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Natural and human-induced carbon storage variability in seagrass meadows
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.ORCID iD: 0000-0003-2016-4857
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrasses are considered highly important CO2 sinks, with the capacity to store substantial quantities of organic carbon in the living biomass and sediments, and thereby acting as a buffer against climate change. In this thesis, I have studied carbon storage variability in temperate and tropical seagrass habitats and identified factors influencing this variation. In addition, as seagrass areas are decreasing worldwide, I have assessed effects of different anthropogenic disturbances on carbon sequestration processes. The result from this thesis showed that there was a large variation in carbon storage within and among temperate, tropical and subtropical regions. The highest organic carbon stocks were found in temperate Zostera marina meadows, which also showed a larger carbon storage variability than the subtropical and tropical seagrass habitats. The tropical and subtropical seagrass meadows had inorganic carbon pools exceeding the organic carbon accumulation, which could potentially weakens the carbon sink function. The variability in organic carbon stocks was generally strongly related to the sediment characteristics of the seagrass habitats. In Z. marina meadows, the strength of the carbon sink function was mainly driven by the settings of the local environment, which in turn indicates that depositional areas will likely have higher organic carbon stocks than more exposed meadows, while in the tropics seagrass biomass was also influencing sedimentary carbon levels. Furthermore, locations with large areas of seagrass were associated with higher carbon storage in tropical and subtropical regions, which could be related to increased accumulation of both autochthonous and allochthonous carbon. In an in situ experiment, impacts on carbon sequestration processes from two types of disturbances (with two levels of intensity) were tested by simulating reduced water quality (by shading) and high grazing pressure (through removal of shoot biomass). At high disturbance intensity, reductions in the net community production and seagrass biomass carbon were observed, which negatively affected carbon sequestration and could impact the sedimentary organic carbon stocks over time. In the treatments with simulated grazing, erosion was also seen, likely due to an increase in near-bed hydrodynamics. When experimentally testing effects of increased current flow on organic carbon suspension in Z. marina sediment, a ten-fold release of organic carbon with higher current flow velocities was measured, which resulted in an increase in the proportion of suspended organic carbon by three times in relation to other sediment particles. Therefore, periods with enhanced hydrodynamic activity could result in a removal of organic carbon and thereby likely reduce the seagrass meadows’ capacity to store carbon. The findings of this thesis add to the emerging picture that there is a large natural variability in seagrasses’ capacity to store carbon, and highlight how human-induced disturbances could negatively affect the carbon sink function in seagrass meadows.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2017.
Keywords [en]
Blue carbon sequestration, Carbon storage variability, Seagrass meadows, Anthropogenic disturbances, Climate change
National Category
Ecology
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-148400ISBN: 978-91-7797-012-5 (print)ISBN: 978-91-7797-013-2 (electronic)OAI: oai:DiVA.org:su-148400DiVA, id: diva2:1152229
Public defence
2017-12-15, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2017-11-22 Created: 2017-10-24 Last updated: 2017-11-15Bibliographically approved
List of papers
1. Sediment Properties as Important Predictors of Carbon Storage in Zostera marina Meadows: A Comparison of Four European Areas
Open this publication in new window or tab >>Sediment Properties as Important Predictors of Carbon Storage in Zostera marina Meadows: A Comparison of Four European Areas
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2016 (English)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 11, no 12Article in journal (Refereed) Published
Abstract [en]

Seagrass ecosystems are important natural carbon sinks but their efficiency varies greatly depending on species composition and environmental conditions. What causes this variation is not fully known and could have important implications for management and protection of the seagrass habitat to continue to act as a natural carbon sink. Here, we assessed sedimentary organic carbon in Zostera marina meadows (and adjacent unvegetated sediment) in four distinct areas of Europe (Gullmar Fjord on the Swedish Skagerrak coast, Asko in the Baltic Sea, Sozopol in the Black Sea and Ria Formosa in southern Portugal) down to similar to 35 cm depth. We also tested how sedimentary organic carbon in Z. marina meadows relates to different sediment characteristics, a range of seagrass-associated variables and water depth. The seagrass carbon storage varied greatly among areas, with an average organic carbon content ranging from 2.79 +/- 0.50% in the Gullmar Fjord to 0.17 +/- 0.02% in the area of Sozopol. We found that a high proportion of fine grain size, high porosity and low density of the sediment is strongly related to high carbon content in Z. marina sediment. We suggest that sediment properties should be included as an important factor when evaluating high priority areas in management of Z. marina generated carbon sinks.

