umu.sePublications
Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Land use effects on greenhouse gas emissions from boreal inland waters
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
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 [en]
greenhouse gas, boreal forest, carbon cycling, whole-ecosystem experiment, limnology, metabolism, forest clearcutting, nitrogen enrichment
National Category
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-134767ISBN: 978-91-7601-716-6 (print)OAI: oai:DiVA.org:umu-134767DiVA: diva2:1094823
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
List of papers
1. Weak response of greenhouse gas emissions to whole lake N enrichment
Open this publication in new window or tab >>Weak response of greenhouse gas emissions to whole lake N enrichment
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Global warming and land use scenarios suggest increased 21st century nitrogen (N) inputs to boreal lakes. Nitrogen affects in-lake processing and, potentially, atmospheric exchange of greenhouse gases. However, we miss evidence from whole-lake experiments addressing the net effect of N inputs on greenhouse gas exchange of lake ecosystems. Here, we test for the first time the effect of a whole-lake experimental increase (three-fold) in external nitrate loads on the atmospheric emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from six N-limited unproductive boreal lakes. Nitrogen enrichment effects were assessed within a paired Before-After/Control-Impact framework based on 2-hourly to biweekly surface-water sampling of dissolved gas concentrations, and monthly whole-lake inventory surveys, carried out over four years. Responses of gas emissions during the summer stratification period to nitrate additions were small in all lakes and not statistically significant; this was also true for changes in summer and winter inventories and emphasizes the modest role of internal carbon fixation for the CO2 dynamics of unproductive boreal lakes. A global synthesis of 54 published studies revealed a wide range of N fertilization effects on greenhouse gas emissions depending on the spatiotemporal scale of the study system and if N was added alone or in combination with phosphorous. Combined, our results suggest that N enrichment effects on greenhouse gas dynamics are small on a whole-lake scale and that greenhouse gas emissions from unproductive boreal lakes are robust against changes in N supply.

National Category
Physical Geography
Identifiers
urn:nbn:se:umu:diva-135002 (URN)
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-18
2. Does clearcut forestry influence aquatic greenhouse gas emissions?
Open this publication in new window or tab >>Does clearcut forestry influence aquatic greenhouse gas emissions?
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Forest clearcutting generally increase exports of carbon and nitrogen to downstream aquatic systems. Although these losses affect the greenhouse gas budget of managed forests, it is unknown if they modify greenhouse gas emissions of recipient inland waters. To assess this question, we quantified atmospheric fluxes of carbon dioxide (CO2) methane (CH4) and nitrous oxide (N2O) of humic lakes and their inlet streams in four boreal catchments of which two were treated with forest clearcuts (18% and 44% of the catchment area) using a Before-After/Control-Impact-experiment. We measured gas concentrations and hydrological and physicochemical water characteristics in hillslope groundwater, along stream transects and at multiple locations in lakes at 2-hourly to biweekly intervals throughout the snow-free season over a four year period. These measurements were combined with atmospheric gas transfer measurements and models to calculate aquatic greenhouse gas emissions. Forest clearcutting did not change greenhouse gas emissions from streams or lakes, despite significant increases of CO2 and CH4 concentrations in hillslope groundwater. Clearcut effects on groundwater were likely buffered in the riparian zone. Hence, the greenhouse gas budget of forests initially after clearcutting is unlikely to be confounded by aquatic greenhouse gas emissions. However, our findings should be extrapolated with caution to other environments. Here, site-specific conditions makes our study system representative for systems where clearcutting causes only a limited initial impact on catchment hydrology and biogeochemistry.

National Category
Physical Geography
Identifiers
urn:nbn:se:umu:diva-135003 (URN)
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-18
3. Neglecting seasonality causes biased view of climate and forestry impacts on lake CO2 cycling
Open this publication in new window or tab >>Neglecting seasonality causes biased view of climate and forestry impacts on lake CO2 cycling
(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:nbn:se:umu:diva-135005 (URN)
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-18
4. Estimates of ecosystem metabolism in unproductive lakes with inclusion of physical oxygen fluxes
Open this publication in new window or tab >>Estimates of ecosystem metabolism in unproductive lakes with inclusion of physical oxygen fluxes
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Whole-lake metabolism estimates based on free-water oxygen measurements may be compromised in unproductive lakes if the transport of oxygen by physical processes is neglected and large relative to biologically induced changes. Here, we quantify how oxygen fluxes associated with atmospheric exchange, vertical mixing and internal waves modify metabolism estimates in five unproductive boreal, brown water lakes. Water temperature and dissolved oxygen concentrations were measured at five minute intervals at 5-8 depths for 8-70 days per lake from May to September 2015. We estimated daily metabolism using three inverse Bayesian models accounting for atmospheric gas exchange using a (1) conventional wind-speed model and a (2) surface renewal model based on near-surface turbulence, and (3) accounting for within lake mixing by deepening of the actively mixing layer and the coefficient of eddy diffusivity. Gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP) ranged from 0 to 0.9, -0.3 to -2.5 and -0.2 to -1.6 g C m-3 d-1, respectively. Metabolism estimates were reduced by up to 400% if oxygen time series were filtered to remove effects of internal waves and thermocline up- and downwelling, which have likely caused large changes in concentrations. Metabolism estimates differed by 0-25% depending on the atmospheric gas transfer model used and by 0-120% depending on whether fluxes from vertical mixing were considered. Almost half of the metabolism estimates were unreasonable (GPP<0 or ER>0), and these largely coincided with enhanced (up to 300%) diel cycles in physical oxygen fluxes. We conclude that physical processes should be explicitly assessed in three dimensions when using the free-water oxygen method to model metabolism in unproductive lakes.

National Category
Physical Geography
Identifiers
urn:nbn:se:umu:diva-135006 (URN)
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-18

Open Access in DiVA

fulltext(1616 kB)58 downloads
File information
File name FULLTEXT01.pdfFile size 1616 kBChecksum SHA-512
2e61780aeea3e61c08e66049a6fcb61102c28044efb54f94a8a2dff720e73a8fba4b8a4117ccba8f0337199482338128fbda784860779149ccda7183e2f8ba98
Type fulltextMimetype application/pdf
spikblad(191 kB)8 downloads
File information
File name SPIKBLAD01.pdfFile size 191 kBChecksum SHA-512
a6484d27203827e3b7cd16cf1ee4682280ce1e023ae25f26d6c1dbe505b0a289c4284d921f67e47950cf2d64a9f2f37d1ce5c33d8f32af1f6be1bbb951e28fc2
Type spikbladMimetype application/pdf

Search in DiVA

By author/editor
Klaus, Marcus
By organisation
Department of Ecology and Environmental Sciences
Physical Geography

Search outside of DiVA

GoogleGoogle Scholar
Total: 58 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 752 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf