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Low sediment-water gas exchange in a small boreal lake
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Swiss Fed Inst Aquat Sci & Technol, Surface Waters Res & Management, Eawag, Kastanienbaum, Switzerland.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
2016 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 121, no 9, 2493-2505 p.Article 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.

Place, publisher, year, edition, pages
2016. Vol. 121, no 9, 2493-2505 p.
Keyword [en]
lake sediment, gas exchange, eddy correlation
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-304299DOI: 10.1002/2016JG003372ISI: 000385712800012OAI: oai:DiVA.org:uu-304299DiVA: diva2:1015055
Funder
Swedish Research CouncilEU, European Research Council, 336642
Available from: 2016-10-04 Created: 2016-10-04 Last updated: 2017-06-09Bibliographically approved
In thesis
1. Gas Exchange over Aquatic Interfaces and its Importance for Greenhouse Gas Emission
Open this publication in new window or tab >>Gas Exchange over Aquatic Interfaces and its Importance for Greenhouse Gas Emission
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. 49 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1457
Keyword
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:nbn:se:uu:diva-307792 (URN)978-91-554-9764-4 (ISBN)
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

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Kokic, JovanaSahlée, ErikSobek, Sebastian

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