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We used a dynamic model to estimate the net carbon dioxide production (NCP) of three experimentally flooded upland areas (FLUDEX) over a period of 4 years and NCP from a flooded wetland (ELARP) over 12 years (2 year pre-flooding, 10 years post-flooding). The 3 flooded upland areas had been chosen to have differing amounts of carbon stored in soils and vegetation. Estimates of NCP ranged from 33–55 mmole·m·d in the first year and decreased steadily to 13–30 mmole·m·d in the fourth year. The NCP from the reservoir with the lowest carbon stock was always lowest, the other two were similar. The NCP estimated for the wetland rose from 45 mmole·m·d in the first year of flooding to 178 mmole·m·d in the years 7–9. A decrease to 126 mmole·m·d was seen in the last year. Overall the model did a good job of simulating the measured results and provided a consistent methodology for comparison of NCP. In this boreal forest area of northwest Ontario flooding of wetland area results in much higher NCP and over a much greater duration than upland flooding.

This paper presents the results of gross carbon dioxide and methane emission measurements in several Brazilian hydro reservoirs. The term ‘gross emissions’ means gas flux measurements from the reservoir surface without correcting for natural pre-impoundment emissions by natural bodies such as the river channel, seasonal flooding and terrestrial ecosystems. The net emissions result from estimating pre-existing emissions by the reservoir. Measurements were carried in the Miranda, Barra Bonita, Segredo, Três Marias, Xingó, Samuel and Tucuruí reservoirs, located in two different climatological regimes. Additional data were used here from measurements taken at the Itaipu and Serra da Mesa reservoirs. Emissions of carbon dioxide and methane in each of the reservoirs selected, whether through bubbles or diffusive exchange between water and atmosphere, were assessed by sampling, with subsequent extrapolation of results to obtain a value for the reservoir. A great variability was found in the emissions, linked to the influence of various factors, including temperature, depth at the point of measurement, wind regime, sunlight, physical and chemical parameters of water, the composition of the local vegetation and the operational regime of the reservoir.

This paper summarizes, in a first part, results of greenhouse gas emissions from the hydroelectric reservoir of Petit Saut in French Guiana obtained during the three first years after impoundment (1994–1997). Results from three years of measurements have been extrapolated to estimate trends in methane emissions and the carbon budget of the reservoir over a 20-year period. Extrapolations were made using the global warming potential concept to calculate cumulative greenhouse gas emissions at a 100-year time horizon and to compare these emissions to potential emissions from thermal alternatives. In a second part, we analyze new data from long term continuous observations (1994–2003) of methane concentrations in the reservoir and flux data obtained during a recent campaign in May 2003. These data confirm predicted trends and show some suitable adjustments. They constitute a unique data base which is used for the development of a model to simulate both water quality and greenhouse gas emissions from tropical artificial reservoirs.

A set of experiments was designed to measure the production of carbon dioxide and methane during decomposition of inundated samples of representative vegetation and soil samples originating from the James Bay territory over a period of approximately one year. Controlled laboratory conditions were set for water temperature (4–22°C), pH (4.5–7.0) and dissolved oxygen concentration (< 2 mg·L and > 2 mg·L). These conditions covered the range of conditions under which vegetation and soil are submitted during permanent flooding in newly created hydroelectric reservoirs. Representative phytomass samples consisted of spruce needles (), alder leaves (), lichen (), green moss () and herbaceous plants (mixed species). Representative forest soil samples consisted of lichen () humus and green mosses () humus with Sphagnum moss () used as a representative ground component (phytomass) for wetlands. Production of carbon dioxide over time was observed from all samples under the given experimental conditions. The quantities of carbon dioxide produced from the vegetation samples were largest under oxic conditions at the higher temperature. The average cumulative quantities produced over 345 days ranged from 201 mg CO·g (dry weight) to 447 mg CO·g (dry weight) with the largest quantities produced from green moss. For the soil samples, the largest quantities of carbon dioxide produced occurred also at the higher temperature but were 15–40% larger under anoxic conditions. Under such conditions, the average cumulative quantities produced over 320 days from lichen humus and green moss humus were 72 g CO·m and 140 g CO·m respectively. Small quantities of methane were produced from the soil samples but only under the most favourable temperature and pH conditions and were higher under anoxic conditions. pH conditions and were higher under anoxic conditions. Under such conditions, the average cumulative quantities of methane produced over 320 days from lichen humus and green moss humus were 0.21 g CH·m and 0.56 g CH·m respectively. Production of methane from vegetation samples was significant only for the higher temperature under anoxic conditions. Under such conditions, the average cumulative quantities produced over 345 days were largest for green moss with a value of 1.72 mg CH·g (dry weight). Results have shown that, under the most favourable conditions for decomposition, the production of carbon dioxide and methane from inundated phytomass and humus soil samples was still very active after 345 and 320 days respectively. Rates of production of CO and CH calculated from the cumulative quantities released from the flooded vegetation and soil samples under the given experimental conditions represent a reference data set from which production of CO and CH emitted from reservoirs under field conditions can be estimated (Thérien and Morisson, Chap. 25).

