| 1. |
- Dahl, Martin, 1984-, et al.
(författare)
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Impacts of land-use change and urban development on carbon sequestration in tropical seagrass meadow sediments
- 2022
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Ingår i: Marine Environmental Research. - : Elsevier BV. - 0141-1136 .- 1879-0291. ; 176
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Tidskriftsartikel (refereegranskat)abstract
- Seagrass meadows store significant carbon stocks at a global scale, but land-use change and other anthropogenic activities can alter the natural process of organic carbon (Corg) accumulation. Here, we assessed the carbon accumulation history of two seagrass meadows in Zanzibar (Tanzania) that have experienced different degrees of disturbance. The meadow at Stone Town has been highly exposed to urban development during the 20th century, while the Mbweni meadow is located in an area with relatively low impacts but historical clearing of adjacent mangroves. The results showed that the two sites had similar sedimentary Corg accumulation rates (22–25 g m−2 yr−1) since the 1940s, while during the last two decades (∼1998 until 2018) they exhibited 24–30% higher accumulation of Corg, which was linked to shifts in Corg sources. The increase in the δ13C isotopic signature of sedimentary Corg (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and Corg accumulation capacity of the meadow after the relocation of a major sewage outlet in the mid–1990s. In contrast, the decrease in the δ13C signatures of sedimentary Corg in the Mbweni meadow since the early 2010s was likely linked to increased Corg run-off of mangrove/terrestrial material following mangrove deforestation. This study exemplifies two different pathways by which land-based human activities can alter the carbon storage capacity of seagrass meadows (i.e. sewage waste management and mangrove deforestation) and showcases opportunities for management of vegetated coastal Corg sinks.
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| 2. |
- Ismail, Rashid O., 1986-
(författare)
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Drivers of carbon sink function in tropical seagrass beds : influence of carbon import, plant composition, seascape configuration and human activities
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Seagrass meadows are effective carbon sinks, sequestering atmospheric CO2 and capturing allochthonous organic material, storing organic carbon (Corg) in their sediments, so called Blue Carbon. In tropical areas, seagrass meadows have a high number of calcareous organisms, which can offset carbon sequestration by releasing CO2 through their calcification. Human activities such as urbanization and land-use change with inadequate management of blue carbon ecosystems are causing fast degradation of tropical blue carbon ecosystems, particularly mangroves and seagrasses. In this thesis, I and colleagues looked at the carbon sequestration process and the impact of marine protected areas (MPAs) on Corg conservation in the blue carbon ecosystems of the western Indian Ocean (WIO) region. This was accomplished by examining the air-water CO2 flux in different plant community compositions (i.e. seagrass and calcifying macroalgae), as well as factors driving air-water CO2 flux and the assessment of Corg stocks within and outside MPAs in tropical and subtropical areas of the WIO. The impact of landscape configuration and modification due to urbanization and mangrove degradation on the accumulation and variability of Corg in seagrass habitats was also investigated. We found that, the sum of the fluxes showed a net efflux of CO2 over the meadows. The CO2 fluxes changed both in rate and direction over the day, and were significantly related to plant community composition and environmental conditions such as pH and CO2 partial pressure, where pH had the strongest influence on CO2 fluxes. Influxes were found only over vegetation with high proportion of seagrass and in the afternoon, whereas calcifying algae appeared to reverse the flow. We found that highly productive seagrass meadows can generate a net CO2 from the water to the atmosphere as plants’ demand for CO2 to a large extent is covered by internal cycling of CO2, both from degradation of autochthonous and allochthonous material and calcification. We found that accumulation of Corg in seagrass meadows is larger than the flow to the atmosphere, indicating that these systems can still be carbon sinks.The inflow of allochthonous carbon, Corg content and stocks in the seagrass meadows was influenced by a combination of landscape metrics and inherent habitat plant- and sediment-properties. We discovered a strong land to sea gradient in terms of Corg content in seagrass seascapes, due to hydrodynamic forces that resulted into unique patterns in sedimentary Corg levels. Seagrass surface sediments closer to a deforested mangrove had higher Corg content and mangrove signal, probably due to increased Corg export from deforested mangrove. In comparison to more diversified and patchy seascapes, seascapes with extensive continuous seagrass meadows have higher sedimentary Corg content. Seagrass meadows located near an area with rapid and short-term urbanization and degraded mangroves had a higher sedimentary Corg content, but similar carbon accumulation rate as an area with long-term progressive urbanization. It was found that tropical and subtropical blue carbon ecosystems store a significant amount of carbon in their sediments, but that many carbon storage hotspots are entirely/partially outside MPAs. This masks their influence on blue carbon conservation. MPAs can still be used to conserve blue carbon if carbon hotspots are properly located and managed.This thesis contributes knowledge of important determining factors influencing primary pathways of tropical coastal ecosystem carbon sequestration and are critical for identifying hotspots of carbon storage to generate conservation prioritizations. Future research should focus on conservation prioritizations that will limit the unsustainable use of coastal resources.
