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Neglecting seasonality causes biased view of climate and forestry impacts on lake CO2 cycling
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Climate Impacts Research Centre, Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden.
Department of Forest Ecology and Management, Swedish University of Agricultural Science, Umeå, Sweden.
Climate Impacts Research Centre, Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Lakes emit carbon dioxide (CO2) at globally significant rates. These emissions may be controlled by land use and climate. Disentangling these interactions is a prerequisite to accurately predicting anthropogenic impacts on carbon cycling in lake rich regions. We used environmental monitoring data of lakes collected in autumn (n = 439) and throughout the whole open-water season (n = 22) from a wet and a dry year in Swedish forest catchments to evaluate direct and indirect effects of temperature, precipitation, wind speed, forest productivity and forest clearcutting on lake CO2 concentrations. We found that trends in CO2 concentrations along climate and forestry gradients do not always correspond between autumn and open-water season averages, implying the need of validation of patterns derived from snap-shot data alone. According to spatially resolved data, autumn CO2 concentrations increased with mean annual air temperature (dry year) or catchment forest productivity (wet year) but were not coupled to colored dissolved organic matter (CDOM) concentrations. In contrast, open-water season averaged CO2 concentrations were constant across temperature and productivity gradients but increased with CDOM. These antagonistic results suggest that our fundamental understanding of the controls of CO2 in lakes depends on whether spatially or temporally resolved data is used. Hence, trade-offs in sampling efforts clearly limit the questions that can be addressed regarding climate and land use effects on lake carbon cycling.

National Category
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-135005OAI: oai:DiVA.org:umu-135005DiVA: diva2:1095742
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-18
In thesis
1. Land use effects on greenhouse gas emissions from boreal inland waters
Open this publication in new window or tab >>Land use effects on greenhouse gas emissions from boreal inland waters
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Anthropogenic activities perturb the global carbon and nitrogen cycle with large implications for the earth’s climate. Land use activities deliver excess carbon and nitrogen to aquatic ecosystems. In the boreal biome, this is mainly due to forestry and atmospheric deposition. Yet, impacts of these anthropogenically mediated inputs of carbon and nitrogen on the processing and emissions of greenhouse gases from recipient streams and lakes are largely unknown. Understanding the ecosystem-scale response of aquatic greenhouse gas cycling to land use activities is critical to better predict anthropogenic effects on the global climate system and design more efficient climate change mitigation measures.

This thesis assesses the effects of forest clearcutting and nitrate enrichment on greenhouse gas emissions from boreal inland waters. It also advances methods to quantify sources and sinks of these emissions. Short-term clearcut and nitrate enrichment effects were assessed using two whole-ecosystem experiments, carried out over four years in nine headwater catchments in boreal Sweden. In these experiments, I measured or modeled air-water fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), combining concentration, ebullition and gas-transfer velocity measurements in groundwater, streams and lakes. By using Swedish national monitoring data, I also assessed broad-scale effects of forest clearcutting by relating CO2 concentrations in 439 forest lakes to the areal proportion of catchment forest clearcuts. To improve quantifications of CO2 sources and sinks in lakes, I analyzed time series of oxygen concentrations and water temperature in five lakes on conditions under which whole-lake metabolism estimates can be inferred from oxygen dynamics given the perturbing influence of atmospheric exchange, mixing and internal waves.

The experiments revealed that aquatic greenhouse gas emissions did not respond to nitrate addition or forest clearcutting. Importantly, riparian zones likely buffered clearcut-induced increases in groundwater CO2 and CH4 concentrations. Experimental results were confirmed by monitoring data showing no relationship between CO2 patterns across Swedish lakes and clearcut gradients. Yet, conclusions on internal vs. external CO2 controls largely depended on whether spatially or temporally resolved data was used. Partitioning CO2 sources and sinks in lakes using time series of oxygen was greatly challenged by physical transport and mixing processes.

Conclusively, ongoing land use activities in the boreal zone are unlikely to have major effect on headwater greenhouse gas emissions. Yet, system- and scale specific effects cannot be excluded. To reveal these effects, there is a large need of improved methods and design of monitoring programs that account for the large spatial and temporal variability in greenhouse gas dynamics and its controls by abiotic and biotic factors.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2017. 46 p.
Keyword
greenhouse gas, boreal forest, carbon cycling, whole-ecosystem experiment, limnology, metabolism, forest clearcutting, nitrogen enrichment
National Category
Physical Geography
Identifiers
urn:nbn:se:umu:diva-134767 (URN)978-91-7601-716-6 (ISBN)
Public defence
2017-06-09, Lilla Hörsalen, KBC huset, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2017-05-19 Created: 2017-05-11 Last updated: 2017-05-19Bibliographically approved

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