Hydroelectric reservoirs and lakes in boreal Québec produce greenhouse gases (GHG) mainly in the form of CO. No method exists, however, which can directly measure the flux of CO across the air-water interface and the methods that are currently used are only representative of a small surface area and a specific time period. The objective of the current study is to improve and validate an isotopic approach to estimate the annual CO flux across the air-water interface. The model requires the calibration of isotopic fluxes into and out of the interface. When applied to the Robert- Bourassa hydroelectric reservoir in boreal Québec, this model estimated an average CO diffusive flux across the air-water interface of 225±51 mg CO·m·d in the summer of 2000 and of 446±93 mg CO·m·d in the summer of 2001. These average fluxes are representative of the whole icefree period.

The FLooded Uplands Dynamics EXperiment (FLUDEX) was initiated to quantify carbon dioxide (CO) and methane (CH) production in boreal reservoirs, and to better understand underlying biogeochemical processes (dissolved inorganic carbon [DIC] production, net primary production [NPP], methanogenesis, and CH oxidation) governing CO and CH production in flooded boreal landscapes. The study experimentally flooded three upland boreal forest sites with different organic carbon (OC) storage in soils and vegetation over three seasons (June to September 1999–2001). Mass budgets of all reservoir inorganic carbon (inorganic C) and CH inputs and outputs were calculated to quantify net reservoir CO and CH production, and isotopic ratio mass budgets were calculated to quantify biogeochemical processes controlling net reservoir CO and CH production.

The three reservoirs produced both CO and CH during each of the three flooding seasons, but neither CO nor CH production was related to overall mass of flooded OC. Net reservoir CO production in the second and third flooding seasons (408 to 479 kg C ha) was lower than in the first flooding season (703 to 797 kg C ha), while reservoir CH production steadily increased with each successive flooding season (from 3.2 to 4.6 kg C ha in 1999 to 29.7 to 35.2 kg C ha in 2001). Over three flooding seasons, NPP ranged from 77 to 273 kg C ha and consumed 15 to 40% of gross reservoir CO production. CH oxidation was negligible during the first flooding season, but reduced gross reservoir CH production by 50% during the second flooding season, and by 70 to 88% during the third flooding season. However, despite decreases in net reservoir CO production and increases in CH oxidation over the study period, the overall total global warming potential (GWP) of the FLUDEX reservoirs remained constant due to successive increases in net CH production.

The objective of this chapter is to establish the functional links between the organic carbon (C) dynamics in soils, the biogeochemical C cycle of forested watershed and the potential changes in the sequestration of atmospheric C by forest soils in response to changing climatic conditions. After an introductory statement on greenhouse gases and their effects on climate, the second part of the chapter describes the properties, functions and biogeochemical cycling of organic carbon (C) in forest soils. The third part of the chapter presents the results of an extensive review of organic C mass balance studies conducted in forested watersheds of northeastern North America. The soil and plant C pools and fluxes are quantified and results are compared to those obtained from other environments, such as southeastern United States and Western and Central Europe. The potential contribution of soils to the emission of greenhouse gases is critically discussed through an evaluation of the net role of soils on organic carbon (C) cycling and on its transport from terrestrial to aquatic environments. Based on the available data and evidences, it appears that the question as to whether soils from northern forests will behave as a net source or sink of C under a warmer climate cannot yet be answer unequivocally.

