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Gas Exchange over Aquatic Interfaces and its Importance for Greenhouse Gas Emission
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.ORCID iD: 0000-0002-8763-3139
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aquatic ecosystems play a substantial role in global cycling of carbon (C), despite covering only about 4% of the earth surface. They emit large amounts of greenhouse gases (GHG) to the atmosphere, comparable to the amount of C stored annually in terrestrial ecosystems. In addition, C can be buried in lake sediments. Headwater systems are located at the interface of the terrestrial and aquatic environment, and are first in line to process terrestrial C and throughout its journey through the aquatic continuum. The uncertainties in global estimates of aquatic GHG emissions are largely related to these headwater systems, as they are highly variable in time and space, and underrepresented in global assessments. The overall aim of this thesis was therefore to study GHG exchange between sediment, water and air in headwater systems, from both an ecosystem perspective and at the small scale of physical drivers of gas exchange.

This thesis demonstrates that carbon dioxide (CO2) emission from headwater systems, especially streams, was the main pathway of C loss from surface waters from a lake catchment. Of the total aquatic CO2-emission of the catchment, 65% originated from stream systems that covered only 0.1% of the total catchment area. The gas transfer velocity (k) was the main driver of stream CO2-emission, but there was a high variability in k on small spatial scales (meters). This variability may have implications for upscaling GHG emissions, especially when using scaled k estimates. Lake sediments only contributed 16% to total lake C emission, but in reality, sediment C emission is probably even lower because experimentally determined sediment C flux returns high estimates that are biased since artificially induced turbulence enhances C flux rates beyond in-situ conditions. When sediment C flux is estimated in-situ, in natural bottom water turbulence conditions, flux rates were lower than those estimated experimentally.

Conclusively, this thesis shows that GHG emissions from small aquatic ecosystems are dominant over other aquatic C fluxes and that our current knowledge regarding the physical processes controlling gas exchange from different small aquatic systems is limited, implying an inherent uncertainty of GHG emission estimates from small aquatic ecosystems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 49
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1457
Keyword [en]
gas exchange, lake, stream, lake sediment, headwaters, carbon dioxide, methane, greenhouse gas emission, carbon, turbulence
National Category
Natural Sciences Ecology Environmental Sciences
Research subject
Biology with specialization in Limnology
Identifiers
URN: urn:nbn:se:uu:diva-307792ISBN: 978-91-554-9764-4 (print)OAI: oai:DiVA.org:uu-307792DiVA, id: diva2:1049414
Public defence
2017-01-20, Ekmansalen, EBC, Norbyvägen 14, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2016-12-21 Created: 2016-11-21 Last updated: 2016-12-28
List of papers
1. Carbon dioxide evasion from headwater systems strongly contributes to the total export of carbon from a small boreal lake catchment
Open this publication in new window or tab >>Carbon dioxide evasion from headwater systems strongly contributes to the total export of carbon from a small boreal lake catchment
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2015 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 120, no 1, p. 13-28Article in journal (Refereed) Published
Abstract [en]

Inland waters are hotspots for carbon (C) cycling and therefore important for landscape C budgets. Small streams and lakes are particularly important; however, quantifying C fluxes is difficult and has rarely been done for the entire aquatic continuum, composed of connected streams and lakes within the same catchment. We investigated carbon dioxide (CO2) evasion and fluvial fluxes of dissolved inorganic carbon and dissolved organic carbon (DIC and DOC) in stream and lake systems within the 2.3km(2) catchment of a small boreal lake. Our results show pronounced spatial and temporal variability in C fluxes even at a small spatial scale. C loss from the catchment through CO2 evasion from headwaters for the total open water-sampling period was 9.7g C m(-2) catchment, dominating the total catchment C loss (including CO2 evasion, DIC, and DOC export from the lake, which were 2.7, 0.2, and 5.2g C m(-2) catchment, respectively). Aquatic CO2 evasion was dominated by headwater streams that occupy similar to 0.1% of the catchment but contributed 65% to the total aquatic CO2 evasion from the catchment. The importance of streams was mainly an effect of the higher gas transfer velocities than compared to lakes (median, 67 and 2.2cmh(-1), respectively). Accurately estimating the contribution of C fluxes from headwater streams, particularly the temporal and spatial dynamics in their gas transfer velocity, is key to landscape-scale C budgets. This study demonstrates that CO2 evasion from headwaters can be the major pathway of C loss from boreal catchments, even at a small spatial scale.

