Vulnerability to sea-level rise and the potential for restoration to enhance blue carbon storage in salt marshes of an urban estuary

Coastal wetlands are vulnerable to sea-level rise (SLR) but are also valued for their potential to provide effective nature-based solutions to climate change mitigation and adaptation. Ecological benefits from these ecosystems can be constrained under urban settings by anthropogenic disturbances and pressures, so restoration activities are promoted as a management approach. Here we report on the potential for restoration of disused commercial salt extraction pans to enhance carbon (C) sequestration in the urban Swartkops Estuary, South Africa. We also considered the impact of SLR to 2100 on the distribution of estuarine habitats, the vulnerability of built infrastructure to tidal flooding, and how C sequestration is projected to change over time using the Sea-Level Affecting Marshes Model (SLAMM). Potential restoration of all salt pans (320 ha) to estuarine habitat was estimated to result in a gain of 67 850 Mg C. Establishing tidal connectivity was investigated as a potential restoration action, but most of the salt pan area was above the elevation of the current tidal range and would require excavation. Although conversion of the salt pans to estuarine habitat was predicted to occur without intervention under SLR, 44% of the original area would remain unchanged. Restoring hydrological connectivity to the estuary for these salt pans would significantly increase the extent of transitional/floodplain marsh, even under SLR to 2100. C sequestration was predicted to be 15% higher (54 614.8 Mg C) by 2100 if the salt pans could be restored, compared to if no action is taken. Overall, restoration of the salt pans has the potential to enhance C sequestration, but SLR will still cause large losses of supratidal marsh due to 'coastal squeeze' and extensive tidal flooding of developed areas by 2100 in the lower reaches of the estuary. A full-scale restoration approach for the Swartkops Estuary could use C sequestration potential to fund the project through carbon offsetting if the revenue exceeds the cost of the restoration activities, but additional social and ecological goals also need to be incorporated if the outcome is to be holistic and beneficial.

Automated summary

Restoring disused commercial salt extraction pans to estuarine habitats has the potential to enhance carbon sequestration, but sea-level rise by 2100 will still lead to significant losses of supratidal marshes and extensive tidal flooding of developed areas in the lower reaches of the estuary.