Methane and carbon dioxide dynamics in natural lakes
Methane (CH4) is a potent greenhouse gas, showing a global warming potential roughly 34 times greater than carbon dioxide (IPCC 2013). Despite the relatively small global area covered by freshwaters, these ecosystems are disproportionally important environmental sources of CH4 to the atmosphere. In lakes, a large amount of organic matter is anaerobically degraded in anoxic sediments and waters with production of CH4. Part of the CH4 produced is then emitted to the atmosphere – via ebullition or diffusion – but another substantial part never reaches the atmosphere due to microbial CH4 oxidation within these ecosystems. In our lab, we are interested in understanding patterns and controls of all processes involved in the CH4 cycle in lakes, from its production to oxidation and emission.
Under ice methane accumulation
Methane cycle in lakes shows a strong seasonal pattern, with major emissions during spring and autumn mixing. In Quebec and other regions of high latitudes or high altitudes, lakes are covered by ice during several months in winter. During this period, methane can accumulate under-ice and is released to the atmosphere shortly after ice melt. However, it is still uncertain which quantity can accumulate in winter, and which fraction of methane is oxidized before escaping to the atmosphere. Therefore, this project aims at quantifying methane accumulation under-ice and its release to the atmosphere during spring ice melt in several lakes of Quebec.
For more informations, see voir Michmerhuizen et al. 1996, Ducharme-Riel et al. 2015, Karlsson et al. 2013.
Fates of methane in lakes: from microbial oxidation to food webs
Lake CH4 emissions are controlled by the balance between CH4 production and oxidation. Oxidation can remove almost all CH4 produced and happens mostly due to consumption by aerobic methane oxidizing bacteria (MOB). These bacteria produce CO2 and biomass from CH4 and thus are key players regulating CH4 and CO2 emissions and generating a potentially important pathway in lake food webs. In this project, I am interested in understanding the regulation of CH4 oxidation in lakes with special emphasis on the role of MOB abundances and composition as well as in assessing the efficiency of CH4 use by MOB and its implications to pelagic C cycling and lake food webs.
Ubiquitous methane oversaturation in lake surface waters
Inland waters are quite consistently oversaturated with methane with respect to the atmosphere. Thus, these waters are constantly emitting methane, a potent carbonic greenhouse gas. Despite this and their potential importance in global carbon cycling, little is known about the constant source of methane to our surface waters, particularly in the oxic central waters of large lakes.
Therefore, within this project, we decided to test one of the initial hypotheses that methane transport from the littoral zone of lakes is the source of central methane. We derived a model accounting for both horizontal dispersion and gas exchange and tested it against methane data from 14 lakes spanning six orders of magnitude in size. We saw that our models under predicted concentrations in the center of most lakes, suggesting that physical processes alone cannot fully explain our observations. Using stable carbon isotope trends of surface methane, we reconciled these discrepancies by quantifying the net biological processing of methane via oxidation, which consumes methane, and the addition of methane from another source in the epilimnion, an often debated process for which a mechanism has not been constrained. We ultimately found that 70% of our northern lakes exhibited net production of methane in their oxic surface waters, further supporting the fact that this process does exist and is prevalent in lake surface waters.
This was recently published in Ecosystems: DelSontro et al. 2017
Using empirical driver models to upscale boreal lake methane emissions globally
Most studies that have upscaled gas emissions, including methane, from lakes have done so by simply multiplying average emission rates by the areal extent of the aquatic surfaces of interest. This inherently assumes that those lakes used to make the average emission rates were representative of all lakes present in the upscaled area. This, however, may not necessarily be true.
Therefore, we have decided to use multiple empirical models derived from a few studies in two different northern regions (i.e., Quebec and Sweden) to estimate diffusive and ebullitive methane emissions from boreal lakes. We found that temperature and total phosphorus best predict ebullition in Quebec, while others found that temperature and solar radiation are the best predictors in Sweden. Diffusive methane emissions from lakes in Quebec are related to lake size as well as temperature. Preliminary results suggest that previous studies overestimated northern aquatic methane emissions and that global boreal methane emissions may represent a small fraction of total aquatic methane emissions. In addition, as temperature is a common variable in all of these models, we can also project future boreal methane emissions according to current trends in increasing lake temperatures. Preliminary findings suggest that a 1°C increase in lake surface temperatures over the next 20 years could result in 10% higher lake methane emissions from the biome with the most freshwater.
For relevant manuscripts, see DelSontro et al. 2016; Rasilo et al. 2015; Wik et al. 2014