- Philip Fearnside Comments to PJCERS on the Teles Pires Hydropower Project (Brazil)
- 1. Introduction
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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.
Philip Fearnside Comments to PJCERS on the Teles Pires Hydropower Project (Brazil)
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 CH 4 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. 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. 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.
CO 2 and CH 4 are respectively monitored at and 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 N 2 O emitted by agricultural soils and other gases present in various environments. 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 CO 2 by vegetation and CO 2 release through soil respiration, a weak consumption of CH 4 by methanotrophic bacteria, and a more or less significant emission of N 2 O from soil, a by-product of nitrification and denitrification reactions.
The increase in solar radiation and growing season length from north to south could be responsible for such a pattern.
In northern forests, some sites have been found to be net sources of CO 2 , particularly during warm and dry years. In wetlands, water-saturated soils are conducive to anaerobic decomposition of organic matter and methane production. CH 4 largely dominates the GHG budget of wetlands. In wetlands, the popular use of the chamber method for CH 4 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 CO 2 fluxes NEE typical of the local variability of environmental conditions within these systems. Sediments were cored from 19 lakes and 4 reservoirs and were analyzed for CO 2 and CH 4. Additionally sediment concentration profiles were measured in cores. Concentrations of porewater CO 2 in oligotrophic lakes were ten times higher than methane concentration with values in the order of 0.
Surface sediment CH 4 concentrations were low in 3 acidotrophic lakes [0. The CO 2 concentrations in acidotrophic lakes were, however, high [0. Diffuse flux of the two greenhouse gases - CH 4 and CO 2 — 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. Dissolved CO 2 fluxes were double the methane fluxes in oligotrophic lakes, about equal for eutrophic lakes [3. Diffuse fluxes of CO 2 in acidotrophic systems [3. There are few data to evaluate the importance of sediment diffuse fluxes of these two greenhouse gases as related to aquatic surface emissions.
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. Available estimates of the soil organic carbon SOC density in northern and tropical forests vary between 8. 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 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. This difference can be explained by slower decomposition rates and a shorter growing season in relatively cold and humid boreal regions.
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. 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.
Estuarine plumes at sea are sites of intense primary production and show large seasonal variations of pCO 2 from undersaturation to oversaturation; on an annual basis, some plumes behave as net sinks of atmospheric CO 2 and some others as net sources; CO 2 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.
CH 4 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. Carbon dioxide CO 2 , methane CH 4 and nitrous oxide N 2 O gross fluxes were measured at the air-water interface of aquatic ecosystems in the Canadian boreal region from to The results show a temporary increase in CO 2 and CH 4 fluxes, followed by a gradual return to values comparable to those observed in natural aquatic ecosystems lakes, rivers and estuaries.
Our results showed a strong similarity between lakes, rivers, and old reservoirs across a 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 CO 2 or CH 4 fluxes are influenced by pH, wind speed, depth at sampling stations and latitude.
Carbon dioxide CO 2 gross fluxes were measured at the air-water interface of 57 aquatic ecosystems in the western semi-arid region of the USA in April 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. These values are similar to those observed in boreal regions. Although several limnological parameters can influence the fluxes of CO 2 , only the pH was significantly related with the CO 2 gross fluxes, which decrease with increasing pH.
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The 3 flooded upland areas had been chosen to have differing amounts of carbon stored in soils and vegetation. The NCP from the reservoir with the lowest carbon stock was always lowest, the other two were similar. 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 net emissions result from estimating pre-existing emissions by the reservoir. 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.
Methane emissions from an Amazon floodplain lake: Enhanced release during episodic mixing and during falling water. Biogeochemistry 51 : Gaseous emissions and oxygen consumption in hydroelectric dams: A case study in French Guiana. Cycles 11 : Significance of pelagic aerobic methane oxidation in the methane and carbon budget of a tropical reservoir. Methane and carbon dioxide emissions from tropical reservoirs: significance of downstream rivers. Gas transfer velocities of CO2 and CH 4 in a tropical reservoir and its river downstream.
Applied Geochemistry 23 : In: Stocker T. On parameters influencing air-water exchange. Methane release below a tropical hydroelectric dam. Buoyancy flux, turbulence, and the gas transfer coefficient in a stratified lake. Pumping methane out of aquatic sediments — ebullition forcing mechanisms in an impounded river, Biogeosciences , 11, —, doi: Influence from surrounding land on the turbulence measurements above a lake.
Greenhouse Gas Emissions — Fluxes and Processes | SpringerLink
Emissions of nitrogen oxides from equatorial rain forest in central Africa: origin and regulation of NO emission from soils. Tellus 46B : Methanogenesis and Methanotrophy in Soil: A Review. Pedosphere 24 : Reservoir surfaces as sources of greenhouse, gases to the atmosphere: A global estimate. The patterns of CH 4 and CO 2 emissions from these three littoral zones were significantly different during the sampling periods, with the eulittoral zone having the highest CH 4 flux and the supralittoral zone having the highest CO 2 flux. Temperature and biomass correlated with CH 4 and CO 2 emissions.
Measurement of CO 2 emissions after removing vegetation varied in each zone and according to time of sampling. A large littoral area of the reservoir sampled herein will be submerged and converted to a pelagic area with deep standing water after the South to North Water Transfer Project is completed, in The results of the present study suggest further research and monitoring are needed, and should focus on likely effects of extreme climate events and the effects of human-mediated factors on greenhouse gas emissions.
Advances in the aquatic sciences.