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Decreased efficiency of sediment carbon burial in boreal lakes at warming lake water temperatures
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
Linköping University. (Department of Thematic Studies – Environmental Change)
Linköping University. (Department of Thematic Studies – Environmental Change)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Organic carbon (OC) burial in lake sediments is a fundamental process for aquatic carbon (C) cycling since it removes C from the active cycling pool. Here, we address the effect of lake warming on the OC burial efficiency (OCBE) of sediments in boreal humic lakes to evaluate the impact of warmer water temperatures and altered water column stratification patterns on the OCBE with respect to future climate change. Present-day water column conditions of temperature and oxygen concentration were monitored in two small (<0.1 km2) boreal lakes in central and southwestern Sweden over an annual cycle. Furthermore, OC burial, sediment OC mineralization, the OCBE, and lake CO2 and CH4 emission to the atmosphere, were quantified on a whole-basin scale and for a whole year. Then, future temperature and stratification patterns were simulated in both lakes by adjusting present-day temperature and oxygen conditions according to projected air temperature warming scenarios, in order to quantify the net effect on the OCBE. Present-day conditions were found to be similar in the two study lakes, with annual OC mineralization (~1 t C yr-1) exceeding annual OC burial (<0.5 t C yr-1) at least by a factor of two, returning whole-basin OC burial efficiencies of 28 and 25%, respectively. Also, annual CO2 and CH4 emission was found to be of similar magnitude in both study lakes (6-7 t C yr-1) surpassing sediment C fluxes by at least 5 times. The different lake warming scenarios (RCP 2.6, 4.5 and 8.5, corresponding to increase in annual mean air temperature of 1.5-4.5 °C) caused moderate decreases in the OCBE of 5-16% and 3-11% for each lake, respectively. The more pronounced decreases in OCBE were obtained for the more shallow lake, related to a higher proportion of sediments being affected by warming in the epilimnion. Our results suggest that warmer surface water temperatures in the future are likely to reduce the efficiency of the lake sediment C sink, thereby affecting the long-term C balance of the boreal forest biome.

National Category
Environmental Sciences
Research subject
Biology with specialization in Limnology
URN: urn:nbn:se:uu:diva-260668OAI: oai:DiVA.org:uu-260668DiVA: diva2:851760
Available from: 2015-09-07 Created: 2015-08-21 Last updated: 2015-10-01
In thesis
1. The role of sediments in the carbon cycle of boreal lakes
Open this publication in new window or tab >>The role of sediments in the carbon cycle of boreal lakes
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Inland waters are active sites of carbon (C) processing and emitters of carbon dioxide (CO2) and methane (CH4) to the atmosphere. In the boreal zone, where surface waters receive large quantities of organic carbon (OC) from surrounding forests and wetlands, lakes and streams act as strong sources of these greenhouse gases. Lake sediments provide the only long-term sink of C in boreal inland waters, through burial of OC. However, mineralization of OC counteracts the efficiency of lake sediments in removing C from the short-term C cycle. In this context, this thesis provides a better insight into the dual role of boreal lake sediments as C source and C sink.

The presented work is based on empirical assessments of OC burial and OC mineralization rates in boreal lakes. The temporal variability of OC burial and the stability of the buried OC was assessed on both centennial and millennial timescales. The quantitative importance of sediment OC burial and mineralization in comparison both to other C fluxes within the lake, and to C fluxes within the tributary stream network, was quantified. By simulating the effect of climate change on water temperature, we also gauged the potential future efficiency of lake sediments in storing C.

The results demonstrate that OC mineralization in sediments dominates three-fold over OC burial when observed at a whole-basin and annual scale. The contribution of sediment OC mineralization to annual C emission from the assessed study lake was, however, found to be small (16%), when compared to OC mineralization in the water column (37%) and catchment import of C (47%). Furthermore, C emission from headwater streams was found to dominate greatly over the lake C emission, mainly triggered by the higher gas transfer velocity of streams compared to lakes.

On a long-term (Holocene) scale, the continuous OC burial flux results in a large amount of C stored in sediments. The temporal variability of this OC accumulation was found to vary across lakes, with, however, time-dependent patterns: On a millennial scale, smaller lakes exhibited a higher variability than larger lakes of the study area. For the last century, similar variability and a trend to increased OC accumulation was found for most study lakes, irrespective of their size. Analysis of lignin phenols in the accumulated OC did not indicated post-depositional degradation, independent of the age of the sediment OC, implying that sediments are a very stable sink for land-derived OC in boreal lakes.

Simulation of warming water temperatures in boreal lakes resulted in declines of the OC burial efficiency BE (OCBE; OC burial/OCdeposition) up to 16%, depending, however, on basin morphometry. Predicted declines in OCBE were higher for the more shallow lake compared to the deeper lake.

In conclusion, this thesis illustrates that sediments play, despite a small quantitative impact on aquatic C cycling, an important role as a very stable C sink in boreal lakes. However, the efficiency of this C sink is likely to be reduced in the future.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 42 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1279
National Category
Environmental Sciences
urn:nbn:se:uu:diva-261157 (URN)978-91-554-9318-9 (ISBN)
Public defence
2015-10-16, Ekmans salen, Norbyvägen, Uppsala, 10:00 (English)
Available from: 2015-09-25 Created: 2015-08-31 Last updated: 2015-10-01

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