies are exploring strategies to lower MeHg contamination. Strategies may involve lowering the source of Hg before flooding, the rate of Hg methylation, or MeHg bioaccumulation and biomagnification. Possible strategies reviewed in this article include selecting a site to minimize impacts, intensive fishing, adding selenium, adding lime to acidic systems, burning before flooding, removing standing trees, adding phosphorus, demethylating MeHg by ultraviolet light, capping and dredging bottom sediment, aerating anoxic bottom sediment and waters, and water level management. A preventative strategy is to limit the flooded area, especially wetland areas. Flooded upland areas that contain less carbon produce MeHg for a shorter time than wetland areas. Run-of-the-river reservoirs contain lower MeHg concentrations than reservoirs that flood vast areas, at the cost of exporting MeHg downstream. Managing water levels to flush systems during times of peak MeHg production may have benefits for the reservoir, but also transports MeHg downstream. Intensive fishing can lower MeHg in food webs by increasing fish growth rate. Additions of selenium can lower MeHg bioaccumulation, but the mechanisms are not well established and excess selenium causes toxicity. Liming can lower fish Hg concentrations in lakes acidified with sulphuric and nitric acid. Burning before flooding can lower the production of MeHg, but MeHg bioaccumulation may increase. The most promising strategy will be one that is agreeable to all affected people.     

Impacts of zooplankton composition and algal enrichment on the accumulation of mercury in an experimental freshwater food web

There is a well documented accumulation of mercury in fish to concentrations of concern for human consumption. Variation in fish Hg burden between lakes is often high and may result from differences in Hg transfer through lower levels of the food web where mercury is bioconcentrated to phytoplankton and transferred to herbivorous zooplankton. Prior research derived patterns of mercury accumulation in freshwater invertebrates from field collected animals. This study provides results from controlled mesocosm experiments comparing the effects of zooplankton composition, algal abundance, and the chemical speciation of mercury on the ability of zooplankton to accumulate mercury from phytoplankton and transfer that mercury to planktivores. Experiments were conducted in 550-L mesocosms across a gradient of algal densities manipulated by inorganic nutrient additions. Enriched, stable isotopes of organic (CH3(200HgCl)) and inorganic (201HgCl2) mercury were added to mesocosms and their concentrations measured in water, seston, and three common zooplankton species. After 2 weeks, monomethylmercury (MMHg) concentrations were two to three times lower in the two copepod species, Leptodiaptomus minutus and Mesocyclops edax than in the cladoceran, Daphnia mendotae. All three zooplankton species had higher MMHg concentrations in mesocosms with low versus high initial algal abundance. However, despite higher concentrations of inorganic mercury (HgI) in seston from low nutrient mesocosms, there were no significant differences in the HgI accumulated by zooplankton across nutrient treatments. Bioaccumulation factors for MMHg in the plankton were similar to those calculated for plankton in natural lakes and a four-compartment (aqueous, seston, macrozooplankton, and periphyton/sediments) mass balance model after 21 days accounted for approximately 18% of the CH3(200Hg) and approximately 33% of the 201Hg added. Results from our experiments corroborate results from field studies and suggest the importance of particular zooplankton herbivores (e.g., Daphnia) in the transfer of Hg to higher trophic levels in aquatic food webs.     

Mercury distribution and exchanges between the Amazon River and connected floodplain lakes

This work presents the distribution and the partition of mercury (Hg) in the Curuai floodplain lakes along the Amazon River. The maximum Total Filtered Hg (T-FHg) concentrations in the floodplain lakes (28 to 52 pmol L^− 1) coincide with the maximum T-FHg concentrations of the Amazon River and are measured during the flooding period. The lowest T-FHg values (3 to 5 pmol L^− 1) are observed during the flood peak of the mainstream, during the rainy season, when waters are diluted by the local rainfall. In this system, Hg is mainly transported in the particulate phase, confirmed by elevated values of the Hg partition coefficient (4.77 < K_d (Hg) < 5.83 L kg^− 1). The highest Total Particulate Hg (T-PHg) concentrations (47–478 pmol L^− 1) in the lakes are measured during the dry season when they are isolated from the mainstream. This enrichment is due to the elevated TSS content associated to the re-suspension of the bottom sediments by the wind action and the bioturbation in shallow water lakes.In the flooded system, the lakes show different geochemical characteristics that control the Hg distribution and partition. In the white water (WW) lakes, characterized by oxidative neutral waters and highest TSS contents (till 2041 mg L^− 1), the T-PHg is associated to the particulate organic matter mainly during the dry season, while the T-FHg and T-FMn concentrations are correlated. In the black water (BW) lakes that show reductive pH conditions and lowest TSS load (2 to 52 mg L^− 1), P-iron and T-PHg display a positive relationship whereas the redox conditions favor the desorption of Hg from the particulate to the filtered phase.The mercury mass budget estimated in this study confirms that the Curuai floodplain system acts as a particulate mercury trap, with a net storage of particulate Hg of 150 kg PHg year^− 1.     

Experimental and modelling study of plutonium uptake by suspended matter in aquatic environments from southern Spain.

