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Über dieses Buch

In a time when an unquestionable link between anthropogenic emissions of greenhouse gases and climatic changes has finally been acknowledged and * widely documented through IPCC reports, the need for precise estimates of greenhouse gas (GHG) production rates and emissions from natural as well as managed ecosystems has risen to a critical level. Future agreements between nations concerning the reduction of their GHG emissions will - pend upon precise estimates of the present level of these emissions in both natural and managed terrestrial and aquatic environments. From this viewpoint, the present volume should prove to a benchmark contribution because it provides very carefully assessed values for GHG emissions or exchanges between critical climatic zones in aquatic en- ronments and the atmosphere. It also provides unique information on the biases of different measurement methods that may account for some of the contradictory results that have been published recently in the literature on this subject. Not only has a large array of current measurement methods been tested concurrently here, but a few new approaches have also been developed, notably laser measurements of atmospheric CO concentration 2 gradients. Another highly useful feature of this book is the addition of - nitoring and process studies as well as modeling.




Ce chapitre a pour but de faire le point sur l’état des connaissances et d’identifier les lacunes relatives à la problématique de l’émission de gaz à effet de serre (GES) par les réservoirs hydroélectriques et les écosystèmes naturels. Il est devenu essentiel d’intégrer nos connaissances du cycle du carbone à des échelles temporelles et spatiales plus vastes de façon à mieux définir l’ampleur des flux de GES associés aux réservoirs1 et aux écosystèmes naturels. Les données disponibles proviennent d’études à petite échelle et de courte durée (1 à 10 ans), effectuées surtout en région boréale, mais aussi en régions semi-aride et tropicale. La variabilité naturelle des flux de GES due à des variations climatiques régionales et leurs impacts sur la production biologique globale est plus importante que celle des méthodes de mesures. Il faut donc garder à l’esprit que les incertitudes concernant les flux de GES sont avant tout le résultat de variations spatiales et temporelles naturelles des flux, et non pas des techniques de mesure disponibles. La présente synthèse se base sur les résultats de plus de dix ans de suivis obtenus par différentes équipes de recherche de plusieurs universités, institutions gouvernementales et compagnies d’électricité.
Alain Tremblay, Louis Varfalvy, Charlotte Roehm, Michelle Garneau

1. Introduction

Without Abstract
Alain Tremblay, Maryse Lambert, Claude Demers

2. Analytical Techniques for Measuring Fluxes of CO2 and CH4 from Hydroelectric Reservoirs and Natural Water Bodies

Hydro-Québec and its partners have been measuring greenhouse gases (GHG) gross fluxes from hydroelectric reservoirs and natural water bodies since 1993. Over the years the methods have changed with a constant aim for improvement. The methods used are: the thin boundary layer, the use of floating chambers with in situ or exsitu laboratory analysis and the use of floating chambers coupled to an automated instrument (NDIR or FTIR). All these methods have their pros and their cons. Over the years many tests were done to compare the methods.
There is no significant difference in the results obtained with the in situ or exsitu laboratory analysis. For CO2 fluxes, the number of results rejected is similar for the NDIR and the laboratory analysis methods. For CH4 fluxes, the number of results rejected is three times lower with the floating chamber with in situ laboratory analysis than with the other methods. The precision for duplicate measurements of fluxes is similar for all methods with floating chambers. In general, the thin boundary layer method tends to measure lower fluxes than the floating chamber method with laboratory analysis and there is no good correlation between the two methods. Fluxes measured with automated instruments (specially for fluxes > 5000 mg·m−2·d−1) tend to be higher compared with the laboratory analysis method but the correlation between the two methods is very good (R2=0.92 for CO2). The method with the less logistical constraints is the floating chamber coupled to an automated instrument. This method enables the sampling of about 5 times more sites in the same amount of time as the method with laboratory analysis. The floating chamber coupled to an automated instrument has therefore been retained as the method of choice by Hydro-Québec for GHG gross flux measurements over water bodies.
Maryse Lambert, Jean-Louis Fréchette

3. Development and Use of an Experimental near Infrared Open Path Diode Laser Prototype for Continuous Measurement of CO2 and CH4 Fluxes from Boreal Hydro Reservoirs and Lakes

