An analysis of ebullition dynamics in agricultural reservoirs using novel automated sensors
Date
2020-01-13
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-6783-8049
Type
Thesis
Degree Level
Masters
Abstract
Freshwater systems are an important component of biogeochemical processing within terrestrial landscapes. Only recently has the importance of these systems for contributions to atmospheric budgets of methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) been recognized at large spatial scales; however, fluxes of the gases remain poorly described. Smaller aquatic systems (≤ 1 ha) may have a greater role in global carbon (C) cycling than their larger counterparts, partly due to the large collective area of small water bodies. Constructed reservoirs — like the headwater reservoirs in South Tobacco Creek Watershed (STCW), Manitoba, investigated herein — are of particular interest as they, among other benefits, trap nutrients and terrestrial C. Trapped materials in these shallow lentic water bodies are subject to enhanced biogeochemical processing and can be released as greenhouse gases (GHG), including CH4 dominated bubble release from sediments (ebullition). Measurement of ebullition using traditional and novel techniques demonstrated that these reservoirs are hotspots of CH4 generation and release. Across eight reservoirs the mean littoral ebullitive CH4 flux was 2.6 (0.1–6.9) mmol m–2 d–1 during the open-water period of 2017 and was stimulated by autochthonous C fixation — showing the strongest relationships with total ammonia nitrogen and chlorophyll a. This highlights the importance of nutrient export to, and eutrophication within, these systems for stimulating methanogenesis. Mean littoral ebullitive CH4 flux increased significantly during the 2018 open-water season to 12.7 (0.6–40.5) mmol m–2 d–1, and these interannual variations were linked to warmer water temperatures, a result of year to year differences in local hydroclimate. Ebullitive fluxes of CH4 from these reservoirs are higher than reported for most other lentic freshwater systems globally, but interestingly the rates varied strongly both across and within reservoirs. The use of novel sensors allowed ebullition rates in deeper zones to be quantified, and these measurements demonstrated that pelagic fluxes were significantly higher than those from littoral zones — an artifact of reservoir morphology. High temporal resolution records from the sensors also permitted detection of diel variations of ebullitive flux, and was significantly synchronous with sediment temperature at that timescale. This work advances our ability to quantify ebullition fluxes through the use of new sensors by allowing more comprehensive investigations of fluxes than previously possible, and also provides a foundation for agricultural reservoir siting and management strategies to minimize trade-offs associated with CH4 emissions while continuing to confer benefits in terms of nutrient retention and flood control.
Description
Keywords
agricultural reservoirs, automated sensors, beneficial management practices, ebullition, greenhouse gases, methane, nutrients
Citation
Degree
Master of Environment and Sustainability (M.E.S.)
Department
School of Environment and Sustainability
Program
Environment and Sustainability