National Category
Climate Research
Identifiers
urn:nbn:se:uu:diva-241585 (URN)10.1002/2014JG002706 (DOI)000349899200002 ()
Available from: 2015-01-13 Created: 2015-01-13 Last updated: 2018-05-21Bibliographically approved
2. The role of sediments in the carbon budget of a small boreal lake
Open this publication in new window or tab >>The role of sediments in the carbon budget of a small boreal lake
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2016 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 61, no 5, p. 1814-1825Article in journal (Refereed) Published
Abstract [en]

We investigated the role of lake sediments as carbon (C) source and sink in the annual C budget of a small (0.07 km2), shallow (mean depth 3.4 m), and humic lake (mean DOC concentration 17 mg L-1) in boreal Sweden. Organic carbon (OC) burial and mineralization in sediments were quantified from 210Pb-dated sediment and laboratory sediment incubation experiments, respectively, and upscaled to the entire basin and to one whole year, by using sediment thickness derived sub-bottom profiling, basin morphometry, and water column monitoring data of temperature and oxygen concentration. Furthermore, catchment C import, open water metabolism, photochemical mineralization as well as carbon dioxide (CO2) and methane (CH4) emissions to the atmosphere, were quantified to relate sediment processes to other lake C fluxes. We found that on a whole-basin and annual scale, sediment OC mineralization was three times larger than OC burial, and contributed about 16% to the annual CO2 emission from the lake to the atmosphere. Remaining contributions to the CO2 emission were attributed to water column metabolism (31%), photochemical mineralization (6%), and catchment imports via inlet streams and inflow of shallow groundwater (47%). We conclude that on an annual and whole-basin scale 1) sediment OC mineralization dominated over OC burial, 2) water column OC mineralization contributed more than sediments to lake CO2 emission, and 3) catchment import of C to the lake was greater than lake-internal C cycling. 

Keyword
mineralization, burial, carbon dioxide, methane, emission, photomineralization
National Category
Environmental Sciences
Research subject
Biology with specialization in Limnology
Identifiers
urn:nbn:se:uu:diva-260666 (URN)10.1002/lno.10336 (DOI)000383621800019 ()
Funder
EU, European Research CouncilSwedish Research Council FormasSwedish Research Council
Available from: 2015-09-07 Created: 2015-08-21 Last updated: 2018-05-21Bibliographically approved
3. Low sediment-water gas exchange in a small boreal lake
Open this publication in new window or tab >>Low sediment-water gas exchange in a small boreal lake
2016 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 121, no 9, p. 2493-2505Article in journal (Refereed) Published
Abstract [en]

Boreal lake sediments are carbon sources by producing CO2. CO2 flux from sediments is partly controlled by turbulence in the water column, which is not given the same attention as CO2production rates in current estimates of CO2 fluxes from sediments. We quantified the in situ CO2flux across the sediment-water interface in a small (0.07 km2) lake in Sweden by measuring the in situ O2 flux with the Eddy Correlation (EC) method and using the apparent respiratory quotient (CO2 production:O2 consumption) derived from sediment incubations. We demonstrate that median CO2 flux estimated by EC was ~70% smaller than estimated by sediment incubations with artificial water mixing (1.0 × 10−2 and 3.6 × 10−2 µmol C m−2 s−1, respectively). Additionally, we show that inducing artificial mixing of supernatant water in the incubation experiment has a positive effect on observed fluxes, enhancing CO2 flux by ~30% compared to not mixing supernatant water. We suggest that the difference between the methods is due to the strong artificial water mixing in sediment incubations compared to the turbulent mixing in this small lake. Additionally, low O2 supply to sediment aerobic heterotrophic microbes during extended periods of low water currents can inhibit respiration and thus CO2 production. These findings suggest that the sediment contribution to total lake CO2 emission might currently be overestimated for small boreal lakes. Care should be taken when upscaling sediment CO2 flux derived from incubation experiments to entire basins of small lakes, as incubation experiments are unlikely to accurately mimic in situ bottom water currents and gas exchange.

Keyword
lake sediment, gas exchange, eddy correlation
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-304299 (URN)10.1002/2016JG003372 (DOI)000385712800012 ()
Funder
Swedish Research CouncilEU, European Research Council, 336642
Available from: 2016-10-04 Created: 2016-10-04 Last updated: 2018-05-21Bibliographically approved
4. High spatial variability in stream k600 revealed by turbulence measurements – implications for scaling GHG emissions
Open this publication in new window or tab >>High spatial variability in stream k600 revealed by turbulence measurements – implications for scaling GHG emissions
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(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences Ecology Environmental Sciences
Research subject
Biology with specialization in Limnology
Identifiers
urn:nbn:se:uu:diva-307785 (URN)
Available from: 2016-11-24 Created: 2016-11-21 Last updated: 2016-11-24

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