Lake Wohlen – Del Sontro et al. (2010)
Researchers from Eawag, the Swiss Federal Institute of Aquatic Science and Technology, found high methane emissions from a small run-of-river hydro reservoir in Switzerland. Taking measurements approximately every month from June 2007 until June 2008, methane emissions averaged >150 mg CH4 m2 d-1, the highest ever documented for a mid-latitude reservoir. The researchers found a positive correlation between methane emissions and temperature. The source of the methane was from bubbling sediments. The two pathways for methane emissions out of the reservoir are via ebullition (bubbling) and diffusion. Ebullition rates are comparative with or exceed estimates from boreal and tropical reservoirs. While annual carbon emissions are several orders of magnitude lower than that of a reservoir located in a tropical region, Lake Wohlen’s emissions are well above that of the average small lake in Europe.
Previously, researchers had suggested that emissions taper ten years after damming, since the organic material that was submerged has decayed (Tremblay et al., 2005 ). Yet Lake Wohlen is 90 years old and still has high emissions. The researchers hypothesize that the high organic carbon load coupled with a fast sedimentation rate explains the high production of methane. While temperate reservoirs emit less than tropical reservoirs, this study indicates that they should be given consideration and taken into account in global estimates.
Three Gorges – Chen et al. (2009)
Researchers found that portions of the Three Gorges reservoir that are partially drained during the summer emit methane. This is the first study that measured emissions from the drawdown region of a reservoir. They usually constitute a small part of the reservoir surface area, so drawdown regions have largely been neglected. But in the case of the Three Gorges Reservoir, one-third of the reservoir is a drawdown region. This study suggests that drawdown regions of reservoirs should be further investigated to better quantify their contribution to GHG emissions from dams.