Greenhouse Gas Production From a Young Boreal Hydroelectric Reservoir (Eastern Canada): A Carbon Isotope Approach

Annie Lalonde, Jean-François Hélie
GEOTOP-UQAM-McGill and Department of Earth Sciences,
Université du Québec à Montréal

It is now accepted that boreal hydroelectric reservoirs and lakes produce greenhouse gases (GHG) mainly in the form of CO2. Much of the research has so far focused on old (> 20 year) reservoirs. However, the problems associated with a newly flooded reservoir are different because after flooding, salts and nutrients from the flooded soils are released into the water column (i.e. the reservoir's effect). It is anticipated that the CO2 fluxes should be higher in young reservoirs than in older ones, but little is known about their magnitude and their sources.

The Eastmain-1 hydroelectric reservoir is a small reservoir of 603 km2 with a mean depth of 11.5m. Flooding began in November 2005 and ended in May 2006. The flooded area was covered with approximately 65% boreal forests, 21% rivers and lakes and 14% peatlands. Here, we make use stable carbon isotopes to constrain carbon sources and cycling in this disturbed environment. Ultimately, the study aims at estimating annual CO2 fluxes at the water-air interface of the reservoir. Sampling was performed four times (June 2006, August 2006, October 2006 and June 2007) to account for seasonality of the carbon cycle. Twelve sites were visited on the reservoir as well as a natural lake near the reservoir. Three sites were also sampled along a depth gradient. At each sampling site, in situ measurements included water and air temperatures, pH, alkalinity, wind speed, conductivity and dissolved oxygen content.

Samples were collected for the analysis of dissolved organic and inorganic carbon (respectively DOC and DIC) and particulate organic carbon (POC) concentrations, for the analysis of the carbon isotopic compositions of DOC, DIC, POC and air CO2 at the water-air interface and finally for the C:N of DOM and POM. DOC concentrations are highest averaging 6.86±1.40 mg*l-1, DIC concentrations average 1.51±0.76 mg*l-1 and POC concentrations are up to 2 orders of magnitude lower averaging 0.036±0.018 mg*l-1. δ13C values of DOC average -27.42±0.32‰ vs V-PDB, close to average C3 plant values and vary little throughout the year as well as throughout the reservoir. δ13C-DIC values vary slightly throughout the reservoir but show large variations from one sampling campaign to the next. Depth profiles show a small decrease in δ13C-DIC with depth, in a well mixed water column. A strong relationship is observed between δ13C-DIC and DIC concentrations. Keeling type regressions (using δ13C-DIC and DIC concentrations) suggest that dissolved CO2 in the reservoir originate from the oxidation of dissolved organic matter within the reservoir.