Diode laser second derivate modulation spectroscopy combined with an open atmospheric path is a well suited technique for trace gas monitoring above wide areas. This paper presents the development of a portable long optical path near infrared spectrometer based on telecommunication laser diodes in order to provide a powerful tool for real time simultaneous measurements of greenhouse gas (GHG) concentrations above lakes and hydro reservoirs. CO2 and CH4 are respectively monitored at 1572 and 1653 nm along optical paths of several hundreds of meters above the area of interest. Simultaneous measurements of the target gases at two different heights above the water surface allows to detect concentration gradients on a continuous basis. A simplified turbulent diffusion model involving both the measured concentration gradients and local wind data has been used to estimate average GHG fluxes emitted by lakes and hydro reservoirs in different regions. Recent optimizations of the developed prototype allow quick on site set-up and operation of the laser device, even in remote areas without local facilities, as well as the continuous measurement of low GHG concentration gradients during long time periods with a minimum of local surveillance. Further improvements of this laser system would allow simultaneous detection of other trace gases such as N2O emitted by agricultural soils and other gases present in various environments.
Michel Larzillière, Denis Roy, Philippe Chrétien, Tommy Ringuette, Louis Varfalvy

4. Greenhouse Gas Fluxes (CO2, CH4 and N2O) in Forests and Wetlands of Boreal, Temperate and Tropical Regions

Current estimates of GHG budgets of forests indicate that these ecosystems act, over all, as C sinks, regardless of climatic region. The sequestering of GHGs by forests is the result of the balance between a substantial uptake of CO2 by vegetation and CO2 release through soil respiration, a weak consumption of CH4 by methanotrophic bacteria, and a more or less significant emission of N2O from soil, a by-product of nitrification and denitrification reactions. According to the estimated GHG budgets, the forest C sink is weaker in the boreal region (mean −236 mgC·m−2·d−1) than in the temperate and tropical regions (−398 and −632 mgC·m−2·d−1, respectively), in agreement with the NEE (Net Ecosystem Exchange in CO2) pattern observed with latitude. The increase in solar radiation and growing season length from north to south could be responsible for such a pattern. The C sinks typical of tropical forests can be offset by nearly 30% by N2O emissions from the soils. In northern forests, some sites have been found to be net sources of CO2, particularly during warm and dry years.
In wetlands, water-saturated soils are conducive to anaerobic decomposition of organic matter and methane production. CH4 largely dominates the GHG budget of wetlands. Overall, northern peatlands are sources of GHGs (mostly as CH4) (173 gCO2-eq.·m−2·yr−1), while accumulating small quantities of CO2 as peat (−22 gC·m−2·yr−1). Our GHG budgets for peatlands do not generally take into account the high CO2 and CH4 emissions from ponds. Tropical wetlands, dominated by marshes and swamps, emit large amounts of CH4 into the atmosphere (mean 71 mgCH4·m−2·d−1) compared to northern peatlands (34 mgCH4·m−2·d−1).
The estimated GHG budget values for forests depends on a) the error in NEE measurements, which dominate the forest GHG budgets and whose percent correction for calm nights can represent up to 50% of their initial value; b) the small number of published data on NEE for boreal and, especially, tropical forests, for which two out of the three available NEE data sets are contested; and c) the lack of NEE data in certain stands, such as in boreal forests post-fire stands, to which a certain potential for GHG emission is attributed. In wetlands, the popular use of the chamber method for CH4 flux measurements, along with a sampling period of less than a year, make it difficult to estimate an annual GHG budget that would integrate these systems' considerable spatial and temporal variability. The use of flux towers in forests, and recently in peatlands, allows for estimations of annual CO2 fluxes (NEE) typical of the local variability of environmental conditions within these systems.
Anne-Marie Blais, Stéphane Lorrain, Alain Tremblay

5. Diffuse Flux of Greenhouse Gases — Methane and Carbon Dioxide — at the Sediment-Water Interface of Some Lakes and Reservoirs of the World

