Forever Blowing Bubbles: Quantifying Methane Discharge from Inland Water Systems (IAPETUS2 Project)

Methane (CH4) is one of the most potent greenhouse gases and a significant regulator of global climate. Significant CH4 degassing occurs from inland aquatic systems, but substantial variability in spatial and temporal flux rates mean quantifying and characterising this pathway is challenging. Critically, a substantial proportion of CH4 degassed occurs via ebullition (bubbling) from surface waters, a stochastic process that is therefore difficult to quantify. In large part down to this measurement uncertainty, estimates of aquatic CH4 fluxes vary from 155 to 235 Tg CH4 yr-1.

Organic-rich anoxic sediments found in wetlands, permafrost regions and peatlands are important contributors to CH4 flux, representing ‘hot-spots’ of evasion. ‘Bottom-up understanding’ developed from field measurements on the strength of local and landscape-scale controls is poorly developed1. Additionally, CH4 emissions from inland waters may increase with warmer temperatures and with a global warming potential (GWP) 23 times greater than CO2 (over 100 years), increases in CH4 efflux can create a significant positive feedback between the changing carbon cycle and warming. Thus, quantification of CH4 emissions from inland waters is crucial for estimating the global carbon and greenhouse gas budgets, and for projecting climate change. However, such assessment is difficult, partially due to the logistical complexities of quantifying a characteristically sporadic process.

Thus, a framework is needed that:
i) develops robust, cost-effective methods for quantifying CH4 ebullition.
ii) captures sufficiently spatial and temporal variability in CH4 flux.
iii) quantifies CH4 flux over a sufficiently diverse landscape spectrum to elucidate ‘bottom-up’ controls.

Making a significant contribution to this framework is the over-arching objective of this Ph.D. but it will also work towards producing a robust, field deployable sensor array based on recent advances. The student will build and expand upon existing designs to incorporate a field-capable system in challenging environments over significant duration. The student will subsequently apply these techniques to various surface water environments representing a range of potential CH4 flux pathways, e.g., peatland draining lakes; artificial reservoirs; urban rivers. This will consequently address the overarching aim of developing a systematic understanding of CH4 ebullition controls and magnitude at local and regional scales.

Classically, CH4 ebullition is quantified by installing a submerged funnel below the water surface for a significant duration (days-weeks) where gas is collected into a vessel to be collected manually, returned to the lab and analysed. While this approach has revealed the significance of ebullitive fluxes, it has provided little insight into fine scale temporal changes and is spatially limited by the logistics of sample retrieval.

To fulfil the overarching aims and research questions this project will build upon recent developments in automated sensor design to produce a series of field deployable, automated ebullition loggers. Existing designs will be improved to allow long-term (3-6 month) field deployments in challenging field conditions (-10 to +30 °C). Utilising this technology the spatial and temporal variability of CH4 ebullition will be explored in the Clyde River catchment, which includes a diverse array of landscapes including peatlands, urban centres, estuaries, agriculture, reservoirs and small shallow ponds.

To provide a full understanding of the aquatic carbon cycles considered, the student will be trained in the measurement of complimentary geochemical parameters (e.g., dissolved in/organic carbon concentrations, CO2 efflux, dissolved oxygen) as this will be a large partial control over the concentration of CH4. Diffusive CH4 flux will be quantified using a floating chamber technique4 and Cavity Ring-down Spectrometry instrumentation will isotopically characterise the CH4. Combined with GIS techniques the student will quantify the importance of CH4 ebullition at local and catchment scales.

This is a jointly supervised project between the University of Glasgow (Dr Bass & Dr Gauchotte-Lindsay) and Stirling (Dr Jens-Arne Subke).