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Carbon metabolism in clear-water and brown-water lakes
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The trophic state of lakes is commonly defined by the concentration of nutrients in the water column. High nutrient concentrations generate high phytoplankton production, and lakes with low nutrient concentrations are considered low-productive. This simplified view of lake productivity ignores the fact that benthic primary producers and heterotrophic bacteria can be important basal producers in lake ecosystems.

In this thesis I have studied clear-water and brown-water lakes with respect to primary production, respiration and bacterial production based on allochthonous organic carbon. These processes were quantified in pelagic and benthic habitats on temporal and spatial scales. I also calculated the net ecosystem production of the lakes, defined as the difference between gross primary production (GPP) and respiration (R). The net ecosystem production indicates whether a lake is net heterotrophic (GPP < R), net autotrophic (GPP > R) or in metabolic balance (GPP = R). Net heterotrophic lakes are sources of carbon dioxide (CO2) to the atmosphere since respiration in these lakes, by definition, is subsidized by an external organic carbon source. External organic carbon is transported to lakes from the terrestrial environment via inlets, and can serve as a carbon source for bacteria but it also limits light availability for primary producers by absorbing light.

On a seasonal scale, four of the clear-water lakes studied in this thesis were dominated by primary production in the soft-bottom benthic habitat and by respiration in the pelagic habitat. Concentrations of dissolved organic carbon (DOC) were low in the lakes, but still high enough to cause the lakes to be net heterotrophic. However, the lakes were not low-productive due to the high production in the benthic habitat. One of the clear-water lakes was studied also during the winter and much of the respiration under ice was supported by the benthic primary production from the previous summer. This is in contrast to brown-water lakes where winter respiration is suggested to be supported by allochthonous organic carbon.

By studying lakes in a DOC gradient (i.e. from clear-water to brown-water lakes) I could draw two major conclusions. The lakes became less productive since benthic primary production decreased with increasing light extinction, and the lakes became larger sources of CO2 to the atmosphere since pelagic respiration was subsidized by allochthonous organic carbon. Thus, lake carbon metabolism can have an important role in the global carbon cycle due to their processing of terrestrial organic carbon and to their possible feedback effects on the climate system.

Place, publisher, year, edition, pages
Umeå: Institutionen för ekologi, miljö och geovetenskap, Department of Ecology and Environmental Sciences , 2010. , p. 31
Keyword [en]
clear-water lakes, brown-water lakes, primary production, bacterial production, benthic, pelagic, net ecosystem production, allochthonous organic carbon, CO2, DOC
National Category
Physical Geography Ecology
Research subject
Limnology; Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-33488ISBN: 978-91-7264-954-5 (print)OAI: oai:DiVA.org:umu-33488DiVA, id: diva2:313605
Public defence
2010-05-21, KBC, Stora Hörsalen (KB3B1), Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2010-04-29 Created: 2010-04-26 Last updated: 2018-06-08Bibliographically approved
List of papers
1. Whole-lake estimates of carbon flux through algae and bacteria in benthic and pelagic habitats of clear-water lakes
Open this publication in new window or tab >>Whole-lake estimates of carbon flux through algae and bacteria in benthic and pelagic habitats of clear-water lakes
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2009 (English)In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 90, no 7, p. 1923-1932Article in journal (Refereed) Published
Abstract [en]

This study quantified new biomass production of algae and bacteria in both benthic and pelagic habitats of clear-water lakes to contrast how carbon from the atmosphere and terrestrial sources regulates whole-lake metabolism. We studied four small unproductive lakes in subarctic northern Sweden during one summer season. The production of new biomass in both benthic and pelagic habitats was calculated as the sum of autotrophic production by algae and heterotrophic production by bacteria using allochthonous organic carbon (OC). Whole-lake production of new biomass was dominated by the benthic habitat (86% +/- 4% [mean +/- SD]) and by primary production (77% +/- 9%). Still, heterotrophic bacteria fueled by allochthonous OC constituted a significant portion of the new biomass production in both benthic (19% +/- 11%) and pelagic habitats (51% +/- 24%). In addition, overall net production (primary production minus respiration) was close to zero in the benthic habitats but highly negative (-163 +/- 81 mg C.m(-2).d(-1)) in pelagic regions of all lakes. We conclude (1) that allochthonous OC supported a significant part of total production of new biomass in both pelagic and benthic habitats, (2) that benthic habitats dominated the whole-lake production of new biomass, and (3) that respiration and net CO2 production dominated the carbon flux of the pelagic habitats and biomass production dominated the benthic carbon flux. Taken together, these findings suggest that previous investigations have greatly underestimated the productivity of clear-water lakes when benthic autotrophic production and metabolism of allochthonous OC have not been measured.