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| 3. |
- Ismail, Rashid O., 1986-, et al.
(författare)
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Effects of calcification on air-water CO2 fluxes in tropical seagrass meadows : A mesocosm experiment
- 2023
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Ingår i: Journal of Experimental Marine Biology and Ecology. - : Elsevier BV. - 0022-0981 .- 1879-1697. ; 561
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Tidskriftsartikel (refereegranskat)abstract
- Seagrass meadows deliver a range of ecosystem services, where one of the more important is the capacity to store carbon and serve as sinks for atmospheric carbon dioxide. The capacity of seagrass meadows for carbon storage might, however, be modified and complicated by several factors; one important factor is the possible effects of calcification within the meadows. In tropical areas, seagrass meadows can contain high proportions of calcareous organisms, which through their calcification may cause release of CO2. To study this aspect of the CO2 balance within tropical seagrass systems, we investigated the air-water CO2 flux in seagrass mesocosms with different plant community compositions, i.e. mixtures of seagrass and calcifying macroalgae, having similar overall photosynthetic oxygen evolution rates. The measured CO2 fluxes changed both in rate and direction over the day and were significantly related to plant community composition. Downward fluxes of CO2 were found only over vegetation with high proportion of seagrass and in the afternoon, whereas occurrence of calcifying algae appeared to reverse the flow. A partial least squares (PLS) regression model indicated that pH, pCO2 and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO2 fluxes. Our findings show that algal calcification might partly counteract the carbon sequestration in seagrass meadows.
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| 4. |
- Ismail, Rashid O., 1986-, et al.
(författare)
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Plant productivity, community composition and carbon import are key drivers of air-water CO2 fluxes in a tropical seagrass meadow : implications for blue carbon science
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Seagrass meadows are considered efficient sinks of Blue Carbon. They capture CO2 by an effective photosynthetic uptake as well as by trapping large amounts of carbon originating from adjacent systems, which in turn can be stored in the sediments. Such import of allochthonous carbon will be partly degraded in the system, increasing the overall community respiration and thus CO2 production and at the same time add to sediment carbon accumulation. Additionally, tropical seagrass meadows can contain a high proportion of calcareous organisms, which (by the pH suppressing the effect of calcification) can further increase the CO2 partial pressure of the seawater if resulting CO2 is not internally used. The scarce literature on actual CO2 fluxes over submerged vegetation in coastal marine areas is reporting partly contrasting data over how coastal areas in general shall be counted in carbon budgets. To better understand the CO2 cycle within a tropical seagrass system, we constructed a simple carbon flux simulation model in which we evaluated the possible fluxes of carbon within the meadow and with regards to the surrounding seascape. We measured air-water CO2 fluxes in seagrass meadows with different plant community compositions (i.e. mixtures of seagrass and calcifying macroalgae) using field measurements, estimated water column productivity, and extracted data for primary productivity, plant composition, and calcification from previous studies in the same area and, traced organic carbon (Corg) sources in seagrass sediment by measuring bulk stable isotope signals of carbon (δ13C) in order to feed the model with the best available data. When needed we supplemented with published data from other regions. The measured fluxes indicated a net efflux of CO2 over the meadows, from sea to air. The fluxes changed both in rate and direction over the day, and were significantly related to plant community composition and environmental conditions, where pH had the strongest influence on CO2 fluxes. Downward fluxes were found only over vegetation in the afternoon. The isotope signals of carbon revealed a strong input of carbon from other habitats. The outcome of the simulation model suggests that highly productive seagrass meadows can generate a net CO2 flux from the water column to the atmosphere since the plants’ demand for CO2 to a large extent is covered by a major internal cycling of CO2, which is from degradation of autochthonous and allochthonous material as well as from CO2 released from calcification. The calculated accumulation3of sedimentary carbon is however larger than the flow to the atmosphere, indicating that these systems can still be carbon sinks.
