The impact of changes in management, climate and sea level on blue carbon and greenhouse gas emissions in coastal wetlands

Coastal wetlands provide numerous critical ecosystem services, including storm surge protection, fisheries production, improvement of water quality, and carbon sequestration, and are potentially one of the most effective ecosystems on the planet for slowing global warming. Blue Carbon refers to organic carbon that is captured and stored by the oceans and coastal ecosystems, particularly by vegetated coastal ecosystems: seagrass meadows, tidal marshes, and mangrove forests [1]. The global average carbon storage in the top metre of salt marsh soils is approximately 255 Mg/ha (with a range of 16-623 Mg/ha) [2].

Unlike terrestrial ecosystems, carbon sequestered in coastal soils can be extensive and remain trapped for very long periods of time (centuries to millennia), resulting in very large carbon stocks. In blue carbon systems, the soil is saturated with water keeping it in an anaerobic state (low to no oxygen), and can accrete vertically at high rates, resulting in continuous build-up of carbon over time [3].

Increasingly, coastal wetlands are being recognized for their important role in carbon sequestration and, when degraded, their potential to become sources of the greenhouse gases, carbon dioxide, methane and nitrous oxide [4]. Because methane and nitrous oxide have global warming potentials that are, respectively, 25 and 310 times greater than carbon dioxide over 100 years, it is important to include the release of these gases as presenting a risk for future climate change. Release of greenhouse gases can be significant in salt marshes that experience reduced soil water salinities, changes in soil oxygen availability, and increases in anthropogenic nutrient loading [4].

Griscom et al. (2017) [5] identified coastal wetland restoration and protection as a gap in mitigation options previously considered and estimated that these options have a global climate change mitigation potential in carbon dioxide equivalents of 1-2 Pg/y. This is higher than the estimated potential of any agricultural or grassland options, and comparable to the combined potential impact of restoration and protection of peatlands. Dynamic, process-based simulation models are needed to accurately predict the impacts of future changes in management and climate on carbon storage and greenhouse gas emissions, but whereas models have been developed to do this for peatlands [6], process-based models for coastal wetlands are missing; it is this gap that the proposed PhD will address.

By modelling the carbon sequestered in coastal wetlands, we can more effectively identify options to reduce greenhouse gas emissions and increase soil carbon stocks, so producing high impact outputs and much needed information to support improved policies on the protection and restoration of coastal wetlands. This research will be under the framework of the newly established "Scottish Blue Carbon Forum" (SBCF) providing a conduit to governmental data and policy in support of model validation and impact.