Assessing the feasibility of mapping changes of ecosystem functional groups in South African estuaries using Landsat and Sentinel images of 1990, 2014, 2018 and 2020

Abstract

This study evaluates the feasibility of using medium-resolution satellite sensors to monitor changes in the extent of ecosystem functional groups (EFGs) in South African estuaries, for reporting on the 2030 targets of the Global Biodiversity Framework (GBF). Landsat and Sentinel-1 and -2 image collections in Google Earth Engine (GEE) were used to generate output layers for each of the national land cover years—1990, 2014, 2018 and 2020. Image composites of each year’s two growth seasons and one dry season, vegetation indices and topographic data were generated. Changes in the extent and accuracies of three estuarine (mangroves, salt marshes and submerged macrophytes) and three freshwater (forested wetlands, freshwater marshes and large macrophytes) EFGs were calculated and compared to a manually mapped through image interpretation, high-confidence layer. Overall, estuarine EFGs comprised between 10 and 18% of the extent of the EFGs, while freshwater EFGs made up 15% of the extent of estuaries. The overall accuracies of detection of EFGs for 1990 were < 64% compared to the > 71% attained for 2014, 2018 and 2020. In comparison to manual delineations of some of these habitats, the outputs generated from these medium-resolution sensors resulted in overestimation of extent for all EFGs; for mangroves by 115% and for salt marshes and submerged macrophytes by 150–230%. Finer spatial resolution images, and time-series mapping would be critical for improved delineation and monitoring of South Africa’s estuarine habitats.