Occurrence and fate modeling of MTBE and BTEX compounds in a Swiss Lake used as drinking water supply

Emissions of fuel components from boating use on multiple-use lakes and reservoirs are of high concern with regard to the drinking water supply from such water bodies. We report results of a detailed study on the occurrence, sources and fate of aromatic hydrocarbons and methyl tert-butyl ether (MTBE) in a typical holomictic lake, Lake Zurich, that supplies drinking water for the largest Swiss city. Emphasis of the investigation was on the fuel oxygenate MTBE, which was found in concentrations up to 1.4 microg/L in the epilimnion and up to 0.05microg/L in the hypolimnion of the lake. The concentration difference was due to the stratification of the lake during the boating season with very limited water exchange across the thermocline. MTBE and BTEX nearly completely volatilized before vertical lake mixing occurred in winter. Spatial and temporal variations of MTBE concentrations in the lake were observed and successfully predicted using two complementary box models (MASAS Light and Aquasim). The drinking water supply from holomictic lakes is not at risk for the scenarios studied if water is extracted from well below the thermocline. Since emissions of unburned gasoline into such water bodies are caused predominantly by boating activities, restrictions of highly emitting two-stroke engines could substantially reduce the MTBE and BTEX load of the epilimnion during the boating season.     

Horizontal and vertical variability of mercury species in pore water and sediments in small lakes in Ontario

Mary Lake, St. George Lake, and Philips Lake are located in the Greater Toronto Area, Ontario, Canada. These lakes are relatively small and have no direct inflow and outflow channels. Mercury (Hg) input to the lakes comes mainly from atmospheric deposition. Sediment cores from the points of the maximum lake depth and surface sediment samples from the points of maximum lake depth to the bank of each lake were collected in October 2005. Total and methyl mercury concentrations in the pore water and sediments of these samples were determined. In these small lakes with high organic content, there was no correlation between organic content and total mercury (THg) in the samples throughout the entire sediment cores while strong positive correlation between these two parameters was observed in all the surface sediments. Compared with typical methylmercury (MeHg) depth-profiles of sediment cores in other studies, where MeHg concentrations and methylation rates decreased sharply with increasing depth, MeHg distributions in the sediment cores in this study showed that MeHg might have been produced not only in the upper sediment but also in the deeper sediments, which resulted in a larger MeHg reservoir in the sediment. Organic matter, to some extent, affected MeHg distributions in the samples throughout the entire sediment cores. Concentrations of MeHg in all the surface sediments, however, were not controlled by organic matter, whereas they were largely a function of water column depths. Total mercury concentrations in pore water were relatively homogenous in both the sediment cores and surface sediment while MeHg in pore water generally deceased with increasing depth in the sediment cores and increasing distance from the centre of the lakes in surface sediments. Methylmercury contributed 1% to 76% of THg in the pore water samples. Concentrations and distributions of MeHg in overlying water and sediment-surface water in Mary Lake and St. George Lake suggested that both in situ production of MeHg in lake water and the release of MeHg from sediment contributed to high MeHg in deep anoxic water.     

Total and methyl mercury transformations and mass loadings within a wastewater treatment plant and the impact of the effluent discharge to an alkaline hypereutrophic lake

Concerns over the fate and bioaccumulation of mercury (Hg) inputs to Onondaga Lake, a hypereutrophic lake in central New York, prompted an investigation into the concentrations and fluxes of Hg discharge from the Onondaga County Metropolitan Wastewater Treatment Plant (METRO WWTP). Discharge of methyl Hg (MeHg) is of concern because it is the form of Hg that readily bioaccumulates along the aquatic food chain. This study incorporated clean protocols for sampling and Hg analysis to evaluate: seasonal patterns in the concentrations of total Hg (THg) and MeHg in the WWTP unit processes; the production of MeHg within the unit processes of the WWTP; the overall fate of THg and MeHg within the WWTP; and the relative impact of the Hg discharged from the WWTP to Onondaga Lake. Concentrations of THg (range: 80-860 ng/L) and MeHg (0.7-17 ng/L) in raw sewage were highly variable, with higher concentrations observed in the summer months. The dynamics of THg though the WWTP were correlated with total suspended solids (TSS). As a result, the majority of the THg removal (55%) occurred during primary treatment. Overall, about 92% of the THg entering the plant was removed as sludge, with volatilization likely a minor component of the overall Hg budget. The transformation of MeHg through the plant differed from THg in that MeHg was not correlated with TSS, and displayed strong seasonal differences between winter (November to April) and summer (May-October) months. During the summer months, substantial net methylation occurred in the activated sludge secondary treatment, resulting in higher MeHg concentrations in secondary effluent. Net demethylation was the dominant mechanism during tertiary treatment, resulting in removal of substantial MeHg from the secondary effluent. The overall MeHg removal efficiency through the plant was about 70% with more efficient removal during summer months. Sediment trap collections made below the epilimnion of Onondaga Lake indicated average deposition rates of 12 μg/m²-day for THg and 0.33 μg/m²-day for MeHg. These deposition rates are more than an order of magnitude higher than the thermocline area normalized external loads from METRO effluent (0.85 μg/m²-day for THg, 0.05 μg/m²-day for MeHg). Our findings indicate that the impact of the discharge from METRO is relatively small, contributing about 10-15% of Hg to the total gross Hg input to the hypolimnion of the lake.     