National Category
Biological Sciences
Research subject
Marine Ecology; Plant Physiology
Identifiers
urn:nbn:se:su:diva-137567 (URN)10.1371/journal.pone.0167493 (DOI)000389587100110 ()
Available from: 2017-01-11 Created: 2017-01-09 Last updated: 2018-04-27Bibliographically approved
2. Blue Carbon Storage in Tropical Seagrass Meadows Relates to Carbonate Stock Dynamics, Plant–Sediment Processes, and Landscape Context: Insights from the Western Indian Ocean
Open this publication in new window or tab >>Blue Carbon Storage in Tropical Seagrass Meadows Relates to Carbonate Stock Dynamics, Plant–Sediment Processes, and Landscape Context: Insights from the Western Indian Ocean
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2018 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 21, no 3, p. 551-566Article in journal (Refereed) Published
Abstract [en]

Globally, seagrass ecosystems are considered major blue carbon sinks and thus indirect contributors to climate change mitigation. Quantitative estimates and multi-scale appraisals of sources that underlie long-term storage of sedimentary carbon are vital for understanding coastal carbon dynamics. Across a tropical–subtropical coastal continuum in the Western Indian Ocean, we estimated organic (Corg) and inorganic (Ccarb) carbon stocks in seagrass sediment. Quantified levels and variability of the two carbon stocks were evaluated with regard to the relative importance of environmental attributes in terms of plant–sediment properties and landscape configuration. The explored seagrass habitats encompassed low to moderate levels of sedimentary Corg (ranging from 0.20 to 1.44% on average depending on species- and site-specific variability) but higher than unvegetated areas (ranging from 0.09 to 0.33% depending on site-specific variability), suggesting that some of the seagrass areas (at tropical Zanzibar in particular) are potentially important as carbon sinks. The amount of sedimentary inorganic carbon as carbonate (Ccarb) clearly corresponded to Corg levels, and as carbonates may represent a carbon source, this could diminish the strength of seagrass sediments as carbon sinks in the region. Partial least squares modelling indicated that variations in sedimentary Corg and Ccarb stocks in seagrass habitats were primarily predicted by sediment density (indicating a negative relationship with the content of carbon stocks) and landscape configuration (indicating a positive effect of seagrass meadow area, relative to the area of other major coastal habitats, on carbon stocks), while seagrass structural complexity also contributed, though to a lesser extent, to model performance. The findings suggest that accurate carbon sink assessments require an understanding of plant–sediment processes as well as better knowledge of how sedimentary carbon dynamics are driven by cross-habitat links and sink–source relationships in a scale-dependent landscape context, which should be a priority for carbon sink conservation.

Keywords
Blue carbon, seagrass meadows, marine sediment, coastal carbon cycle, organic carbon, carbonate, source–sink relationships, landscape configuration, Western Indian Ocean
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-148276 (URN)10.1007/s10021-017-0170-8 (DOI)000428556200012 ()
Available from: 2017-10-20 Created: 2017-10-20 Last updated: 2018-05-02Bibliographically 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, p. 654-664Article 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.

Keywords
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: 2018-04-30Bibliographically approved
4. Increased current flow enhances the risk of organic carbon loss from Zostera marina sediments: Insights from a flume experiment
Open this publication in new window or tab >>Increased current flow enhances the risk of organic carbon loss from Zostera marina sediments: Insights from a flume experiment
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-148277 (URN)
Available from: 2017-10-20 Created: 2017-10-20 Last updated: 2017-10-24Bibliographically approved

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