Sediments were cored from 19 lakes and 4 reservoirs and were analyzed for CO2 and CH4. Additionally sediment concentration profiles were measured in 107 cores. Concentrations of methane in the surface (2 ± 1.7 cm, n=107) sediment porewater in 4 oligotrophic lakes was in the order of 0.04 ± 0.03 mM (0–0.14, n=25), 0.24 mM in one oligotrophic reservoir to high values of 1.4 ± 0.96 mM (0–2.8, n=18) in 5 eutrophic-hypereutrophic lakes and 1.4 ± 1.0 mM (0–3.5, n=24) in 2 eutrophic reservoirs. Concentrations of porewater CO2 in oligotrophic lakes were ten times higher than methane concentration with values in the order of 0.4 ± 0.36 mM but slightly lower for the same eutrophic lakes with values around 1.35 ± 0.9 mM and reservoirs [1.0 ± 0.8 (0.1–3.2, n=23)]. Surface sediment CH4 concentrations were low in 3 acidotrophic lakes [0.2 ± 0.3 (0–1, n=22)] and very high in geothermal lakes [2.4 ± 0.6 (1.8-3, n=3)]. The CO2 concentrations in acidotrophic lakes were, however, high [0.9 ± 0.7 (0.1–2, n=14)]. Diffuse flux of the two greenhouse gases - CH4 and CO2 — from the surficial sediments across the sediment-water interface (SWI) were calculated from Fick's first law of diffusion. These resulted in low methane fluxes from oligotrophic systems (lakes and reservoirs) of 0.2–0.4 mM CH4·m−2·d−1 (3–6 mg CH4·m−2·d−1) to much higher fluxes from eutrophic systems [3.9 mM CH4·m−2·d−1 (62 mg CH4·m−2·d−1) for lakes and 5.2 mM (83 mg) for reservoirs] to very high fluxes at the two geothermal lakes [14 mM CH4·m−2·d−1 (220 mg CH4·m−2·d−1)]. Dissolved CO2 fluxes were double the methane fluxes in oligotrophic lakes, about equal for eutrophic lakes [3.8 mM CO2·m−2·d−1 (167 mg CO2·m−2·d−1)] and slightly lower than methane fluxes in eutrophic reservoirs [4.3 mM CO2·m−2·d−1 (190 mg CO2·m−2·d−1)]. Diffuse fluxes of CO2 in acidotrophic systems [3.3 mM CO2·m−2·d−1 (143 mg CO2·m−2·d−1)] were almost the same as observed in eutrophic lakes. Even though it is unclear why there are such great differences between temperate and tropical ecosystems, CO2 gas fluxes at the SWI in one tropical reservoir [Lobo Broa at 16 mM CO2·m−2·d−1 (700 mg CO2·m−2·d−1)] were much higher than temperate ecosystems while CH4 diffuse fluxes [9 mM CH4·m−2·d−1 (140 mg CH4·m−2·d−1)] are only slightly higher than temperate ones. There are few data to evaluate the importance of sediment diffuse fluxes of these two greenhouse gases as related to aquatic surface emissions. One study in an 11-m deep tropical high elevation reservoir observed that surface losses represented 10% of the sediment diffuse flux of CH4 (thus, 90% was oxidized at the SWI or in the water column). For CO2 it is suspected that 20% of the surface emissions come from sediment sources. The sediments represent an important repository of carbon which contributes gases to overlying waters. These fuel the activities of microorganisms and substantially contribute to oxygen depletion in overlying waters as well as contributing to climate change.
Donald D. Adams