Influences of iron, manganese, and dissolved organic carbon on the hypolimnetic cycling of amended mercury

The biogeochemical cycling of iron, manganese, sulfide, and dissolved organic carbon were investigated to provide information on the transport and removal processes that control the bioavailability of isotopic mercury amended to a lake. Lake profiles showed a similar trend of hypolimnetic enrichment of Fe, Mn, DOC, sulfide, and the lake spike ((202)Hg, purity 90.8%) and ambient of pools of total mercury (HgT) and methylmercury (MeHg). Hypolimnetic enrichment of Fe and Mn indicated that reductive mobilization occurred primarily at the sediment-water interface and that Fe and Mn oxides were abundant within the sediments prior to the onset of anoxia. A strong relationship (r(2)=0.986, n=15, p<0.001) between filterable Fe and Mn indicated that reduction of Fe and Mn hydrous oxides in the sediments is a common in-lake source of Fe(II) and Mn(II) to the hypolimnion and that a consistent Mn:Fe mass ratio of 0.05 exists in the lake. A strong linear relationship of both the filterable [Fe] (r(2)=0.966, n=15, p<0.001) and [Mn] (r(2)=0.964, n=15, p<0.001) to [DOC] indicated a close linkage of the cycles of Fe and Mn to DOC. Persistence of iron oxides in anoxic environments suggested that DOC was being co-precipitated with Fe oxide and released into solution by the reductive dissolution of the oxide. The relationship between ambient and lake spike HgT (r(2)=0.920, n=27, p<0.001) and MeHg (r(2)=0.967, n=23, p<0.001) indicated that similar biogeochemical processes control the temporal and spatial distribution in the water column. The larger fraction of MeHg in the lake spike compared to the ambient pool in the hypolimnion suggests that lake spike may be more available for methylation. A linear relationship of DOC to both filterable ambient HgT (r(2)=0.406, n=27, p<0.001) and lake spike HgT (r(2)=0.314, n=15, p=0.002) suggest a role of organic matter in Hg transport and cycling. However, a weak relationship between the ambient and lake spike pools of MeHg to DOC indicated that other processes have a major role in controlling the abundance and distribution of MeHg. Our results suggest that Fe and Mn play important roles in the transport and cycling of ambient and spike HgT and MeHg in the hypolimnion, in part through processes linked to the formation and dissolution of organic matter-containing Fe and Mn hydrous oxides particles.     

Vertical distribution of sediment enzyme activities involved in the cycling of carbon, nitrogen, phosphorus and sulphur in three boreal rural lakes

Vertical distribution of eleven hydrolytic enzyme activities were investigated with fluorogenic model substrates in boreal rural lake sediments, Lake Uurainen (13 km(2)), Lake J?mij?rvi (9 km(2)) and Lake Pyylampi (0.068 km(2)), in Finland. The eleven hydrolytic enzyme activities were high into deep sediment layers indicating potential for turnover of organic matter in the permanently anoxic zones. The activities of beta-glucosidase, sulphatase and N-acetyl-glucosaminidase were similar in the three lakes independent on the ecological status of the lake. Acetate- and butyrate-esterase were more active than the other enzymes in the three lakes. These unspecific esterases had sediment activities in forest Lake Pyylampi close to those reported for boreal coniferous forest soils in Finland. Similar beta-cellobiosidase activities throughout the sediment depths indicated constant depolymerisation potential for cellulose.     

