Salt marsh elevation and responses to future sea-level rise in the Knysna Estuary, South Africa

Abstract

Sea-level rise is a significant threat to salt marshes. However, these habitats can respond by increasing their relative elevation, or by migrating into adjacent areas. Here we provide the first report on surface elevation change for salt marsh at the Knysna Estuary, South Africa, and predict the responses of these habitats to sea-level rise using the Sea-Level Affecting Marshes Model (SLAMM). Recent (2018) surface elevation measurements at lower intertidal salt marsh sites were compared with those from 2009–2011. The long-term (1960–2017) relative sea-level rise (RSLR) was estimated as 2.19 mm y−1 and only two of the seven sites assessed are keeping pace with this RSLR. The SLAMM, which was applied to a focus area near Thesen’s Island, predicted up to 40% loss of upper intertidal salt marsh by 2100, because development in the adjacent area prevents habitat migration. Currently, salt marsh habitats cover a total of 667 ha, and artificial structures are present along 20.7 km of the perimeter. Protecting areas for salt marsh migration should be a conservation priority, because this estuary supports the third largest area of intertidal salt marsh in the country. Monitoring the responses of coastal wetlands should be prioritised so that natural capital contributes towards future resilience.

Keywords: coastal squeeze, Rod Surface Elevation Table, SLAMM, Spartina maritima, South Africa