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Carbon sequestration processes in tropical seagrass beds
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. University of Dodoma.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrass meadows may play a substantial role in climate change mitigation as they are capable to sequester and store substantial amounts of anthropogenic carbon in plant biomass and, more importantly, in their underlying sediments. In this PhD thesis, the carbon-burial potential was assessed by quantifying the amount of organic carbon stored in different seagrass meadows, each dominated by one of the four major seagrass species in the Western Indian Ocean region. Impacts of anthropogenic disturbances on biomass carbon allocation, greenhouse gas emission (methane and nitrous oxide) and production of sulphide were investigated in Chwaka Bay, Zanzibar. The findings showed that east African seagrass meadows generally have high carbon sink capacity. The storage of sedimentary organic carbon, however, varied among seagrass habitats and across sites, and was up to five-fold higher in seagrass sediment to those of nearby unvegetated sediments. Seagrass meadows in eutrophicated sites had higher sedimentary organic carbon content, and substantially higher emission rates of nitrous oxides and methane, compared to more pristine meadows. Disturbances in terms of shading and simulated grazing of seagrass affected several processes, with major decreases in seagrass primary productivity, net community production and biomass carbon, in turn influencing seagrass carbon sequestration as well as stimulating anaerobic microbial processes. In addition, production of sulphide in the sediment and methane emissions from the sediment surface increased significantly when disturbed. At present, seagrass meadows in the Western Indian Ocean have high carbon sink capacity. This important ecosystem service is, however, highly threatened due to regional anthropogenic pressure, which may change the role of blue carbon rich habitats, such as seagrass meadows, from being a sink to a source of greenhouse gases.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2016. , 49 p.
Keyword [en]
Seagrass, Carbon sequestration, Carbon sink, Eutrophication, Productivity, Nitrous oxide, Methane, Greenhouse gases, Tropical, Eastern Africa
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-128201ISBN: 978-91-7649-369-4 (print)OAI: oai:DiVA.org:su-128201DiVA: diva2:913477
Public defence
2016-05-04, Vivi Täckholm-salen, NPQ-huset, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 2: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2016-04-11 Created: 2016-03-21 Last updated: 2017-02-20Bibliographically approved
List of papers
1. Carbon sequestration capacity in tropical and subtropical seagrass meadows of the Western Indian ocean
Open this publication in new window or tab >>Carbon sequestration capacity in tropical and subtropical seagrass meadows of the Western Indian ocean
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-128145 (URN)
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2016-03-21Bibliographically approved
2. Emission of nitrous oxide and methane from tropical seagrass meadows: effects of gas transport through seagrass plant and eutrophication
Open this publication in new window or tab >>Emission of nitrous oxide and methane from tropical seagrass meadows: effects of gas transport through seagrass plant and eutrophication
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-128148 (URN)
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2016-03-21Bibliographically approved
3. Effects of shading and simulated grazing on carbon sequestration in a tropical seagrass meadow
Open this publication in new window or tab >>Effects of shading and simulated grazing on carbon sequestration in a tropical seagrass meadow
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2016 (English)In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 104, no 3, 654-664 p.Article in journal (Refereed) Published
Abstract [en]

1. There is an ongoing world-wide decline of seagrass ecosystems, one of the world's most efficient carbon sink habitats. In spite of this, there is a clear lack of studies experimentally testing the effects of anthropogenic disturbances on carbon sequestration of seagrass systems. 2. We assessed the effects of two disturbances of global concern on the carbon sink function in a five-month in situ experiment within a tropical seagrass (Thalassia hemprichii) meadow by testing the impacts of shading and simulated grazing at two levels of intensity using shading cloths and clipping of shoot tissue. We measured the effects of these disturbances on the carbon sequestration process by assessing the net community production (NCP), carbon and nitrogen content in tissue biomass, and organic matter and THAA (total hydrolysable amino acids) in the sediment down to 40 cm depth. 3. Treatments of high-intensity shading and high-intensity clipping were similarly impacted and showed a significantly lower NCP and carbon content in the below-ground biomass compared to the seagrass control. No significant effects were seen in organic carbon, total nitrogen, C:N ratio and THAA in the sediment for the seagrass treatments. However, both clipping treatments showed different depth profiles of carbon and THAA compared to the seagrass control, with lower organic carbon and THAA content in the surface sediment. This can be explained by the clipping of shoot tissue causing a less efficient trapping of allochthonous carbon and reduced input of shredded seagrass leaves to the detritus sediment layer. In the clipping plots, erosion of the surface sediment occurred, which was also most likely caused by the removal of above-ground plant biomass. 4. Synthesis. Our findings show that during the course of this experiment, there were no impacts on the sedimentary carbon while the high-intensity disturbances caused a clear depletion of carbon biomass and reduced the seagrass meadow's capacity to sequester carbon. From a long-term perspective, the observed effect on the carbon biomass pool in the high-intensity treatments and the sediment erosion in the clipping plots may lead to loss in sedimentary carbon.

Keyword
aquatic plant ecology, blue carbon sequestration, changing climate, disturbances, ecosystem production, in situ experiment, marine vegetation, natural carbon sinks
National Category
Ecology
Research subject
Plant Physiology; Marine Ecology
Identifiers
urn:nbn:se:su:diva-128154 (URN)10.1111/1365-2745.12564 (DOI)000379014900005 ()
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2017-10-24Bibliographically approved
4. Productivity of a tropical seagrass meadow under stress: effects of prolonged shading and simulated grazing
Open this publication in new window or tab >>Productivity of a tropical seagrass meadow under stress: effects of prolonged shading and simulated grazing
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-128151 (URN)
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2016-03-21Bibliographically approved
5. Shading and simulated grazing of seagrass leaves increases sulphide production and methane emission in a tropical seagrass meadow
Open this publication in new window or tab >>Shading and simulated grazing of seagrass leaves increases sulphide production and methane emission in a tropical seagrass meadow
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-128152 (URN)
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2016-03-21Bibliographically approved

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