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| 5. |
- Kalokora, Olivia J, et al.
(författare)
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Seagrass meadows mixed with calcareous algae have higher plant productivity and sedimentary blue carbon storage
- 2022
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Ingår i: Ecology and Evolution. - : John Wiley & Sons. - 2045-7758. ; 12:2
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Tidskriftsartikel (refereegranskat)abstract
- Seagrass meadows capture and store large amounts of carbon in the sediment beneath, thereby serving as efficient sinks of atmospheric CO2. Carbon sequestration levels may however differ greatly among meadows depending on, among other factors, the plant community composition. Tropical seagrass meadows are often intermixed with macroalgae, many of which are calcareous, which may compete with seagrass for nutrients, light, and space. While the photosynthetic CO2 uptake by both seagrasses and calcareous algae may increase the overall calcification in the system (by increasing the calcium carbonate saturation state, Ω), the calcification process of calcareous algae may lead to a release of CO2, thereby affecting both productivity and calcification, and eventually also the meadows' carbon storage. This study estimated how plant productivity, CaCO3 production, and sediment carbon levels were affected by plant community composition (seagrass and calcareous algae) in a tropical seagrass-dominated embayment (Zanzibar, Tanzania). Overall, the patterns of variability in productivity differed between the plant types, with net areal biomass productivity being highest in meadows containing both seagrass and calcareous algae. Low and moderate densities of calcareous algae enhanced seagrass biomass growth, while the presence of seagrass reduced the productivity of calcareous algae but increased their CaCO3 content. Sedimentary carbon levels were highest when seagrasses were mixed with low or moderate cover of calcareous algae. The findings show that plant community composition can be an important driver for ecosystem productivity and blue carbon sequestration.
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| 6. |
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| 7. |
- Dahl, Martin, et al.
(författare)
-
Impacts of land-use change and urban development on tropical seagrass carbon sinks
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Seagrass meadows store significant carbon stocks at a global scale, but land-use change and anthropogenic activities can alter the natural process of organic carbon (Corg) accumulation. Here, we assessed the carbon accumulation history of two seagrass meadows in Zanzibar (Tanzania) that experienced different degrees of disturbance. The meadow at Stone Town has been highly exposed to urban development during the 20th century, while the Mbweni meadow is located in an area with relatively low impacts but historical clearing of adjacent mangroves. The results showed that the two sites had similar sedimentary Corg accumulation rates (22–25 g m-2 yr-1) since the 1940s, while during the last two decades (~1998 until 2018) they exhibited 24–30% higher accumulation of Corg, which was linked to shifts in Corg sources. The increase in the δ13C isotopic signature of sedimentary Corg (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and Corg accumulation capacity of the meadow after the relocation of a major sewage outlet in the mid–1990s. In contrast, the decrease in the δ13C signatures of sediment Corg in the Mbweni meadow since the early 2010s was likely linked to Corg transport from mangrove/terrestrial material run-off following the mangrove deforestation. This study exemplifies two different pathways by which land-based human activities can alter the carbon storage capacity of seagrass meadows (i.e. sewage waste management and mangrove deforestation) and showcases opportunities for management of vegetated coastal Corg sinks
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| 8. |
- Deyanova, Diana, et al.
