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Greenhouse Gas Dynamics in Ice-covered Lakes Across Spatial and Temporal Scales
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lakes play a major role in the global carbon (C) cycle, despite making up a small area of earth’s surface. Lakes receive, transport and process sizable amounts of C, emitting a substantial amount of the greenhouse gases, carbon dioxide (CO2) and methane (CH4), into the atmosphere. Ice-covered lakes are particularly sensitive to climate change, as future reductions to the duration of lake ice cover will have profound effects on the biogeochemical cycling of C in lakes. It is still largely unknown how reduced ice cover duration will affect CO2 and CH4 emissions from ice-covered lakes. Thus, the primary aim of this thesis was to fill this knowledge gap by monitoring the spatial and temporal dynamics of CO2 and CH4 in ice-covered lakes. The results of this thesis demonstrate that below ice CO2 and CH4 were spatially and temporally variable. Nutrients were strongly linked to below ice CO2 and CH4 oxidation variations across lakes. In addition, below ice CO2 was generally highest in small shallow lakes, and in bottom waters. Whilst below ice CH4 was elevated in surface waters near where bubbles from anoxic lake sediment were trapped. During the ice-cover period, CO2 accumulation below ice was not linear, and at ice-melt incomplete mixing of lake waters resulted in a continued CO2 storage in bottom waters. Further, CO2 transported from the catchment and bottom waters contributed to high CO2 emissions. The collective findings of this thesis indicate that CO2 and CH4 emissions from ice-covered lakes will likely increase in the future. The strong relationship between nutrients and C processes below ice, imply that future changes to nutrient fluxes within lakes will influence the biogeochemical cycling of C in lakes. Since catchment and lake sediment C fluxes play a considerable role in below ice CO2 and CH4 dynamics, changes to hydrology and thermal stability of lakes will undoubtedly alter CO2 and CH4 emissions. Nevertheless, ice-covered lakes constitute a significant component of the global C cycle, and as such, should be carefully monitored and accounted for when addressing the impacts of global climate change.  

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 53 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1341
Keyword [en]
carbon cycle, climate change, cryosphere, carbon dioxide, methane, lakes, winter limnology, methane oxidation, nutrients, catchment
National Category
Natural Sciences
Research subject
Limnology
Identifiers
URN: urn:nbn:se:uu:diva-275018ISBN: 978-91-554-9467-4 (print)OAI: oai:DiVA.org:uu-275018DiVA: diva2:898624
Public defence
2016-03-18, Friessalen, Evolutionary Biology Centre (EBC), Norbyvägen 14, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-02-26 Created: 2016-01-28 Last updated: 2016-03-09
List of papers
1. Regional Variability and Drivers of Below Ice CO2 in Boreal and Subarctic Lakes
Open this publication in new window or tab >>Regional Variability and Drivers of Below Ice CO2 in Boreal and Subarctic Lakes
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2016 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 19, no 3, 461-476 p.Article in journal (Refereed) Published
Abstract [en]

Northern lakes are ice-covered for considerable portions of the year, where carbon dioxide (CO2) can accumulate below ice, subsequently leading to high CO2 emissions at ice-melt. Current knowledge on the regional control and variability of below ice partial pressure of carbon dioxide (pCO(2)) is lacking, creating a gap in our understanding of how ice cover dynamics affect the CO2 accumulation below ice and therefore CO2 emissions from inland waters during the ice-melt period. To narrow this gap, we identified the drivers of below ice pCO(2) variation across 506 Swedish and Finnish lakes using water chemistry, lake morphometry, catchment characteristics, lake position, and climate variables. We found that lake depth and trophic status were the most important variables explaining variations in below ice pCO(2) across the 506 lakes(.) Together, lake morphometry and water chemistry explained 53% of the site-to-site variation in below ice pCO(2). Regional climate (including ice cover duration) and latitude only explained 7% of the variation in below ice pCO(2). Thus, our results suggest that on a regional scale a shortening of the ice cover period on lakes may not directly affect the accumulation of CO2 below ice but rather indirectly through increased mobility of nutrients and carbon loading to lakes. Thus, given that climate-induced changes are most evident in northern ecosystems, adequately predicting the consequences of a changing climate on future CO2 emission estimates from northern lakes involves monitoring changes not only to ice cover but also to changes in the trophic status of lakes.

Keyword
CO2; winter limnology; ice cover; carbon; nutrients; lake depth
National Category
Ecology
Identifiers
urn:nbn:se:uu:diva-275017 (URN)10.1007/s10021-015-9944-z (DOI)000373018200007 ()
Funder
Swedish Research CouncilSwedish Research Council FormasEU, European Research Council
Available from: 2016-01-27 Created: 2016-01-27 Last updated: 2017-11-30Bibliographically approved
2. Temporal and spatial carbon dioxide concentration patterns in a small boreal lake in relation to ice cover dynamics
Open this publication in new window or tab >>Temporal and spatial carbon dioxide concentration patterns in a small boreal lake in relation to ice cover dynamics
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2015 (English)In: Boreal environment research, ISSN 1239-6095, E-ISSN 1797-2469, Vol. 20, no 6, 679-692 p.Article in journal (Refereed) Published
Abstract [en]

