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Flocculation of Allochthonous Dissolved Organic Matter – a Significant Pathway of Sedimentation and Carbon Burial in Lakes
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Evolution, Limnology.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Inland waters receive substantial amounts of organic carbon from adjacent watersheds. Only about half of the carbon exported from inland waters reaches the oceans, while the remainder is lost en route. This thesis identifies flocculation as an important and significant fate of carbon in the boreal landscape. Flocculation reallocates organic carbon from the dissolved state into particles which are prone to settle. Thus, flocculation relocates organic carbon from the water column to the sediment.

The dissolved organic carbon (DOC), mainly originating from terrestrial sources, in a set of Swedish lakes was found to determine the extent of sedimentation of particulate organic carbon. A major fraction of the settling particles were of allochthonous origin. This implies that allochthonous DOC was the precursor of the settling matter in these lakes. The gross sedimentation was of the same magnitude as the evasion of carbon dioxide to the atmosphere.

Sunlight, especially in the photosynthetically active region, stimulated flocculation of DOC. The effect of light appeared to involve a direct photochemical reaction. Iron was involved in the flocculation but it could not be unravelled whether the iron catalyzes the flocculation or just co-precipitates with the settling matter. Microbial activity was identified as the main regulator of the flocculation rates. Accordingly, alteration of temperature, oxygen concentration and pH did not affect flocculation only indirectly, via their effects on microbial metabolism.

A comparison of fluorescence characteristics of organic matter collected in sediment trap and in the sediment surface layer revealed that autochthonous organic carbon was preferentially lost in the sediments while allochthonous matter increased. The recalcitrant nature of the flocculated matter could favour sequestration of this matter in the lake sediment. Hence, the lakes will act as sinks of organic carbon due to a slower mineralization of the flocculated matter in the sediments.

Place, publisher, year, edition, pages
Uppsala: Universitetsbiblioteket , 2008. , p. 42
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 586
Keywords [en]
flocculation, dissolved organic carbon, allochthonous, carbon sequestration, carbon cycle, boreal lakes
National Category
Ecology
Identifiers
URN: urn:nbn:se:uu:diva-9500ISBN: 978-91-554-7374-7 (print)OAI: oai:DiVA.org:uu-9500DiVA, id: diva2:173034
Public defence
2009-01-21, Ekmansalen, Kärnhuset, EBC, Norbyvägen 18, Uppsala, 10:00
Opponent
Supervisors
Available from: 2008-12-17 Created: 2008-12-17 Last updated: 2016-04-28Bibliographically approved
List of papers
1. Sedimentation in Boreal Lakes: the role of flocculation of allochthonous dissolved organic matter in the water column
Open this publication in new window or tab >>Sedimentation in Boreal Lakes: the role of flocculation of allochthonous dissolved organic matter in the water column
2008 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 11, no 5, p. 803-814Article in journal (Refereed) Published
Abstract [en]

We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (< , 0.48 km(2)), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l(-1). Stable isotope analysis suggests that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m(2) lake area was correlated to DOC concentration in the water (R-2 = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R-2 = 0.58). The net efflux of C as CO2 from the water to the atmosphere was likewise correlated to DOC concentration (R-2 = 0.52). The losses of organic carbon from the water column via mineralization to CO2 and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.

Keywords
sedimentation, carbon cycle, DOC, flocculation, allochthonous, boreal lakes
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-97913 (URN)10.1007/s10021-008-9162-z (DOI)000258958700012 ()
Available from: 2008-12-17 Created: 2008-12-17 Last updated: 2017-12-14Bibliographically approved
2. Linking allochthonous dissolved organic matter and boreal lake sediment carbon sequestration: The role of light-mediated flocculation
Open this publication in new window or tab >>Linking allochthonous dissolved organic matter and boreal lake sediment carbon sequestration: The role of light-mediated flocculation
2008 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 53, no 6, p. 2416-2426Article in journal (Refereed) Published
Abstract [en]

We measured flocculation of dissolved organic carbon (DOC) in the water from a humic lake (DOC = 14.9 mg C L-1) and from an adjacent mire (DOC = 25.7 mg C L-1), in in situ enclosure experiments with different light regimes. Light stimulated the formation of organic particles in both waters, and organic particle formation was observed at all incubation depths, even in the dark controls. Production of phytoplankton biomass was negligible, and allochthonous DOC was the most important precursor of the sinking particles. 8-22% and 25-60% of the loss of DOC in lake and mire water, respectively, could be accounted for by flocculation. Depth-integrated flocculation based on the enclosure experiments was 14.7 mg C m(-2) d(-1). Lake-water DOC concentration and water color has been increasing during the last decade, and sediment trap studies show that gross sedimentation of organic carbon also increased. Thus flocculation of allochthonous DOC, stimulated by light, constitutes a pathway for the sequestration of carbon in lake sediments.

National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-97914 (URN)000261355500006 ()
Available from: 2008-12-17 Created: 2008-12-17 Last updated: 2018-05-21
3. Microbially induced flocculation of allochthonous dissolved organic carbon in lakes
Open this publication in new window or tab >>Microbially induced flocculation of allochthonous dissolved organic carbon in lakes
2009 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 54, no 5, p. 1811-1818Article in journal (Refereed) Published
Abstract [en]

The flocculation of allochthonous dissolved organic carbon (DOC) in lakes was investigated in a series of laboratory experiments. Flocculation was enhanced by increased temperatures, and addition of glucose increased the flocculation further. A change in pH within the range of 3.3 to 7.3 resulted in decreased flocculation with increasing deviation from the original pH. Flocculation was similar under oxic and anoxic conditions. For all treatments (i.e., temperature and glucose, pH, and O2 regime), occulation was positively correlated to bacterial respiration. However, bacterial biomass made up a negligible fraction of the formed flocs, suggesting that the formation of detrital particles was a result of bacterial activity, but there was no significant contribution of bacteria to the mass of the particles formed. In all experiments, both the concentration of DOC and concentration of colored dissolved organic matter (CDOM) decreased concomitantly with flocculation, suggesting that CDOM is the precursor of the flocs. Bacteria mediate a translocation of DOC in the water column into particles prone to gravitational settling.

National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-97915 (URN)000268325300036 ()
Available from: 2008-12-17 Created: 2008-12-17 Last updated: 2017-12-14Bibliographically approved
4. Preferential sequestration of allochthonous organic matter in boreal lake sediments
Open this publication in new window or tab >>Preferential sequestration of allochthonous organic matter in boreal lake sediments
Manuscript (Other academic)
Identifiers
urn:nbn:se:uu:diva-97916 (URN)
Available from: 2008-12-17 Created: 2008-12-17 Last updated: 2017-11-30Bibliographically approved

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