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.

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 (C_org) 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 C_org 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 C_org, which was linked to shifts in C_org sources. The increase in the δ¹³C isotopic signature of sedimentary C_org (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and C_org 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 C_org in the Mbweni meadow since the early 2010s was likely linked to increased C_org 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 C_org sinks.

Seagrass meadows are effective carbon sinks, sequestering atmospheric CO₂ and capturing allochthonous organic material, storing organic carbon (C_org) 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 C_org 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 C_org 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 C_org 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 C_org 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, C_org 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 C_org content in seagrass seascapes, due to hydrodynamic forces that resulted into unique patterns in sedimentary C_org levels. Seagrass surface sediments closer to a deforested mangrove had higher C_org content and mangrove signal, probably due to increased C_org export from deforested mangrove. In comparison to more diversified and patchy seascapes, seascapes with extensive continuous seagrass meadows have higher sedimentary C_org content. Seagrass meadows located near an area with rapid and short-term urbanization and degraded mangroves had a higher sedimentary C_org 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.

Context Seagrass meadows act as efficient natural carbon sinks by sequestering atmospheric CO2 and through trapping of allochthonous organic material, thereby preserving organic carbon (C-org) in their sediments. Less understood is the influence of landscape configuration and transformation (land-use change) on carbon sequestration dynamics in coastal seascapes across the land-sea interface. Objectives We explored the influence of landscape configuration and degradation of adjacent mangroves on the dynamics and fate of C-org in seagrass habitats. Methods Through predictive modelling, we assessed sedimentary C-org content, stocks and source composition in multiple seascapes (km-wide buffer zones) dominated by different seagrass communities in northwest Madagascar. The study area encompassed seagrass meadows adjacent to intact and deforested mangroves. Results The sedimentary C-org content was influenced by a combination of landscape metrics and inherent habitat plant- and sediment-properties. We found a strong land-to-sea gradient, likely driven by hydrodynamic forces, generating distinct patterns in sedimentary C-org levels in seagrass seascapes. There was higher C-org content and a mangrove signal in seagrass surface sediments closer to the deforested mangrove area, possibly due to an escalated export of C-org from deforested mangrove soils. Seascapes comprising large continuous seagrass meadows had higher sedimentary C-org levels in comparison to more diverse and patchy seascapes. Conclusion Our results emphasize the benefit to consider the influence of seascape configuration and connectivity to accurately assess C-org content in coastal habitats. Understanding spatial patterns of variability and what is driving the observed patterns is useful for identifying carbon sink hotspots and develop management prioritizations.

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 (C_org) 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 20^th 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 C_org 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 C_org, which was linked to shifts in C_org sources. The increase in the δ¹³C isotopic signature of sedimentary C_org (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and C_org 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 C_org in the Mbweni meadow since the early 2010s was likely linked to C_org 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 C_org sinks

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 (C_org) 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.