Global carbon dioxide (CO2) emission estimates from inland waters commonly neglect the ice-cover season. To account for CO2 accumulation below ice and consequent emissions into the atmosphere at ice-melt we combined automatically-monitored and manually- sampled spatially-distributed CO2 concentration measurements from a small boreal ice-covered lake in Sweden. In early winter, CO2 accumulated continuously below ice, whereas, in late winter, CO2 concentrations remained rather constant. At ice-melt, two CO2 concentration peaks were recorded, the first one reflecting lateral CO2 transport within the upper water column, and the second one reflecting vertical CO2 transport from bottom waters. We estimated that 66%–85% of the total CO2 accumulated in the water below ice left the lake at ice-melt, while the remainder was stored in bottom waters. Our results imply that CO2 accumulation under ice and emissions at ice-melt are more dynamic than previously reported, and thus need to be more accurately integrated into annual CO2 emission estimates from inland waters.

National Category
Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-267129 (URN)000366959400002 ()
Funder
Swedish Research CouncilSwedish Research Council FormasEU, European Research Council
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2017-12-01Bibliographically approved
3. Methane oxidation at the water-ice interface of an ice-covered lake
Open this publication in new window or tab >>Methane oxidation at the water-ice interface of an ice-covered lake
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2016 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 61, no S1, S78-S90 p.Article in journal (Refereed) Published
Abstract [en]

Lakes are important components of the global methane (CH4) cycle. In seasonally ice-covered lakes, CH4 transported by ebullition (bubbling) from anoxic sediments gets trapped at the water-ice interface. If not oxidized by methane-oxidizing bacteria (MOB), this can potentially lead to high episodic CH4 emissions at ice-melt. To understand the fate of CH4 trapped below ice, we measured depth-distributions of CH4 concentrations in the water column near bubbles trapped below ice in Lake Erken. We also performed a 21 d incubation experiment at low temperature (2.3 ± 0.2°C) to investigate the potential for CH4 oxidation. During most sampling occasions, we found steep CH4 concentration gradients just below the ice with a 13-fold decrease from the surface to a depth of 20 cm. In vitro incubations revealed that CH4oxidation can occur at low temperatures typical for the water-ice interface. CH4 oxidation was observed as a significant decrease in CH4 concentration, a significant increase in stable isotope 13C signature, and an increase in MOB during the incubation. Thus, CH4 accumulating in the top 20 cm of the water column, fed by diffusion from CH4 in trapped bubbles, may fuel significant CH4 oxidation. Since northern latitude lakes can be ice-covered for many months of the year and significant amounts of CH4 accumulate below the ice, the extent of CH4oxidation under these low temperature-conditions is important for understanding the potential CH4 emissions to the atmosphere during ice-melt.

National Category
Oceanography, Hydrology, Water Resources
Identifiers
urn:nbn:se:uu:diva-274392 (URN)10.1002/lno.10288 (DOI)000388560900007 ()
Funder
Swedish Research CouncilSwedish Research Council Formas
Available from: 2016-01-21 Created: 2016-01-21 Last updated: 2017-11-30Bibliographically approved
4. Constraints on methane oxidation in ice-covered boreal lakes
Open this publication in new window or tab >>Constraints on methane oxidation in ice-covered boreal lakes
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2016 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 121, no 7, 1924-1933 p.Article in journal (Refereed) Published
Abstract [en]

Boreal lakes can be ice covered for a substantial portion of the year at which time methane (CH4) can accumulate below ice. The amount of CH4 emitted at ice melt is partially determined by the interplay between CH4 production and CH4 oxidation, performed by methane-oxidizing bacteria (MOB). Yet the balance between oxidation and emission and the potential for CH4 oxidation in various lakes during winter is largely unknown. To address this, we performed incubations at 2 degrees C to screen for wintertime CH4 oxidation potential in seven lakes. Results showed that CH4 oxidation was restricted to three lakes, where the phosphate concentrations were highest. Molecular analyses revealed that MOB were initially detected in all lakes, although an increase in type I MOB only occurred in the three lake water incubations where oxidation could be observed. Accordingly, the increase in CO2 was on average 5 times higher in these three lake water incubations. For one lake where no oxidation was measured, we tested if temperature and CH4 availability could trigger CH4 oxidation. However, regardless of incubation temperatures and CH4 concentrations, ranging from 2 to 20 degrees C and 1-500M, respectively, no oxidation was observed. Our study indicates that some lakes with active wintertime CH4 oxidation may have low emissions during ice melt, while other and particularly nutrient poor lakes may accumulate large amounts of CH4 below ice that, in the absence of CH4 oxidation, will be emitted following ice melt. This variability in CH4 oxidation rates between lakes needs to be accounted for in large-scale CH4 emission estimates.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-274393 (URN)10.1002/2016JG003382 (DOI)000382581900015 ()
Funder
Swedish Research CouncilSwedish Research Council FormasCarl Tryggers foundation
Note

De två första författarna delar förstaförfattarskapet.

Available from: 2016-01-21 Created: 2016-01-21 Last updated: 2017-11-30Bibliographically approved

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