6. Organic Carbon Densities of Soils and Vegetation of Tropical, Temperate and Boreal Forests

Available estimates of the soil organic carbon (SOC) density in northern and tropical forests vary between 8.5 and 13.9 kgC·m−2 for the top first meter. Values of SOC for boreal forests are higher when considering the carbon stored as peat and in the forest floor. Overall, current SOC estimates often underestimate the total soil carbon content of boreal and tropical forests, because in many cases sampling is often limited to the soil's first meter.
The estimates of organic carbon sequestered in the vegetation of Amazonian forests (15.2 to 23.3 kgC·m−2) are two to five times higher compared to boreal (4.0 to 6.4 kgC·m−2) and temperate forests (4.8 to 5.7 kgC·m−2). Apart from their greater productivity, the more stable natural conditions prevailing in tropical rainforests have contributed to the accumulation of large amounts of carbon as biomass. On the other hand, the frequent recurrence of forest fires and insect outbursts in northern forests greatly limit carbon storage in the biomass.
The distribution of the total organic carbon stock between soil and vegetation varies with latitude. In northern forests, 72% of the organic carbon is found in the soil, with the remainder (28%) in the plant biomass. In tropical forests, the distribution is reversed, with 38% of the organic carbon stored in the soil and 62% in the vegetation. This difference can be explained by slower decomposition rates and a shorter growing season in relatively cold and humid boreal regions.
The boreal peatland carbon pool (98 to 335 PgC) is comparable to that of the whole boreal forest (180 to 330 PgC), but its surface area is five times smaller with an organic carbon content two to 10 times greater (39 to 134 kgC·m−2).
Export rates of organic carbon from terrestrial to aquatic ecosystems are small compared to their total stock but could be significant in the global forest carbon balance.
Anne-Marie Blais, Stéphane Lorrain, Yanick Plourde, Louis Varfalvy

7. Carbon Dioxide and Methane Emissions from Estuaries

Carbon dioxide and methane emissions from estuaries are reviewed in relation with biogeochemical processes and carbon cycling. In estuaries, carbon dioxide and methane emissions show a large spatial and temporal variability, which results from a complex interaction of river carbon inputs, sedimentation and resuspension processes, microbial processes in waters and sediments, tidal exchanges with marshes and flats and gas exchange with the atmosphere. The net mineralization of land- and marsh-derived organic carbon leads to high CO2 atmospheric emissions (10–1000 mmol·m−2·d−1 i.e. 44–44 000 mg·m−2·d−1) from inner estuarine waters and tidal flats and marsh sediments. Estuarine plumes at sea are sites of intense primary production and show large seasonal variations of pCO2 from undersaturation to oversaturation; on an annual basis, some plumes behave as net sinks of atmospheric CO2 and some others as net sources; CO2 atmospheric fluxes in plumes are usually one order of magnitude lower than in inner estuaries. Methane emissions to the atmosphere are moderate in estuaries (0.02–0.5 mmol·m−2·d−1 i.e. 0.32–8 mg·m−2·d−1), except in vegetated tidal flats and marshes, particularly those at freshwater sites, where sediments may be CH4-saturated. CH4 emissions from subtidal estuarine waters are the result of lateral inputs from river and marshes followed by physical ventilation, rather than intense in-situ production in the sediments, where oxic and suboxic conditions dominate. Microbial oxidation significantly reduces the CH4 emissions at low salinity (<10) only.
Gwenaël Abril, Alberto Vieira Borges

8. GHG Emissions from Boreal Reservoirs and Natural Aquatic Ecosystems

Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) gross fluxes were measured at the air-water interface of 205 aquatic ecosystems in the Canadian boreal region from 1993 to 2003. Fluxes were obtained with a floating chamber connected to an automated NDIR or a FTIR instrument. The results show a temporary increase in CO2 and CH4 fluxes, followed by a gradual return to values comparable to those observed in natural aquatic ecosystems (lakes, rivers and estuaries). Mean values for CO2 and CH4 measured in Québec's reservoirs older than 10 years were 1508±1771 mg CO2·m−2·d−1 and 8.8±12 mg CH4·m−2·d−1. Our results showed a strong similarity between lakes, rivers, and old reservoirs across a 5000 km transect from the west coast to the east cost of Canada. These values are comparable to those observed in Finland or in the sub-tropical semi-arid western USA. Although several limnological parameters can influence these fluxes, none showed a statistical relationship. However, levels of CO2 or CH4 fluxes are influenced by pH, wind speed, depth at sampling stations and latitude.
Alain Tremblay, Jean Therrien, Bill Hamlin, Eva Wichmann, Lawrence J. LeDrew