Temporal coherence in water temperature and chemistry under the ice of boreal lakes (Finland)

Temporal coherence was assessed for 11 limnological variables--water temperature, oxygen, conductivity, alkalinity, pH, colour, calcium (Ca), iron, aluminium, total phosphorus and total nitrogen--between 28 boreal lakes in southern Finland for the winter ice-covered period. The lakes were mainly small (<0.2 km2) and brown-coloured, and located within a circle of 10-km radius. A mean Pearson correlation coefficient for all lake pairs and variables averaged 0.37. Temporal coherence of variables across lake pairs was highest for conductivity, Ca, water temperature and alkalinity. The lake pairs with a direct surface water channel connection had a higher coherence than the lake pairs not connected by a stream. The size of the lake or catchment area had little effect on the coherence between the lakes. Temporal coherence was not strongly related to the difference in water colour (dystrophy) between the lakes. However, between polyhumic lakes (colour > 100 g Pt m(-3)) the coherence was generally higher than between less coloured lakes. Year-to-year variation in limnological characteristics could be partly explained by the variation in local weather. In March, water temperature and chemistry were infrequently related to winter weather, rather they correlated with the weather conditions of the previous autumn, while the ambient late winter weather seemed to have a stronger influence on lake conditions in April. Temporal variation in some variables was related to the atmospheric pressure changes over the North Atlantic (the North Atlantic Oscillation, NAO). Our results suggest that the potential effects of climatic change on lakes can be generalised regionally for brown-coloured dystrophic lakes.     

Grundwasseranbindung von alten Tagebaurestseen im Niederlausitzer Braunkohlentagebaugebiet

Groundwater with high acidity often leads to strongly acidic residual lakes in the vicinity of abandoned open pit lignite mines. Residual lake 77 (RL77) in Lower Lusatia (Germany) belongs to a sequence of residual lakes accomplishing groundwater drainage. A two-part high resolution three-dimensional groundwater model was set-up for the estimation of groundwater discharge into RL77. The objective of the simulations was to quantify the hydraulic component of the mass balance of the lake. The calculated total discharge could be distinguished with respect to water entering from the dump areas (11.3 l s ^?1 ) and from the undisturbed sediments (3.1 l s ^?1 ). The discharge was further distinguished with respect to the vertical entrance location into the temporarily stratified lake. It showed that groundwater discharged into the epilimnion of the lake at a rate of 9.7 l s ^?1 and into the hypolimnion at a rate of 4.7 l s ^?1 . Both the large fraction of water from the dump areas and the preferential entrance into the oxygen rich epilimnion of the lake have an adverse effect on lake water quality.     

Influence of intensive fishing on the partitioning of mercury and methylmercury in three lakes of Northern Québec

It has been demonstrated that intensive fishing, i.e., removing more than 25% of the fish biomass, can reduce mercury levels in predator fish in a lake. We test here the hypothesis that, by removing an important part of the fish biomass from a lake, a significant amount of methylmercury can be eliminated, therefore reducing the mercury available to the remaining biota, at least in the short term. A mass burden approach is used to evaluate the partitioning of total mercury and methylmercury in natural lake ecosystems. Three small natural lakes from the James Bay territory, in northern Québec, Canada, were selected for intensive fishing. Mercury (Hg) and methylmercury (MeHg) concentrations were evaluated for sediments, water column (dissolved fraction and suspended particulate matter), plankton, aquatic invertebrates, and fish. Biomasses were determined for fish, plankton, and aquatic invertebrates. Two case scenarios are presented using different mercury contributions from the sediment component (1 cm depth, and no sediment). Our results for the scenario including the sediment contribution show that lake sediments represent over 98% of the total mercury while the biotic components represent less than 0.1% of the same burden. For methylmercury, fish account for up to 5% of the burden, while sediments make up 84.6% to 93.1%. If we put aside the sediment contribution, the methylmercury in fish partitioning can represent up to 48%. As for invertebrates, they can account for up to 48% of the total MeHg burden. We do not observe any change in the partitionings or the quantities of Hg and MeHg before and after fishing in either of the two case scenarios even when we do not take into account dynamics of the ecosystems. This will be all the more the case when the dynamics of the system are included in the analyses. Therefore, biological parameters such as growth rates or fish diet must be considered.     

