The utility of Sentinel-2 MSI to assess wetland vegetation chlorophyll content and leaf area index in wetland areas in Pietermaritzburg, KwaZulu-Natal.

Abstract

Wetland ecosystems are being modified and threatened due to anthropogenic activities and climate change, hence the urgent need for wetland restoration. Wetland rehabilitation is important in the reversal of these dire conditions, through restoring damaged wetland ecosystems and recovering wetland vegetation. Wetland biophysical properties such as leaf area index and chlorophyll content are important indicators of vegetation productivity and stress. Therefore, the overall aim of this study was to assess the variations in wetland vegetation productivity between wetlands under different management regimes in Pietermaritzburg, South Africa using Sentinel-2 MSI data. Chlorophyll and leaf area index were used as proxies of wetland Cyperus dives and Typha capensis productivity in this study. The first objective was to test the ability of Sentinel-2 MSI data and vegetation indices in estimating leaf area index of wetland vegetation across natural and rehabilitated wetlands. The second objective was to assess the utility of the high-spatial resolution Sentinel-2 MSI data in the estimation of chlorophyll content of Cyperus dives and Typha capensis species across natural and rehabilitated wetlands. Results showed that vegetation indices derived from red-edge bands produced better LAI estimation accuracies for both wetlands with a root mean square error (RMSE) of 0.32 m2/m2 and 0.51 m2/m2 as well as R2 ‘s of 0.61 and 0.75 for the natural and rehabilitated wetlands, respectively. The optimal model for predicting LAI across natural and rehabilitated wetlands was attained based on red-edge bands centered at 705 nm (Band 5), 740 nm (Band 6), 783 nm (Band 7) as well as 865 nm (Band 8a) yielding a RMSE of 0.51 m2/m2 and R2 of 0.75. In addition, the combination of all spectral variables in estimating chlorophyll across different wetland management regimes and species exhibited a relatively low RMSE of 9.11 μg cm2 (12%) and R2 value of 0.88 based on red-edge bands centered at 705 nm (Band 5), 740 nm (Band 6), 783 nm (Band 7) as well as 865 nm (Band 8a). The findings of this study indicate that Sentinel-2 MSI data can be optimally used to estimate productivity (chlorophyll content and LAI) of wetland plant species such Cyperus dives and Typha capensis growing under different management regimes, with the rehabilitated wetland exhibiting improved productivity. Results of this study underscores the unique potential of new generation earth observation sensors in wetland vegetation monitoring and management, this has implications on other ecosystem processes such as wetland water use and carbon sequestration.