Health assessment and restoration options for the degraded Swartkops Estuary, South Africa

Abstract

The Global Biodiversity Framework and UN Decade of Ecosystem Restoration have focused attention on the need for health assessments and restoration options for estuaries. This study focused on the Swartkops Estuary because of its biodiversity and socio-economic importance that are threatened by pressures from surrounding development and human activities. The ‘Present Ecological State’ (PES) was assessed using an estuarine health index to determine the health score of the estuary compared to historical reference conditions, using both abiotic and biotic indices. Results showed that nutrient-rich freshwater from upstream wastewater treatment works and stormwater canals has increased freshwater inflow to the estuary by 41% compared to natural, leading to eutrophication and persistent harmful algal blooms. Development and disturbance have transformed the estuary functional zone, impacting on macrophyte and bird abundances. Invertebrate bait organisms and linefish species are overexploited. As a result, the health of the Swartkops Estuary has continued its downward trajectory from 53% of its natural state in 2015 to 47% at present. This study is the first to identify potential remediation measures aimed at improving the current ecological health of the estuary. These include the removal of wastewater inputs and the restoration of salt marsh habitat, which would improve the ecological status from a largely modified to moderately modified condition. This study highlights how difficult it is to restore an estuary once deteriorated, while emphasising the need for an implemented estuary management plan with well-defined management, conservation, and restoration goals.