Coastal blue carbon ecosystems (seagrasses, mangroves and tidal marshes) have recently been found to match or even exceed the capability of terrestrial ecosystems to sequester organic carbon (OC) per unit area. This has led to the development of a promising new strategy for climate change mitigation, termed Blue Carbon, based on the conservation and restoration of these habitats. But including blue carbon sequestration as a component in schemes and policies for climate change mitigation and adaptation requires determining precisely how much OC these ecosystems hold, how it accumulates over time, and which is its fate after habitat conversion and degradation. The aim of this thesis was to resolve these questions by quantifying OC sequestration rates and their controls over contemporary (∼100 yr) time scales in sediments of natural and degraded coastal blue carbon ecosystems, with a focus on seagrass and mangrove habitats. To accomplish this aim, we used the 210Pb dating technique to assess the rate of OC accumulation in sediments and evaluated its potentials and limitations. We reassessed OC sequestration rates in seagrass ecosystems by compiling and analyzing 167 new OC accumulation rates in seagrass sediments worldwide, and assessed the vulnerability of sediment OC stores in seagrass meadows and mangrove forests to be remineralized after ecosystem disturbance. Results re-considered some of the accepted paradigms of 'blue carbon' science: that OC stocks are a measure of OC sequestration efficiency, that the sequestration rates of seagrass ecosystems are orders of magnitude higher than those of terrestrial counterparts, or that habitat disturbance will result in the loss of most or all of the OC stock previously sequesterd. We found that sedimentation rates were a better predictor of OC sequestration efficiency in sediments of coastal blue carbon ecosystems than it was the sediment OC content. Considering 210Pb dating models and associated uncertainties, contemporary global OC accumulation rates in seagrass beds were reassessed and estimated at 20 - 30 g C m-2 yr-1 (or 6 - 18 Tg C yr-1, globally). This was 7 times lower than previously acknowledged, nonetheless, the revised estimates, based on the best available contemporary data, are still extraordinarily high. On a global scale, OC burial in seagrass sediments contributes between 4 and 8% to the total OC buried in ocean sediments. Additionally, its significance as C sinks could be larger, while only 10% of their net community production is buried in immediate sediments, the remaining 90% (32 - 65 Tg C yr-1) production is exported to adjacent systems, a fraction of which could be buried beyond the meadows or preserved in the deep sea. Physical disturbance and habitat loss caused losses of sediment OC over the course of months to years depending on the type of disturbance and the size of the OC stock. Using satellite imagery and a published model of OC decomposition, we quantified that between 4 and 20% of the C stock in the upper meter of degraded seagrass sediments was remineralized 3 years following a marine heatwave in Shark Bay, Western Australia. In mangroves, direct measurements of sediment OC revealed that a 20% of the top meter C stock was lost 10 years following deforestation. In both studies, the rates of OC loss in degraded habitats were several (> 4) times higher than the rates of OC sequestration under intact conditions suggesting that the real potential of blue carbon ecosystems to mitigate greenhouse gas emissions is towards the preservation of existing habitats and restoration of lost habitats, which can result in avoided significant GHG emissions.
|Date of Award||29 Mar 2019|
|Supervisor||Pere Masque Barri (Director), Carlos Duarte (Director) & Jordi Garcia Orellana (Director)|
- Vegetated coastal ecosystems
- Carbon sequestration
- Climate change mitigation