Total arsenic, lead, cadmium, copper, and zinc in some salt rivers in the northern Andes of Antofagasta, Chile

Lake Velenje is located in one of the most polluted regions in Slovenia, the Salek Valley. The major source of pollution in the valley is the coal-fired thermal power plant in Sostanj (STPP, capacity 775 MW). It has five separate units. All units have electrostatic precipitators for fly ash removal. Unit 4 also has installed a wet flue gas desulfurisation system (FGD system). Total mercury (THg) concentrations were measured in lignite, slag and ash samples from the STPP. In flue gas, different mercury species (THg, MeHg, Hg2+, Hg0) were determined separately for unit 4 and unit 5 which use different flue gas cleaning technology. Mercury and methyl mercury (MeHg) concentrations were also measured in lake water at different depths, in inflow water, outflow water, rain, snow and lake sediments in order to establish the influence of the power plant on the lake. Most mercury emitted from the power plant is in the elemental form. The ratio between oxidised and elemental Hg depends on the flue gas cleaning technology. Mass balance calculations have been performed for the STPP. The results show that the major sources of mercury in Lake Velenje are wet deposition and lake inflows. Total and MeHg concentrations in the water column are very low and can be compared to other non-contaminated freshwater lakes in the world.     

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.     

Evidence for flocculation in glacier-fed Lillooet Lake, British Columbia

This study provides evidence of in situ flocculation in a glacier-fed freshwater lake. Direct observation of flocculated particles using microscopy provides the strongest evidence to establish that clastic suspended sediment in Lillooet Lake is flocculated. Flocs in Lillooet Lake exhibit a hierarchical size structure, with primary particles (<4 microm) joining to create microflocs (10-35 microm), and both primary particles and microflocs joining to create macroflocs (200-280 microm). In situ laser particle sizing reveals primary particles and microflocs dominate in the epilimnion, a zone susceptible to shear induced by wind or inflow and exhibiting the highest suspended sediment loads. Macroflocs dominate in the hypolimnion, a more quiescent zone and exhibiting the lowest suspended sediment loads. Flocculated particles are invisible to traditional methods of sediment analysis in glaciolacustrine settings. Flocculation is a plausible explanation for accelerated sedimentation of the clay-sized sediments known to dominate the size distribution of varves in other glacier-fed lakes.     

A dynamic model of caesium transport in lakes and their catchments

A mathematical model has been developed to predict radiocaesium concentrations over time within individual compartments of the lake and its catchment. The lake has been divided into five compartments; catchment, lake water (epilimnion and hypolimnion during stratification), lake sediment and fish. Radiocaesium enters the lake via contaminated rainfall and catchment runoff. A proportion of this radiocaesium absorbs onto suspended solids in the lake. This proportion is represented by a distribution coefficient. Sedimentation of the suspended solids occurs at a rate defined by the areal removal coefficient and results in increased caesium concentrations in the sediment. The ingestion of radiocaesium by either water column or benthic feeding fish is described by transfer functions. The model has been tested against data collected from Esthwaite water and Windermere shortly after the Chernobyl reactor accident from May 1986 to December 1987. The model simulates observed radiocaesium concentrations in Esthwaite lake water and sediment and also in lake water, sediment and fish in Windermere. The model could form the basis of a valuable management tool for the water industry should a major airborne pollution event occur again.     

Formation of dissolved gaseous mercury in a tropical lake (Petit-Saut reservoir, French Guiana)

Formation of dissolved gaseous mercury (DGM) and its volatilization from aquatic systems can be considered as a natural attenuation process, which limits the methylation of mercury (Hg) and Hg accumulation in fish. Although gold-mining activity and erosion of tropical soils lead to increased Hg concentration in aquatic systems, little is known about DGM production and distribution in tropical aqueous systems. Our work explores the vertical distribution and principal sources of DGM in a meromictic Amazonian reservoir. Dissolved gaseous mercury measurements carried out in Petit-Saut reservoir (French Guiana, South America) revealed DGM increase in the surface waters and at the bottom layers of the reservoir during the dry season. As in arctic and temperate lakes, high DGM concentrations in surface waters were attributed to sunlight-mediated photochemical processes. Dissolved gaseous mercury concentrations in the anaerobic hypolimnion were larger than in temperate or arctic lakes. In order to elucidate Hg(II) reduction pathways in the bottom layer of tropical reservoir, laboratory Hg(II) reduction experiments were performed with anoxic aqueous suspensions of surface sediments either untreated or treated by gamma-ray and NaN3. Our results indicated that DGM production at the bottom layer of Petit-Saut reservoir was biologically mediated. Dissolved gaseous mercury formation rates in the surface sediment suspensions were of the same order of magnitude as formation rates in freshwater lakes reported in literature.     

