The influence of pelagic metabolism on CO2 dynamics in 3 young boreal cascade-reservoirs (La Romaine complex, Québec)
Reservoirs have been considered net sources of biogenic gases to the atmosphere, part of this carbon excess being associated to the decomposition of terrestrial OM from surrounding soils and vegetation. The balance between ecosystem primary production and ecosystem respiration, together with inputs of soil derived CO2 and carbon burial will define if a reservoir is a net source or a net sink of carbon. Ecosystem metabolism is known to play a key-role in CO2 dynamics and emissions in a variety of aquatic environments, including reservoirs, but the magnitude of this contribution has seldomly been addressed, especially in higher latitudes. Here we propose to use oxygen isotopes and concentrations to measure gross primary production (GPP), respiration (R) and the resulting net ecosystem production (NEP) in a series of young cascade-reservoirs in Northern Quebec and estimate how much of the observed CO2 emissions can be attributed to metabolism. We further compared the metabolic rates from the reservoirs with those from the main river, its tributaries, and surrounding lakes. There was a high degree of variability in metabolic rates both within and between reservoirs, and in time. Most measured sites, however, had net heterotrophic metabolism, and ambient CO2 concentrations were negatively related to the magnitude of NEP, suggesting an influence of aquatic metabolism on CO2 fluxes. NEP explained between 25 and 90% of the observed CO2 emissions, the latter occurring during autumn turnover. These results highlight the important role played by aquatic metabolism on the carbon balance of Northern reservoirs.
Pedro Barbosa, Felipe Rust, Pascal Bodmer and Paul del Giorgio
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