9. CO2 Emissions from Semi-Arid Reservoirs and Natural Aquatic Ecosystems

Carbon dioxide (CO2) gross fluxes were measured at the air-water interface of 57 aquatic ecosystems in the western semi-arid region of the USA in April 2003. Fluxes were obtained with a floating chamber connected to an automated NDIR instrument. The results showed a strong similarity between lakes and reservoirs, as is also the case in boreal regions. The values ranged from −1500 to 10800 mg CO2·m−2·d−1 for both natural systems and reservoirs. These values are similar to those observed in boreal regions. Although several limnological parameters can influence the fluxes of CO2, only the pH was significantly related with the CO2 gross fluxes, which decrease with increasing pH.
Jean Therrien, Alain Tremblay, Robert B. Jacques

10. A Comparison of Carbon Dioxide Net Production in Three Flooded Uplands (FLUDEX, 1999–2002) and a Flooded Wetland (ELARP, 1991–2002) Using a Dynamic Model

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−2·d−1 in the first year and decreased steadily to 13–30 mmole·m−2·d−1 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−2·d−1 in the first year of flooding to 178 mmole·m−2·d−1 in the years 7–9. A decrease to 126 mmole·m−2·d−1 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.
Raymond H. Hesslein, Rachel A. Dwilow, Kenneth G. Beaty, Mark E. Lyng

11. Gross Greenhouse Gas Emissions from Brazilian Hydro Reservoirs

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.
Marco Aurelio dos Santos, Bohdan Matvienko, Luiz Pinguelli Rosa, Elizabeth Sikar, Ednaldo Oliveira dos Santos

12. Long Term Greenhouse Gas Emissions from the Hydroelectric Reservoir of Petit Saut (French Guiana) and Potential Impacts

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.
Robert Delmas, Sandrine Richard, Frédéric Guérin, Gwénaël Abril, Corinne Galy-Lacaux, Claire Delon, Alain Grégoire

13. Production of GHG from the Decomposition of in vitro Inundated Phytomass and Soil

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−1 and > 2 mg·L−1). 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 (Picea mariana sp.), alder leaves (Alnus sp.), lichen (Cladonia sp.), green moss (Pleurosium sp.) and herbaceous plants (mixed species). Representative forest soil samples consisted of lichen (Cladonia sp.) humus and green mosses (Pleurosium sp.) humus with Sphagnum moss (Sphagnum sp.) 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 CO2·g−1 (dry weight) to 447 mg CO2·g−1 (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 CO2·m−2 and 140 g CO2·m−2 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 CH4·m−2 and 0.56 g CH4·m−2 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 CH4·g−1 (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 CO2 and CH4 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 CO2 and CH4 emitted from reservoirs under field conditions can be estimated (Thérien and Morisson, Chap. 25).
Normand Thérien, Ken Morrison

14. Diffusive CO2 Flux at the Air-Water Interface of the Robert-Bourassa Hydroelectric Reservoir in Northern Québec : Isotopic Approach (13C)

Hydroelectric reservoirs and lakes in boreal Québec produce greenhouse gases (GHG) mainly in the form of CO2. No method exists, however, which can directly measure the flux of CO2 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 CO2 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 CO2 diffusive flux across the air-water interface of 225±51 mg CO2·m−2·d−1 in the summer of 2000 and of 446±93 mg CO2·m−2·d−1 in the summer of 2001. These average fluxes are representative of the whole icefree period.
Jean-François Hélie, Claude Hillaire-Marcel

15. The Use of Carbon Mass Budgets and Stable Carbon Isotopes to Examine Processes Affecting CO2 and CH4 Production in the Experimental FLUDEX Reservoirs

