[1] Six reservoirs located in the Western United States (F. D. Roosevelt, Dworshak, Wallula, Shasta, Oroville, and New Melones) were sampled in order to estimate their greenhouse gas (GHG) emissions. Two types of fluxes were assessed: (1) diffusive fluxes of methane (CH 4 ) and carbon dioxide (CO 2 ) at the air/water interface and (2) degassing fluxes of CH 4 and CO 2 from water passing through the turbine spillways. Diffusive flux measurements indicated that the surface of the reservoirs were a source of CH 4 during the sampling period (from +3.2 to +9.5 mg CH 4 m À2 d À1 ). Oroville (+1026 mg CO 2 m À2 d À1 ) and Shasta (+1247 mg CO 2 m À2 d À1 ) surfaces were also sources of CO 2 . In contrast, the surface of all the other reservoirs constituted sinks for CO 2 (from À349 to À1195 mg CO 2 m À2 d À1 ). Degassing fluxes ranged from +0.003 to +0.815 t CH 4 d À1 , and from +16 to +324 t CO 2 d À1 . Daily GHG budgets ranged from +0.146 to +2.228 t CH 4 d À1 , and from À15 to +224 t CO 2 d À1 . Degassing fluxes represented an important term of these budgets. A significant correlation was observed between the magnitude of CO 2 diffusive fluxes and the water pH (R 2 = 0.81; p < 0.0001). All other correlations between GHG diffusive fluxes and independent variables tested were weak and/or not significant. Finally, while attempting to resolve the spatial variability in diffusive fluxes, we were able to cluster reservoirs neither according to geological nor ecological criteria.

International audienceSeasonal snow is an active media and an important climate factor that governs nutrient transfer in Arctic ecosystems. Since the snow stores and transforms nutrients and contaminants, it is of crucial importance to gain a better understanding of the dynamics of contaminant cycling within the snowpack and its subsequent release to catchments via meltwater. Over the course of a two-month field study in the spring of 2008, we collected snow and meltwater samples from a seasonal snowpack in Ny-Ålesund, Norway (78°56′N, 11°52′E), which were analyzed for major inorganic ions and some organic acids, as well as total, dissolved, bioavailable mercury (THg, DHg, BioHg, respectively) and monomethylmercury (MMHg) species. We observe a seasonal gradient for ion concentrations, with surface samples becoming less concentrated as the season progressed. A significant negative correlation between BioHg and MMHg was observed in the snowpack. MMHg was positively and significantly correlated to methanesulfonate concentrations. Based on these results, we propose a new model for aerobic methylation of mercury involving species in the dimethylsulfoniopropionate cycle

Most studies dealing with greenhouse gas (GHG) emissions from large boreal reser voirs were conducted during the ice-free period. In this paper, the potential methane (CH 4 ) and carbon dioxide emissions are estimated for two hydroelectric reservoirs, as well as for a small experimental reservoir from boreal latitudes (northern Quebec, Canada) at the ice breakup event through diffusion (diffusive fluxes) and release of bubbles (bubbling fluxes). The results of this preliminary study suggest that the winter diffusive fluxes at the air-water interface of the sampled reser voirs represent < 7% of their cumulative carbon emissions during the ice-free period. Furthermore, the release upon ice-break of CH 4 bubbles accumulated under the ice cover during the winter could represent ≤ 2% of the summer carbon emissions from hydroelectric reservoirs in northern Quebec. The results presented herein suggest that the GHG emissions upon ice break-up from the boreal reservoirs investigated are a small, but non-negligible, component of their annual GHG emissions.

The ever‐increasing demand for energy over the recent development of societies has spurred the construction of hydroelectric facilities. Since dams were first used to generate hydropower around 1890, their construction rate increased tremendously to peak during the 1950s and the 1980s . Today, about 25% of the 33,105 large dams (≥15 m height) listed by the International Commission on Large Dams (ICOLD) are used for hydropower generation and currently provide 19% of the world's electricity supply . Although over 150 countries operate hydroelectric plants, Brazil, China, Canada, Russia, and the United States produce more than 50% of the world's hydropower . According to data from 1996, hydroelectric reservoirs worldwide cover an estimated 600,000 km 2 .