(författare)
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Contribution of seagrass plants to CO2 capture in a tropical seagrass meadow under experimental disturbance
- 2017
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 12:7
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Tidskriftsartikel (refereegranskat)abstract
- Coastal vegetative habitats are known to be highly productive environments with a high ability to capture and store carbon. During disturbance this important function could be compromised as plant photosynthetic capacity, biomass, and/or growth are reduced. To evaluate effects of disturbance on CO2 capture in plants we performed a five-month manipulative experiment in a tropical seagrass (Thalassia hemprichii) meadow exposed to two intensity levels of shading and simulated grazing. We assessed CO2 capture potential (as net CO2 fixation) using areal productivity calculated from continuous measurements of diel photosynthetic rates, and estimates of plant morphology, biomass and productivity/respiration (P/R) ratios (from the literature). To better understand the plant capacity to coping with level of disturbance we also measured plant growth and resource allocation. We observed substantial reductions in seagrass areal productivity, biomass, and leaf area that together resulted in a negative daily carbon balance in the two shading treatments as well as in the high-intensity simulated grazing treatment. Additionally, based on the concentrations of soluble carbohydrates and starch in the rhizomes, we found that the main reserve sources for plant growth were reduced in all treatments except for the low-intensity simulated grazing treatment. If permanent, these combined adverse effects will reduce the plants' resilience and capacity to recover after disturbance. This might in turn have long-lasting and devastating effects on important ecosystem functions, including the carbon sequestration capacity of the seagrass system.
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| 9. |
- George, Rushingisha, et al.
(författare)
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High midday temperature stress has stronger effects on biomass than on photosynthesis : A mesocosm experiment on four tropical seagrass species
- 2018
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Ingår i: Ecology and Evolution. - : Wiley. - 2045-7758. ; 8:9, s. 4508-4517
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Tidskriftsartikel (refereegranskat)abstract
- The effect of repeated midday temperature stress on the photosynthetic performance and biomass production of seagrass was studied in a mesocosm setup with four common tropical species, including Thalassia hemprichii, Cymodocea serrulata, Enhalus acoroides, and Thalassodendron ciliatum. To mimic natural conditions during low tides, the plants were exposed to temperature spikes of different maximal temperatures, that is, ambient (29-33 degrees C), 34, 36, 40, and 45 degrees C, during three midday hours for seven consecutive days. At temperatures of up to 36 degrees C, all species could maintain full photosynthetic rates (measured as the electron transport rate, ETR) throughout the experiment without displaying any obvious photosynthetic stress responses (measured as declining maximal quantum yield, Fv/Fm). All species except T.ciliatum could also withstand 40 degrees C, and only at 45 degrees C did all species display significantly lower photosynthetic rates and declining Fv/Fm. Biomass estimation, however, revealed a different pattern, where significant losses of both above- and belowground seagrass biomass occurred in all species at both 40 and 45 degrees C (except for C.serrulata in the 40 degrees C treatment). Biomass losses were clearly higher in the shoots than in the belowground root-rhizome complex. The findings indicate that, although tropical seagrasses presently can cope with high midday temperature stress, a few degrees increase in maximum daily temperature could cause significant losses in seagrass biomass and productivity.
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| 10. |
- Gullström, Martin, et al.
(författare)
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Assessment of vegetation changes in seagrass communities of tropical Chwaka Bay (Zanzibar) using satellite remote sensing
- 2006
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Ingår i: Estuarine, Coastal and Shelf Science. - : Elsevier BV. - 0272-7714 .- 1096-0015. ; 67:3, s. 399-408
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Tidskriftsartikel (refereegranskat)abstract
- Spatial and temporal dynamics of submerged aquatic vegetation (SAV) cover were studied in the relatively pristine and seagrass-dominated area of Chwaka Bay, Zanzibar (Tanzania) by using satellite remote sensing. Through complementary field work the potential of the technique for change detection was verified. The general changes in SAV cover were examined using Landsat images from 1986, 1987, 1998, 2001 and 2003. Two of these images, from 1987 (Landsat TM) and 2003 (Landsat ETM+), were specifically analysed to create a map of the change in SAV cover. Overall, the general distribution of SAV stayed fairly stable over the period investigated, but the result also showed regions where significant alterations, both losses and gains, had occurred between the two years. Based on our findings and anecdotal information from local fishermen and seaweed farmers potential causative factors are discussed. It was concluded that a repeated mapping with satellite remote sensing is a suitable tool to monitor changes of seagrass and seaweed distribution in shallow tropical environments.
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