The FLooded Uplands Dynamics EXperiment (FLUDEX) was initiated to quantify carbon dioxide (CO2) and methane (CH4) production in boreal reservoirs, and to better understand underlying biogeochemical processes (dissolved inorganic carbon [DIC] production, net primary production [NPP], methanogenesis, and CH4 oxidation) governing CO2 and CH4 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 CH4 inputs and outputs were calculated to quantify net reservoir CO2 and CH4 production, and isotopic ratio mass budgets were calculated to quantify biogeochemical processes controlling net reservoir CO2 and CH4 production.
The three reservoirs produced both CO2 and CH4 during each of the three flooding seasons, but neither CO2 nor CH4 production was related to overall mass of flooded OC. Net reservoir CO2 production in the second and third flooding seasons (408 to 479 kg C ha−1) was lower than in the first flooding season (703 to 797 kg C ha−1), while reservoir CH4 production steadily increased with each successive flooding season (from 3.2 to 4.6 kg C ha−1 in 1999 to 29.7 to 35.2 kg C ha−1 in 2001). Over three flooding seasons, NPP ranged from 77 to 273 kg C ha–1 and consumed 15 to 40% of gross reservoir CO2 production. CH4 oxidation was negligible during the first flooding season, but reduced gross reservoir CH4 production by 50% during the second flooding season, and by 70 to 88% during the third flooding season. However, despite decreases in net reservoir CO2 production and increases in CH4 oxidation over the study period, the overall total global warming potential (GWP) of the FLUDEX reservoirs remained constant due to successive increases in net CH4 production.
Cory J.D. Matthews, Jason J. Venkiteswaran, Vincent L. St. Louis, Sherry L. Schiff

16. Mass Balance of Organic Carbon in the Soils of Forested Watersheds from Northeastern North America

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.
François Courchesne, Marie-Claude Turmel

17. Planktonic Community Dynamics over Time in a Large Reservoir and their Influence on Carbon Budgets

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. a).
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.
Jérôme Marty, Dolors Planas, Bernadette Pinel-Alloul, Ginette Méthot

18. Production and Consumption of Methane in Soil, Peat, and Sediments from a Hydro-Electric Reservoir (Robert-Bourassa) and Lakes in the Canadian Taiga

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 (Fe3+, SO42-). 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 CH4 production was close to the lower range of values observed in this study. This parallel evolution between the potential rate of methanogenesis and CH4 oxidation suggests an association between CH4 oxidation activity and CH4 supply. Methanogenesis appeared more sensitive to temperature increases than methanotrophy.
The nucleotide sequences of PCR amplified and cloned mcrA 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.
Louis B. Jugnia, Réal Roy, Maria C. Pacheco-Oliver, Carlos B. Miguez, Charles W. Greer

19. Bacterial Activity in the Water Column and its Impact on the CO2 Efflux

As part of a comprehensive study intended to elucidate mechanisms that drive carbon dioxide (CO2) 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 a, phytoplankton production and CO2 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 per se 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 CO2 emissions at least in reservoirs, where it explained 38% of the variance of CO2 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 CO2 emissions from aquatic ecosystems, and suggest that the relative importance of factors driving bacterial activities and CO2 efflux may be quite different in lakes versus reservoirs.
Rémy D. Tadonléké, Dolors Planas, Serge Paquet

20. Production-Consumption of CO2 in Reservoirs and Lakes in Relation to Plankton Metabolism

We present data on metabolism, primary production and respiration, and their relationship to CO2 effluxes of a three-year study conducted in northeastern Canada, in two distinct boreal regions where large hydroelectric reservoirs have been established. In each region, the youngest and oldest reservoirs were sampled, as well as three to four natural lakes surrounding each reservoir.
The trophic status of all the sampled ecosystems ranged from oligotrophic, in lakes and the oldest (23 and 35 years-old) reservoirs, to mesotrophic in the youngest (1 and 7 years-old) reservoirs. The areal gross primary production (AGP) to areal planktonic respiration (APR) ratio varied from lower than 1 to higher than 1 in any given system. Differences in the AGP/APR ratio were related to season; lower in spring, higher in summer, both in reservoirs and in lakes. Within reservoirs differences in the ratio were also a function of the maximum depth of the sampled station. The AGP/APR ratio tended to be higher in the deeper than in the shallower stations. A very strong relationship (r2=0.93) was found between CO2 evasive fluxes and the total respiration of the system. The contribution of gross primary production (GPP) to total planktonic respiration (TPR) was higher in lakes (from > 50 to 200%) than in reservoirs. In reservoirs, the % of GPP to TPR varied by < 10% in the spring in the older oligotrophic reservoir, to more than 100% in the 7 year-old mesotrophic reservoir in summer.
Dolors Planas, Serge Paquet, Annick Saint-Pierre