In order to evaluate the role of photochemistry in the carbon dioxide (CO 2 ) generation from a 10-year-old boreal reservoir, the photomineralization of dissolved organic matter (DOM) was assessed and compared to a boreal river as well as to boreal and temperate lakes during July and August, 2003. Sterile water samples were irradiated by sunlight over the whole photoperiod and subsequently analyzed for CO 2 . Mean energy-normalized apparent photochemical yield of CO 2 (an index of DOM photoreactivity normalized for the energy absorbed by samples) was significantly higher in the reservoir (27.7 ± 13.0 mg CO 2 Ám -3 ÁkJ -1 ) and the boreal river (35.8 ± 2.3 mg CO 2 Ám -3 ÁkJ -1 ) than in the boreal lakes (15.5 ± 5.1 mg CO 2 Ám -3 ÁkJ -1 ). The DOM photoreactivity of the temperate lakes (20.9 ± 8.1 mg CO 2 Ám -3 ÁkJ -1 ) was not statistically different from any type of boreal water bodies. There was no significant difference in either the integrated photoproduction of CO 2 (273-433 mg CO 2 Ám -2 Ád -1 ) or the potential photochemical contribution to CO 2 diffusive fluxes (56-92%) among these water bodies. DOM photoreactivity was significantly affected by the cumulative hydrological residence time (CHRT) when considering the whole data set. However, when considering only the boreal water bodies, iron (Fe) and manganese (Mn) also intervened. The fact that DOM photoreactivity was related to CHRT as well as to Fe and Mn concentrations, which are respectively permanent and long-lasting features of the reservoir, suggests that the photoproduction of CO 2 is not likely to decrease over time. This process may therefore play a substantial role in the long-term CO 2 emissions from boreal reservoirs during the summer, its potential contribution to CO 2 diffusive fluxes being estimated at 56 ± 29 %.

[1] Most natural freshwater lakes are net greenhouse gas (GHG) emitters. Compared to natural systems, human perturbations such as watershed wood harvesting and long-term reservoir impoundment lead to profound alterations of biogeochemical processes involved in the aquatic cycle of carbon (C). We exploited these anthropogenic alterations to describe the C dynamics in five lakes and two reservoirs from the boreal forest through the analysis of dissolved carbon dioxide (CO 2 ), methane (CH 4 ), oxygen (O 2 ), and organic carbon (DOC), as well as total nitrogen and phosphorus. Dissolved and particulate organic matter, forest soil/litter and leachates, as well as dissolved inorganic carbon were analyzed for elemental and stable isotopic compositions (atomic C:N ratios, d 13 C org , d 13 C inorg and d 15 N tot ). We found links between the export of terrestrial organic matter (OM) to these systems and the dissolved CO 2 and O 2 concentrations in the water column, as well as CO 2 fluxes to the atmosphere. All systems were GHG emitters, with greater emissions measured for systems with larger inputs of terrestrial OM. The differences in CO 2 concentrations and fluxes appear controlled by bacterial activity in the water column and the sediment. Although we clearly observed differences in the aquatic C cycle between natural and perturbed systems, more work on a larger number of water bodies and encompassing all four seasons should be undertaken to better understand the controls, rates, and spatial as well as temporal variability of GHG emissions, and to make quantitatively meaningful comparisons of GHG emissions (and other key variables) from natural and perturbed systems.

The dry ice sowing experiment (DISE) consisted in adding dry ice to a lake and monitoring the subsequent evasion of carbon dioxide (CO2). DISE allowed us to evaluate two approaches commonly used for measuring aquatic CO2 diffusive fluxes: the boundary layer equation (BLE) from Cole and Caraco (1998) and a particular model of static chamber (SC). CO2 evasion measurements with both approaches were compared to CO2 mass budgets as a relative reference to define their recovery coefficients (p). p for the BLE and the SC over the whole measurement period were 101 +/- 14% and 115 +/- 56%, respectively. Results from discrete sampling intervals revealed that the BLE generally provided estimations in good agreement (80-130%) with the mass budgets during both daytime and nighttime. Variations in p for the BLE were related to wind speed and, consequently, piston velocity (k600). The SC overestimated CO2 evasion during daytime (149 +/- 39%), and underestimated it during nighttime (57 +/- 18%). Variations in p for the SC were related to k600, stemming mainly from the alteration of the air/ water temperature gradient.

Trace levels of three organophosphate insecticides (OPI) were detected in eight fish species from the region of Santarém, State of Pará, Brazil. Individual concentrations of OPI in fish ranged from less than the detection limit to 2,1 ppb. Mean concentrations of chlorpyrifos, malathion, and methyl-parathion were 0,3 ± 0,3, 0,1 ± 0,1, and 0,3 ± 0,3 ppb, respectively. Pellona flavipinnis, the largest and fattest piscivorous species analyzed, was the most contaminated. Since an inhabitant of this Amazonian region consumes 220 g of fish per day on average, ingested doses of chlorpyrifos, malathion, and methyl-parathion may reach up to 308, 220, and 462 ng·d-1, respectively. Compared to acceptable daily intakes (ADI), quantities of OPI absorbed via fish consumption on a daily basis are far below deleterious levels. We estimated that even considering the highest OPI contents detected, the average daily fish consumption of anadult of 60 kg would have to increase by ca. 1 950, 5 450, and 2 600 times to reach ADI of chlorpyrifos, malathion, and methyl-parathion, respectively. Neither fish diet nor fish lipid content enabled us to completely explain the interspecific differences observed.