Kinetic transfer coefficients are important parameters to understand and reliably model the behaviour of non-conservative radionuclides in aquatic environments. This report pertains to a series of five radiotracer experiments on Pu uptake in natural aqueous suspensions of unfiltered waters from three aquatic systems in the south of Spain (Gergal reservoir, the Guadalquivir river, and the estuary of the Tinto river). The experimental procedure ensured the simulation of environmental conditions. Pu activity was measured by a liquid scintillation technique. The uptake curves, covering a period up to one week, are discussed with respect to numerical uptake kinetics models. The data suggest that in our experimental setting the main pathways for Pu uptake consist of two parallel and reversible reactions followed by a consecutive non-reversible reaction. Kinetic transfer coefficients were estimated by a fitting procedure and a comparative study was followed.     

Apparent rates of production and loss of dissolved gaseous mercury (DGM) in a southern reservoir lake (Tennessee, USA)

Apparent rates of dissolved gaseous mercury (DGM) concentration changes in a southern reservoir lake (Cane Creek Lake, Cookeville, Tennessee) were investigated using the DGM data collected in a 12-month study from June 2003 to May 2004. The monthly mean apparent DGM production rates rose from January (3.2 pg L(-1)/h), peaked in the summer months (June-August: 8.9, 8.0, 8.6 pg L(-1)/h), and fell to the lowest in December (1.6 pg L(-1)/h); this trend followed the monthly insolation march for both global solar radiation and UVA radiation. The monthly apparent DGM loss rates failed to show the similar trend with no consistent pattern recognizable. The spring and summer had higher seasonal mean apparent DGM production rates than the fall and winter (6.8, 9.0, 3.9, 5.0 pg L(-1)/h, respectively), and the seasonal trend also appeared to closely follow the solar radiation variation. The seasonal apparent DGM loss featured similar rate values for the four seasons (5.5, 4.3, 3.3, and 3.9 pg L(-1)/h for spring, summer, fall, and winter, respectively). Correlation was found of the seasonal mean apparent DGM production rate with the seasonal mean morning solar radiation (r=0.9084, p<0.01) and with the seasonal mean morning UVA radiation (r=0.9582, p<0.01). No significant correlation was found between the seasonal apparent DGM loss rate and the corresponding afternoon solar radiation (r=0.5686 for global radiation and 0.6098 for UVA radiation). These results suggest that DGM production in the lake engaged certain photochemical processes, either primary or secondary, but the DGM loss was probably driven by some dark processes.     

Mercury and methylmercury in the Gulf of Trieste (northern Adriatic Sea)

The distribution, sources and fate of mercury (Hg) in the water column of the Gulf of Trieste (northern Adriatic Sea), affected by the Hg polluted river Soca/Isonzo for centuries draining the cinnabar-rich deposits of the Idrija mining district (NW Slovenia), were studied in terms of total and dissolved Hg, reactive Hg, total and dissolved methylmercury (MeHg), mesozooplankton Hg and MeHg, and sedimentation rates of particulate Hg. Higher total Hg concentrations in the surface layer were restricted to the area of the Gulf in front of the river plume expanding in a westerly direction. Higher concentrations in bottom water layers were the consequence of sediment resuspension. Dissolved Hg exhibited higher concentrations in the surface layer in the area in front of the river plume. Higher bottom concentrations of dissolved Hg observed at some stations were probably due to remobilization from sediments, including resuspension and benthic recycling. The relationship between dissolved Hg in the surface layer and salinity showed nonconservative mixing in June 1995 during higher riverine inflow and nearly conservative mixing in September 1995 during lower riverine inflow. Both mixing curves confirm the river Soca/Isonzo to be the most important source of total and dissolved Hg, which are significantly correlated, in the Gulf. Reactive Hg is significantly correlated with dissolved Hg, indicating that the majority of dissolved Hg is reactive and potentially involved in biogeochemical transformations. The higher total MeHg in the bottom layer is the result of remobilization of MeHg from sediments including benthic fluxes. Strong seasonal variation of sedimentation rates of particulate Hg was found during a 2-year study in the central part of the Gulf. These variations followed those of total sedimented matter, indicating that sedimented Hg is mostly associated with inorganic matter. About a 2.5-fold higher fluxes of particulate Hg were observed at the depth of 20 m relative to 10 m which is attributed to bottom sediment resuspension. Temporal variability of mesozooplankton Hg and MeHg is the consequence of biomass and species variations, and grazing behaviour. From the preliminary Hg mass balance it appears that the Gulf is an efficient trap for total Hg and a net source of MeHg.     

Is the Negro River Basin (Amazon) impacted by naturally occurring mercury?

In order to investigate the major sources and cycling of mercury in the Negro River Basin (Amazon), total metal measurements were carried out in soils, in river and lake waters, in the atmosphere, and in bulk precipitation during the period 1995 throughout 1998. Median values of 1.3 ng m(-3) in the atmosphere, 172 microg kg(-1) in soils, 4.6-7.5 ng l(-1) in three different lakes, 4.5 ng l(-1) in 17 different Negro River tributaries and 20.3 microg m(-2) year(-1) in bulk precipitation were found. Mercury concentrations in rivers and lakes waters, as well as in soils and bulk precipitation were high, considering the scarcity of anthropogenic point sources in the region. Mercury leaching from soil, the largest regional reservoir of this metal, was considered to be the major pathway to mercury enrichment in the region.