Effects of whole lake mixing on water quality and phytoplankton

This paper describes the effect of artificial mixing of two Oklahoma lakes with a downflow pump on water quality and algal biomass. Artificial pumping in Arbuckle Lake (951 ha), advanced autumnal turnover, but never destratified the lake completely. Ammonia decreased in the epilimnion, while sulfide (H₂S) declined and dissolved oxygen (DO) increased in the hypolimnion. Other water quality parameters did not change. Near-bottom concentrations of manganese (Mn²⁺) increased, indicating pumping did not affect water chemistry near deep sediments (> 16 m). Pumping did not change significantly the depth of the Secchi disc or algal biomass as measured by chlorophyll a. The algal flora was dominated by diatoms at all times, and the density of blue-green algae was always low. Pumping kept Ham's Lake (41 ha) destratified, but seldom produced completely isothermal conditions or isochemical concentrations of DO. There was no drastic change in other water quality parameters. However, artificial mixing decreased water clarity and increased algal biomass by a factor of about 2.5, probably by reducing sinking rates of the phytoplankton. Artificial mixing apparently eliminated a fall pulse of Microcystis.     

Mercury and methylmercury concentrations in high altitude lakes and fish (Arctic charr) from the French Alps related to watershed characteristics

Total mercury (THg) and methylmercury (MeHg) concentrations were measured in the muscle of Arctic charr (Salvelinus alpinus) and in the water column of 4 lakes that are located in the French Alps. Watershed characteristics were determined (6 coverage classes) for each lake in order to evaluate the influence of watershed composition on mercury and methylmercury concentrations in fish muscle and in the water column. THg and MeHg concentrations in surface water were relatively low and similar among lakes and watershed characteristics play a major role in determining water column Hg and MeHg levels. THg muscle concentrations for fish with either a standardized length of 220 mm, a standardized age of 5 years or for individualuals did not exceed the 0.5 mg kg^− 1 fish consumption advisory limit established for Hg by the World Health Organization (WHO, 1990). These relatively low THg concentrations can be explained by watershed characteristics, which lead to short Hg residence time in the water column, and also by the short trophic chain that is characteristic of mountain lakes. Growth rate did not seem to influence THg concentrations in fish muscles of these lakes and we observed no relationship between fish Hg concentrations and altitude. This study shows that in the French Alps, high altitude lakes have relatively low THg and MeHg concentrations in both the water column and in Arctic charr populations. Therefore, Hg does not appear to present a danger for local populations and the fishermen of these lakes.     

Effect of nitrate concentration in lake water on phosphate release from the sediment

Thirty-one Danish eutrophic lakes were investigated routinely over 1 year during the period 1978–1980. Nine lakes were dimictic with anoxic hypolimnia and 22 were very productive and shallow, polymictic lakes. Phosphate release from the sediment resulted in large increases in phosphate concentrations in anoxic hypolimnion, if concentrations of oxidized nitrogen in hypolimnion were less than about 0.1 g N m⁻³. If concentration of oxidized nitrogen (mainly nitrate) in hypolimnion was about 1 g N m⁻³ or higher, no release of phosphate from the sediment to anoxic hypolimnion occurred. In lakes with no summer stratification a release of phosphate from the sediment to the well oxygenated water resulted in summer maxima of phosphate in the lake water, when nitrate concentration in the water was less than about 0.5 g N m⁻³, but no release took place if nitrate concentration exceeded about 0.5 g N m⁻³. This effect of oxidized nitrogen in preventing phosphate release from the sediment demonstrates the ability of oxidized nitrogen to buffer the redox potential of the surface sediment at a level high enough to prevent a release of phosphate. Thus, among the efforts to limited phytoplankton biomass in lakes an artificial enrichment with nitrate may in some cases be an important supplement to the usually necessary reduction in phosphorus loading, but the possibility of a stimulation of phytoplankton growth through the addition of nitrate must be carefully considered.     

