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We present data on metabolism, primary production and respiration, and their relationship to CO 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 (r=0.93) was found between CO 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.

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 (CO) and carbon monoxide (CO). Lower molecular weight DOM can be more easily utilized by microorganisms. Secondary microbial production and the production of CO from aerobic respiration are, therefore, favored. However, the harmful effects UV have on phytoplankton diminish the rate of CO fixation. The fluxes of CO 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 CO 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.

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 CH 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 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, CH oxidation velocity) will be used to improve the models developed to simulate both water quality and greenhouse gas emissions in a tropical reservoir environment.

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.

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 CO 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.

A mechanistic model has been constructed to compute the fluxes of CO and CH 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 CO and CH between zones and water layers, the rates of CO and CH produced from the decomposition of flooded vegetation and soil in the reservoir, and, mass transfer of CO and CH 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 CO and CH as a function of time in the surface and bottom water layers of each of the zones of the reservoir. The rates of CO and CH 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 CO concentration in the surface water layer. Secondly, results from the model were compared with fluxes of CO and CH emitted from that reservoir as calculated from the experimental determination of dissolved CO in water. Also, they were compared with direct measurements of the fluxes at the water-air interface. It has been observed that concentrations of CO 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 CH. Emissions of CO computed by the model were in the range of fluxes calculated from the experimental determination of dissolved CO in water. The computed flux as a function of reservoir age was also coherent with the CO flux measurements data. The transitional emissions of CO 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 CO 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 CO concentration in the influent water to be at equilibrium with the atmospheric CO. 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 CO 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 CH computed from the model are grossly underestimating the values reported from the direct measurements of CH emissions. Analysis of the results have indicated the source of the discrepancies which lies with the very low production of CH as indicated from the vegetation and soil decomposition data used by the model. Suggestions to improve the model forecasting of CH emissions are indicated.

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.