Place, publisher, year, edition, pages
Washington, DC, USA: Ecological Society of America, 2009
Identifiers
urn:nbn:se:umu:diva-25554 (URN)10.1890/07-1855.1 (DOI)
Available from: 2009-08-19 Created: 2009-08-19 Last updated: 2018-06-08Bibliographically approved
2. Winter respiration of allochthonous and autochthonous organic carbon in a subarctic clear-water lake
Open this publication in new window or tab >>Winter respiration of allochthonous and autochthonous organic carbon in a subarctic clear-water lake
2008 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 53, no 3, p. 948-954Article in journal (Refereed) Published
Abstract [en]

We studied a small subarctic lake to assess the magnitude of winter respiration and the organic carbon (OC) source for this respiration. The concentration and stable isotopic composition (d13C) of dissolved inorganic carbon (DIC) accumulating in the lake water under ice was analyzed over one winter (7 months). The DIC concentration increased and the d13C of DIC decreased over time, with the greatest changes at the lake bottom. Winter respiration was 26% of annual respiration in the lake. Keeling plot analysis demonstrated that the d13C of respired DIC varied spatially, high d13C values occurring at shallow (2.5 m, 21.7‰) compared with intermediate (4 m, 25.1‰) and deep (6 m, 27.8‰) locations in the lake. The variation in the d13C of respired DIC was related to the variation in the d13C of the sediments between locations, suggesting that sediment OC supported much of the winter respiration and that the dominant OC source for respiration was OC from benthic algae at shallow locations and settled OC, of predominately terrestrial origin, at deep locations. The respiration of OC from benthic algae constituted 55% of the winter respiration, equaling 54% of the primary production by benthic algae the previous summer. The study indicates the importance of temporal and spatial variation in respiration for the metabolism and net DIC production in unproductive high-latitude lakes; both allochthonous and autochthonous carbon can contribute to winter DIC accumulation and, consequently, to spring CO2 emissions from lakes.

Place, publisher, year, edition, pages
American Society of Limnology and Oceanography, 2008
Identifiers
urn:nbn:se:umu:diva-11454 (URN)
Available from: 2009-01-08 Created: 2009-01-08 Last updated: 2018-06-09Bibliographically approved
3. Terrestrial organic matter and light penetration: Effects on bacterial and primary production in lakes
Open this publication in new window or tab >>Terrestrial organic matter and light penetration: Effects on bacterial and primary production in lakes
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2009 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 54, no 6, p. 2034-2040Article in journal (Refereed) Published
Abstract [en]

We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear-water to brown-water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear-water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole-lake (benthic + pelagic) energy mobilization was negatively correlated to the light-extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light-absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.

Place, publisher, year, edition, pages
American Society of Limnology and Oceanography, Inc., 2009
Identifiers
urn:nbn:se:umu:diva-26800 (URN)
Available from: 2009-10-27 Created: 2009-10-27 Last updated: 2018-06-08Bibliographically approved
4. Net ecosystem production in clear-water and brown-water lakes
Open this publication in new window or tab >>Net ecosystem production in clear-water and brown-water lakes
2012 (English)In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 26, p. GB1017-Article in journal (Refereed) Published
Abstract [en]

We studied 15 lakes in northern Sweden with respect to primary production and respiration in benthic and pelagic habitats. The lakes were characterized by different concentrations of colored dissolved organic carbon (DOC) of terrestrial origin, forming a gradient ranging from clear-water to brown-water lakes. Primary production decreased and respiration increased on a whole-lake scale along the gradient of increasing DOC. Thus, the lakes became more net heterotrophic, i.e., had lower net ecosystem production (NEP = gross primary production - community respiration), with increasing terrestrial DOC and this change coincided with increasing partial pressure of carbon dioxide (pCO(2)) in the surface waters. The single most important process for the increasing net heterotrophy along the DOC gradient was pelagic respiration of terrestrial organic carbon. In spite of high metabolic activity in the benthic habitat, benthic primary production and benthic respiration decreased simultaneously with increasing DOC, showing that the benthic habitat was in metabolic balance throughout the gradient. Therefore, the net heterotrophic states of the lakes depended on the terrestrial DOC export to lakes and the concomitant respiration of terrestrial organic carbon in the pelagic habitat.

National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:umu:diva-53875 (URN)10.1029/2010GB003951 (DOI)000301129200001 ()
Available from: 2012-04-05 Created: 2012-04-04 Last updated: 2018-06-08Bibliographically approved

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