21. Impacts of Ultraviolet Radiation on Aquatic Ecosystems: Greenhouse Gas Emissions and Implications for Hydroelectric Reservoirs

Ultraviolet radiation (UV) affects carbon dynamics in aquatic ecosystems. Photooxidation of dissolved organic matter (DOM) can produce greenhouse gases (GHG) and the effects of UV on primary and secondary production can influence the flux of carbon (C) between aquatic ecosystems and the atmosphere. Products of photooxidation include: DOM of lower molecular weight, carbon dioxide (CO2) and carbon monoxide (CO). Lower molecular weight DOM can be more easily utilized by microorganisms. Secondary microbial production and the production of CO2 from aerobic respiration are, therefore, favored. However, the harmful effects UV have on phytoplankton diminish the rate of CO2 fixation. The fluxes of CO2 in reservoirs are influenced in the same manner as natural lakes since reservoirs older than 10 years are comparable to lakes. The largest distinction to be made between reservoirs and lakes is their surficial aerial coverage, which is generally much larger for reservoirs. This factor may help explain the differences observed in carbon fluxes from natural lakes and reservoirs. Following the literature review, a first gross estimate was made of the importance of the production of GHGs resulting from photooxidation relative to other processes. The results show that when making a calculation of the balance of net CO2 emissions from aquatic ecosystems and hydroelectric reservoirs, photooxidation needs to be taken into consideration as it can account for between 6 and 28% of total emissions.
Julie Bastien

22. Impact of Methane Oxidation in Tropical Reservoirs on Greenhouse Gases Fluxes and Water Quality

This chapter presents a summary of water quality data (physico-chemical) from 10 years of measurements in the Petit Saut hydroelectric reservoir in French Guiana. Methane oxidation in and downstream of the reservoir are of particular interest. In the first part of the paper we discuss both the primary factors influencing the water quality and the patterns of stratification, methane production and oxidation in the reservoir. Secondly, we present data of methane emissions and oxidation downstream of the dam. We demonstrate that the oxidation of the dissolved CH4 was a major oxygen consumer downstream of the dam. The results indicate that the aerating weir built in the plant outlet canal guarantees the minimum regulatory concentration of 2 mg·L−1 of dissolved oxygen as delineated by the scientific community of Petit Saut, following observations of the resistance to hypoxia in a tropical environment. This long term database, which helped in detecting changes over time (dissolved gases concentrations, CH4 oxidation velocity) will be used to improve the models developed to simulate both water quality and greenhouse gas emissions in a tropical reservoir environment.
Sandrine Richard, Philippe Gosse, Alain Grégoire, Robert Delmas, Corinne Galy-Lacaux

23. Using Gas Exchange Estimates to Determine Net Production of CO2 in Reservoirs and Lakes

The net contributions to the atmosphere of GHG’s from reservoirs and lakes are made up of fluxes in inflows, outflows, and gas exchange. The rates of production, which are primarily due to bacteria and algae, and the transport coefficients due to hydrologic flows, wind velocity etc. can both vary considerably over time. Achieving accurate estimates of the net production requires an understanding of the variability of these various functions and requires sampling protocols adequate to define the parameters over the period of study. Several sampling protocols have been used each with strengths and weaknesses. This paper discusses the methods used for data collection and data interpretation for the gas exchange fluxes. Serious potential errors in estimates are identified for data based on infrequent sampling. Alternate protocols are recommended which use models of wind histories and estimates of diurnal changes due to photosynthesis. The recommended approach is to use high resolution measurements of parameters supplemented by models to understand the variability prior to designing programs to estimate fluxes.
Raymond H. Hesslein

24. A One-Dimensional Model for Simulating the Vertical Transport of Dissolved CO2 and CH4 in Hydroelectric Reservoirs

The goal of this project consisted in developing a mathematical model capable of simulating the physical processes responsible for the vertical transport of dissolved greenhouse gases (GHG) in hydroelectric reservoirs. This combined approach of measurement and numerical modeling confirmed certain hypotheses concerning the missing CO2 source and the amount of methane oxidation. Moreover, a relation between differences in GHG emission patterns and reservoir depth was revealed. The numerical model developed in this study can be used to determine sampling strategies based on the temporal and spatial distributions of a given reservoir.
Nathalie Barrette, René Laprise

