Ecosystem Metabolism Is the Dominant Source of Carbon Dioxide in Three Young Boreal Cascade-Reservoirs (La Romaine Complex, Quebec)

The impoundment of rivers for multipurpose reservoirs has significant consequences to the carbon cycle, one of the most relevant being the increase in greenhouse gases emissions. Reservoirs have been shown to be net sources of such gases to the atmosphere, emitting between 0.8 and 1.08 Pg carbon dioxide (CO2) equivalents per year. Even though emission estimates have become common, less is known about the processes driving this CO2 excess, a prerequisite for understanding and ultimately predicting and managing CO2 emissions from reservoirs. In the present study, we aimed at exploring ecosystem metabolism (gross primary production, ecosystem respiration, and net ecosystem production [NEP]) and its environmental drivers in three young hydroelectric reservoirs in a cascade configuration but with distinctive morphometries (shape, depth, and size). By combining our metabolic measurements with a hydrological mass balance approach, we quantified the relative contributions of internal (ecosystem metabolism) versus external sources (tributaries and groundwater) to the reservoir surface diffusive CO2 emissions. There was a predominance of net heterotrophy in all sites, and metabolism played a key role in fueling CO2 fluxes in all three reservoirs, NEP alone being able to account for the measured fluxes in approximately 50% of all sites. Internal production was thus the main process explaining total reservoir CO2 diffusive emissions (similar to 100%), with groundwater and tributaries contributing similarly but more modestly (similar to 9% each). Our results contribute to our understanding of the processes underlying boreal reservoir carbon footprints, and in particular, for apportioning the emissions can be attributed to the reservoir itself.