9:00 am - 10:00 am
Event details: A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by McKenzie Kuhn. This seminar is open to the general public to attend via Zoom:
Thesis Topic: Greenhouse Gas Emissions from Northern Wetlands and Lakes: Circumpolar and Local Perspectives
Methane (CH4) emissions from Boreal-Arctic wetlands and lakes are likely to increase in a warming climate, and thus add to the atmospheric burden of greenhouse gases (GHG). However, there are large uncertainties in current estimates of CH4 emissions from northern ecosystems, and we have a limited understanding of the sensitivity of northern lake CH4 and carbon dioxide (CO2) emissions to warming and the thawing of perennially frozen ground (i.e. permafrost)- a critical gap in our ability to predict the future global atmospheric GHG budget. To address these knowledge gaps, I integrated meta-data analysis of CH4 fluxes from wetlands and lakes across the northern study domain with intensive, multi-year (2017-2019) field studies of lake biogeochemistry and GHG exchange across a 1600 km transect of western Canada.
First, I compiled a comprehensive dataset of small-scale, ground-based CH4 flux data from 540 terrestrial sites (wetland and non-wetland) and 1247 aquatic sites (i.e. lakes), from 189 studies (Chapter 2). The dataset was built in parallel with a novel, CH4-specific land cover dataset for the circumpolar north- the Boreal-Arctic Wetland and Lake Dataset (BAWLD), allowing for flux observations and spatial distribution of land cover features to be classified under the same criteria for the first time at a Boreal-Arctic scale. Most of the observed CH4 flux variability from terrestrial and aquatic ecosystems could be explained by a land cover classification system focused on splits in permafrost presence and hydrology in wetlands and lake size and genesis in lakes. Using land cover class as the main predictor variable, I arrive at new estimate of annual CH4 emissions from the Boreal-Arctic region of ~36.5 Tg CH4 yr-1. This estimate is on the low end, but within similar ranges of annual emissions reported from other bottom-up studies and is closer to the total emission estimated by inverse models- suggesting distinguishing between land cover class is an important step towards reconciling circumpolar emission estimates.
To further constrain current estimates of CH4 from northern lakes and assess potential changes in lake CH4 and CO2 emissions with warming and permafrost thaw, I measured seasonal GHG emissions from 20 peatland lakes across a climate and permafrost gradient in western Canada (Chapter 3). Both CO2 and CH4 emissions followed opposing trends across the gradient and were associated with different drivers. Less permafrost in the south was associated with greater hydrological connectivity, nutrient availability, and thus increased primary productivity and uptake of CO2. Conversely, positive trends in CH4 emissions were driven by higher temperatures towards the south and augmented by shifts in microbial communities. Using a space-for-time approach, the results show that net radiative forcing from altered GHG emissions of boreal peatland lakes this century will be dominated by increasing CH4 emissions and only partially offset by reduced CO2 emissions. The influence of permafrost thaw on lake productivity and reduced CO2 emissions, and the high-temperature sensitivity of CH4 emissions, are likely associated with characteristics of peatland lakes and the hydrology/landscape history of their surrounding landscape.
Finally, I examined the direct effects of permafrost thaw on CH4 and CO2 emissions from one peatland thaw lake in northern Alberta (Chapter 4). Spatial variation across the lake was the primary control on CH4 emissions with highest ebullitive emissions from the thaw edge and the highest diffusive emissions from the thaw edge and stable. Ebullitive CH4 and CO2 from the thaw edge was older than the stable edge and lake center, signifying GHG’s from the thaw edge may be sourced from recently thawed permafrost carbon. While the age of CH4 emitted from the thaw age is similar to other peatland thaw lakes, ebullitive CH4 emissions were an order of magnitude higher than other lakes. This suggests not all peatland lakes have similar responses in GHG emissions to permafrost thaw and that other factors including local peat history/quantity and lake morphology must be considered.
Together, the results of this thesis suggest that future research, including GHG emission models, should consider wetland and lake types and regional variability when estimating current emissions and projecting changes in future emissions with warming.