25. Modelling the GHG emission from hydroelectric reservoirs

A mechanistic model has been constructed to compute the fluxes of CO2 and CH4 emitted from the surface of hydroelectric reservoirs. The structure of the model has been designed to be adaptable to hydroelectric reservoirs of different sizes and configurations and the reservoir can be partitioned into one, two or three vertical volumetric zones. Each zone may accommodate a number of influents and effluents including turbined flow and discharged flow. Each zone consists of a surface water layer (0–10 m) and a bottom water layer (>10 m). The model considers advective and diffusive mass transfers of dissolved CO2 and CH4 between zones and water layers, the rates of CO2 and CH4 produced from the decomposition of flooded vegetation and soil in the reservoir, and, mass transfer of CO2 and CH4 at the water-air interface. Global mass balance equations are solved to compute the magnitude of the advective flows between zones and water layers. Component mass balance equations are solved to compute the concentrations of CO2 and CH4 as a function of time in the surface and bottom water layers of each of the zones of the reservoir. The rates of CO2 and CH4 emitted from the surface water layer are computed using the two-film theory. Data from the Robert-Bourassa reservoir, a large operational hydroelectric reservoir, has been used as input data to the model. Results from the model were first compared with experimental data available for the calculation of dissolved CO2 concentration in the surface water layer. Secondly, results from the model were compared with fluxes of CO2 and CH4 emitted from that reservoir as calculated from the experimental determination of dissolved CO2 in water. Also, they were compared with direct measurements of the fluxes at the water-air interface. It has been observed that concentrations of CO2 computed by the model are in the range of values reported for the surface water layer. No data was available for comparison with concentration of CH4. Emissions of CO2 computed by the model were in the range of fluxes calculated from the experimental determination of dissolved CO2 in water. The computed flux as a function of reservoir age was also coherent with the CO2 flux measurements data. The transitional emissions of CO2 resulting from the decomposition of flooded vegetation and soil were found to be significant during not more than 6 to 8 years depending of the volumetric zones of the reservoir considered. Simulations were done under two distinct scenarios for the CO2 content of the influents to the reservoir. The first scenario used data which reflected the contribution of carbon originating from the drainage basin. The second scenario assumed the CO2 concentration in the influent water to be at equilibrium with the atmospheric CO2. From the simulation results and the data available an important finding is that the main source of carbon contributing to the GHG emission from the hydroelectric reservoir after the transitional emissions of CO2 due to the decomposition of the flooded vegetation and soil have faded away appears to be essentially the carbon originating from the drainage basin.
The results have also indicated that fluxes of CH4 computed from the model are grossly underestimating the values reported from the direct measurements of CH4 emissions. Analysis of the results have indicated the source of the discrepancies which lies with the very low production of CH4 as indicated from the vegetation and soil decomposition data used by the model. Suggestions to improve the model forecasting of CH4 emissions are indicated.
Normand Thérien, Ken Morrison

26. Synthesis

The objectives of this chapter are to present a comprehensive review of the current state of knowledge and to identify the gaps in the greenhouse gas issues in hydroelectric reservoirs and natural ecosystems. It has become essential to integrate our knowledge of the carbon cycle at the larger temporal and spatial scales in order to properly assess the magnitude of GHG fluxes from reservoirs1 and natural ecosystems. The information available comes from small scale and short term (1 to 10 years) studies mostly from boreal regions but also from semi-arid and tropical regions. Natural variability of GHG fluxes due to regional climatic variations and their impacts on whole biological production are far more important than the techniques available to measure them. Therefore, one must keep in mind that the uncertainties in the GHG fluxes are related to natural spatial and temporal variations of fluxes and not necessarily from the measurement techniques themselves. This synthesis is based on the findings of over ten years of studies reported by research teams from many universities, governmental agencies and power utilities.
Alain Tremblay, Louis Varfalvy, Charlotte Roehm, Michelle Garneau


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