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Carbon Dioxide in Inland Waters: Drivers and Mechanisms Across Spatial and Temporal Scales
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.ORCID iD: 0000-0002-0078-2810
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Inland waters are an essential component of the global carbon cycle as they are very active sites for carbon transformation processes. Much of this carbon is transformed into the greenhouse gas carbon dioxide (CO2) and emitted into the atmosphere. The biogeochemical and hydrological mechanisms driving CO2 concentrations in inland waters are manifold. Although some of them have been studied in detail, there are still knowledge gaps regarding the relative importance of the different CO2-driving mechanisms, both on a spatial and a temporal scale. The main aim of this thesis was to fill some of the knowledge gaps by studying long- and short-term effects of enhanced dissolved organic carbon (DOC) concentrations on surface water partial pressure of CO2 (pCO2) as well as to investigate both internal (i.e., within the water body) and external (i.e., catchment) drivers of pCO2 in inland waters. Based on analyses of long-term data from more than 300 boreal lakes and streams and on results from two mesocosm experiments as well as a detailed catchment study, one of the main results of the thesis was that DOC concentrations were, on a temporal scale, generally uncoupled to pCO2. Indeed, additions of allochthonous DOC to lake water could result in increased pCO2 in waters but not as originally expected by stimulation of bacterial activity but instead by light driven suppression of primary production, at least in mesotrophic waters. Changes in the carbonate system was also found to be a main driver for surface water pCO2. Finally, also external processes such as groundwater inputs contributed substantially to variations of surface water pCO2. In a detailed study on carbon in groundwater, pCO2 in groundwater was found to decrease with soil depth and correlated negatively with pH, which increased with soil depth. Conclusively, this thesis show that pCO2 does not follow the trends of increased DOC in boreal surface waters but instead correlates with changes in primary production and shifts in the carbonate system. Additionally, the dominating mechanisms driving pCO2 clearly differ between lakes and streams. Consequently, simulations of future CO2 dynamics and emissions from inland waters cannot rely on DOC concentrations as a pCO2 predictor, but rather need to incorporate several pCO2 driving mechanisms, and consider the difference between lakes and streams.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 50
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1838
Keywords [en]
carbon dioxide, dissolved organic carbon, inland water, lake, stream, groundwater, mesocosm, carbonate system, carbon
National Category
Biological Sciences
Research subject
Biology with specialization in Limnology
Identifiers
URN: urn:nbn:se:uu:diva-390882ISBN: 978-91-513-0720-6 (print)OAI: oai:DiVA.org:uu-390882DiVA, id: diva2:1343035
Public defence
2019-10-04, Friessalen, Evolutionsbiologiskt Centrum, Norbyvägen 18, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2019-09-12 Created: 2019-08-15 Last updated: 2019-10-15
List of papers
1. No long-term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams and rivers
Open this publication in new window or tab >>No long-term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams and rivers
2017 (English)In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 31, no 6, p. 985-995Article in journal (Refereed) Published
Abstract [en]

Concentrations of dissolved organic carbon (DOC) from terrestrial sources have been increasing in freshwaters across large parts of the boreal region. According to results from large-scale field and detailed laboratory studies, such a DOC increase could potentially stimulate carbon dioxide (CO2) production, subsequently increasing the partial pressure of CO2 (pCO2) in freshwaters. However, the response of pCO2 to the presently observed long-term increase in DOC in freshwaters is still unknown. Here we tested whether the commonly found spatial DOC-pCO2 relationship is also valid on a temporal scale. Analyzing time series of water chemical data from 71 lakes, 30 streams, and 4 river mouths distributed across all of Sweden over a 17 year period, we observed significant DOC concentration increases in 39 lakes, 15 streams, and 4 river mouths. Significant pCO2 increases were, however, only observed in six of these 58 waters, indicating that long-term DOC increases in Swedish waters are disconnected from temporal pCO2 trends. We suggest that the uncoupling of trends in DOC concentration and pCO2 are a result of increased surface water runoff. When surface water runoff increases, there is likely less CO2 relative to DOC imported from soils into waters due to a changed balance between surface and groundwater flow. Additionally, increased surface water runoff causes faster water flushing through the landscape giving less time for in situ CO2 production in freshwaters. We conclude that pCO2 is presently not following DOC concentration trends, which has important implications for modeling future CO2 emissions from boreal waters.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2017
National Category
Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-323413 (URN)10.1002/2016GB005539 (DOI)000405103600004 ()
Funder
EU, Horizon 2020, 643052Knut and Alice Wallenberg Foundation, KAW 2013.0091Swedish Research Council, 2016-04153
Available from: 2017-06-07 Created: 2017-06-07 Last updated: 2019-08-15Bibliographically approved
2. Colored organic matter increases CO2 in meso-eutrophic lake water through altered light climate and acidity
Open this publication in new window or tab >>Colored organic matter increases CO2 in meso-eutrophic lake water through altered light climate and acidity
Show others...
2019 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 64, no 2, p. 744-756Article in journal (Refereed) Published
Abstract [en]

Many surface waters across the boreal region are browning due to increased concentrations of colored allochthonous dissolved organic carbon (DOC). Browning may stimulate heterotrophic metabolism, may have a shading effect constraining primary production, and may acidify the water leading to decreased pH with a subsequent shift in the carbonate system. All these effects are expected to result in increased lake water carbon dioxide (CO2) concentrations. We tested here these expectations by assessing the effects of both altered allochthonous DOC input and light conditions through shading on lake water CO2 concentrations. We used two mesocosm experiments with water from the meso‐eutrophic Lake Erken, Sweden, to determine the relative importance of bacterial activities, primary production, and shifts in the carbonate system on CO2 concentrations. We found that DOC addition and shading resulted in a significant increase in partial pressure of CO2 (pCO2) in all mesocosms. Surprisingly, there was no relationship between bacterial activities and pCO2. Instead the experimental reduction of light by DOC and/or shading decreased the photosynthesis to respiration ratio leading to increased pCO2. Another driving force behind the observed pCO2 increase was a significant decrease in pH, caused by a decline in photosynthesis and the input of acidic DOC. Considering that colored allochthonous DOC may increase in a warmer and wetter climate, our results could also apply for whole lake ecosystems and pCO2 may increase in many lakes through a reduction in the rate of photosynthesis and decreased pH.

National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-366220 (URN)10.1002/lno.11072 (DOI)000461865500022 ()
Available from: 2018-11-18 Created: 2018-11-18 Last updated: 2019-08-15Bibliographically approved
3. Groundwater carbon within a boreal catchment – spatiotemporal variability of a hidden aquatic carbon pool
Open this publication in new window or tab >>Groundwater carbon within a boreal catchment – spatiotemporal variability of a hidden aquatic carbon pool
(English)In: Article in journal (Other academic) Submitted
National Category
Biological Sciences
Research subject
Biology with specialization in Limnology
Identifiers
urn:nbn:se:uu:diva-390878 (URN)
Available from: 2019-08-15 Created: 2019-08-15 Last updated: 2019-08-15
4. Highly variable explanations of long-term pCO2 increases in boreal lakes and streams
Open this publication in new window or tab >>Highly variable explanations of long-term pCO2 increases in boreal lakes and streams
(English)In: Article in journal (Other academic) Submitted
National Category
Biological Sciences
Research subject
Biology with specialization in Limnology
Identifiers
urn:nbn:se:uu:diva-390881 (URN)
Available from: 2019-08-15 Created: 2019-08-15 Last updated: 2019-08-15

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