Effects of intensive fishing on the structure of zooplankton communities and mercury levels

Following the impoundment of hydroelectric reservoirs, a small fraction of the mercury (Hg) in the flooded soils is transformed and released into the ecosystem. This causes an increase in the mercury level in the food chain, particularly in piscivorous fish, and represents a potential risk for human health. In 1998, Hydro-Québec carried out an intensive fishing campaign to examine the feasibility of using intensive fishing as a mitigation tool. The goal of this particular part of the project was to evaluate the impact of intensive fishing on the zooplanktonic communities' structure and mercury levels. Specifically, the effects of intensive fishing on: (1) total biomass and zooplankton size structure (>500, 200-500, 100-200 and 53-100 microm); (2) species composition; and (3) total mercury and methylmercury (MeHg) concentrations in zooplankton of different size fractions were studied. Although important decreases were observed in total zooplankton biomass or its size structure, it cannot be related to the intensive fishing. The results, however, show major changes in the dominance of macrozooplanktonic species in fished lakes as opposed to reference lakes. Similarly, in contrast to the reference lakes, mercury and MeHg concentrations in the four size fractions of the zooplanktonic communities changed from 1998 to 2000 in the fished lakes. The MeHg concentration increased from approximately 20 to 200 ng/gdw from the 53 to 500 microm mesh size fraction, showing a biomagnification in the food chain. The canonical correspondence analysis showed that lakes dominated by Holopedium gibberum presented higher concentrations of Hg and MeHg than lakes dominated by Daphnia spp.     

Factors affecting enhanced mercury bioaccumulation in inland lakes of Isle Royale National Park,USA

We investigated factors causing mercury (Hg) concentrations in northern pike to exceed the consumption advisory level (>500 ng/g) in some inland lakes of Isle Royale National Park. Using Hg-clean techniques, we collected water, zooplankton, macro invertebrates, and fishes in 1998 and 1999 from one advisory lake, Sargent Lake, for analysis of total mercury (Hg(T)) and methylmercury (MeHg). For comparison, samples were also collected from a non-advisory lake, Lake Richie. Concentrations of Hg(T) in northern pike were significantly higher in Sargent Lake (P<0.01). Counter to expectations, mean concentrations of both Hg(T) and MeHg in open water samples were slightly higher in Lake Richie. However, zooplankton in Sargent Lake contained higher average concentrations of Hg(T) and MeHg than in Lake Richie. Mercury concentrations in macro invertebrates were similar between lakes, but different between taxa. The two lakes exhibited similar Hg(T) concentrations in age-1 yellow perch and adult perch but concentrations in large adult perch (>160 mm) in Sargent Lake were twice the concentrations in Lake Richie. Analysis of stable isotopes (delta(13)C and delta(15)N) in biota showed that pike from the two lakes are positioned at the same trophic level (4.2 and 4.3), but that the food web is more pelagic-based in Sargent and benthic-based in Richie. Factors causing concentrations in large pike to be higher in Sargent Lake may include higher bioavailability of methylmercury and a food web that enhances bioaccumulation.     

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.     

Can flooded organic matter from sediments predict mercury concentrations in zooplankton of a perturbed lake?

Mercury (Hg) and the composition of organic matter (OM) in sediments and large zooplankton (> 190 microm) have been analysed in a perturbed lake (Lake 154) in northern Quebec, Canada. We investigated whether methylmercury concentrations ([MeHg]) in flooded sediments can predict [MeHg] in zooplankton. [MeHg] at the uppermost sediment layers located immediately downstream of an inundated system (LA-40) are 10 times higher than at other sediment stations. Zooplankton organisms sampled downstream of LA-40 contain higher [MeHg] (183+/-50 ng g(-1) dry wt.) than those of neighbouring natural lakes (66+/-37 ng g(-1) dry wt.). OM in sediment and zooplankton samples has been characterised applying atomic carbon/nitrogen ratios (C/N) and stable carbon isotopic ratios (delta13C). While C/ N ratios in zooplankton from Lake 154 (5.6+/-0.4) do not differ from samples of natural lakes (5.7+/-0.1), delta13C signatures are more depleted at stations close to the discharge of LA-40 (-33+/-0.4/1000) with respect to zooplankton from unperturbed lakes (-30.6+/-0.7/1000). Results of this tracer study suggest that the biochemical composition of OM in flooded sediments did not modify the biochemical composition of zooplankton and can therefore not predict [MeHg] in zooplankton.