The aim of this chapter was to determine the influence of zooplankton organisms on carbon cycling within reservoirs and lakes from Northern Quebec. The first part of the paper presents results from LG-2 reservoir where zooplankton dynamics were followed from 1 year prior to impoundment to 6 years after flooding. In terms of community structure, flooding was associated with an increase in zooplankton biomass with the strongest effects observed for cladocerans and rotifers. This increase was related to changes in the physical characteristics of the sampled sites (water residence time, temperature and turbidity), chemical characteristics of the water (total phosphorus) and the abundance of resources (Chl. ).

The second part of the chapter is a comparison of zooplankton community structure expressed as limnoplankton (AFDW) for several reservoirs of different age (1 to 35 years old). We related the average size of organisms to the algal biomass and finally to the carbon fluxes measured between the water and the atmosphere. We found that part of the larger carbon fluxes observed in young reservoirs compared to older reservoirs may be explained by a top-down control of primary producers by zooplankton.

Functional and structural aspects of the indigenous methanogenic and methanotrophic microbial populations were assessed in soil, peat and sediment from a hydroelectric reservoir (Robert-Bourassa) located in the subarctic Taiga. Three locations (un-flooded, seasonal flooding, and permanent flooding) in the reservoir were selected for sampling of forest soil and peat soil. Lakes located near the reservoir were also sampling for comparison with nearby unperturbed aquatic systems. Using samples incubated in microcosms at 5, 10 and 25°C, methane production and oxidation were quantified by gas chromatography. Structural aspects included bacterial counts of methanotrophic bacteria, and PCR amplification using 16S rDNA universal primers and primers specific for genes involved in methanogenesis or methanotrophy.

Methanogenesis in the different systems appeared to depend on a combination of environmental factors, including the amounts and quality of organic carbon, and the abundance of oxidizing ions (Fe, SO). Periodically flooded or flooded peats contributed more to methane production than unflooded peats, soils and natural lake sediments. Similarly, methane oxidation rates were higher in peat soils than in flooded soils or lake sediments. The lowest rate of methane oxidation was observed in the forest soil, which was a typically undisturbed soil where the rate of CH production was close to the lower range of values observed in this study. This parallel evolution between the potential rate of methanogenesis and CH oxidation suggests an association between CH oxidation activity and CH supply. Methanogenesis appeared more sensitive to temperature increases than methanotrophy.

The nucleotide sequences of PCR amplified and cloned fragment, a gene specific to methanogenesis revealed that many of the sequences obtained from the soils in this study were closely related to only uncultured clones of methanogens. Methanotrophic bacterial abundance was higher in flooded peat and lake sediment than in flooded soil, but abundance of methanotrophic bacteria in unflooded peat was lower than in unflooded forest soil. PCR amplification of genes specific to methanotrophic bacteria suggested that flooding of soil leads to a shift in populations of methanotrophic bacteria.

A comparison of methane production and oxidation values obtained during this study indicated that essentially all of the methane produced in peat, forest soil and sediment could be oxidized within the system with little net atmospheric emission.

As part of a comprehensive study intended to elucidate mechanisms that drive carbon dioxide (CO) emissions from hydroelectric reservoirs, we examined bacterial abundance and production in the water column of three hydroelectric reservoirs of different ages and their nearby lakes, in relation to temperature, dissolved organic carbon (DOC), Chlorophyll , phytoplankton production and CO fluxes from these ecosystems to the atmosphere. The summer values of bacterial production and bacterial specific production in each reservoir were similar to those in the nearby lakes. There was no clear evidence that the age of the reservoir had a strong effect on the measured bacterial activities, even though the highest values of these activities were found in the youngest reservoir. DOC and nutrient availability were among the major factors driving bacterial activities. DOC was indeed positively related to bacterial production, bacterial specific production and the proportion of bacteria with high nucleic acid content (i.e. bacteria with higher activity = % HNA) in these sites, where nutrients were, most of the time, found to be limiting for bacterial growth. Among the bacterial variables tested, the % HNA appeared to be important in determining changes in CO emissions at least in reservoirs, where it explained 38% of the variance of CO fluxes to the atmosphere. Such a relationship was not found in lakes. These results indicate that examining different aspects of the functioning of bacterial communities may help to understand the mechanisms underlying CO emissions from aquatic ecosystems, and suggest that the relative importance of factors driving bacterial activities and CO efflux may be quite different in lakes versus reservoirs.