Exploring the potential of remote sensing to detect marine plastic debris in the South African Ocean region

Abstract. The ocean provides various economic, environmental, and social benefits for society. In recent years there has been a threat to the ocean through the millions of tonnes of marine pollution that has been disposed into the ocean. Due to these detrimental consequences of plastic pollution on the marine environment, several governments and international governing bodies are proposing solutions to this global issue. It is therefore important to develop methods for the detection of marine plastics for more informed and targeted clean-up or prevention operations for the oceans. However, this can be challenging due to the vastness of the ocean. Remote sensing techniques such as optical and Synthetic Aperture Radar offer a unique opportunity to detect marine debris and a larger extent. In this study, we present a method to use both optical and SAR data to detect marine debris in the South African Exclusive Economic Zone (EEZ). The use of spectral indices, Plastics Index (PI), Floating Debris Index (FDI) and Normalized Difference Vegetation Index (NDVI) was derived from Sentinel-2 and RapidEye imagery. The results indicated that the higher resolution RapidEye imagery was more sensitive in detecting potential floating plastics than Sentinel-2 due to the higher resolution. Positive NDVI and PI values indicate potential floating vegetation and associated plastics. The results revealed that18% and 30% of the pixels in the NDVI and PI images respectively were positive. A threshold of 1 standard deviation showed few outliers that were confined to the edges of the study area and in the harbour, with only 0.12 % of the study area being classified as an outlier in the FDI image. Sentinel-1A GRD data was used to derive backscatter in VV, VH and VV/VH polarisations. The results showed that the VV polarisation highlighted the potential floating more significantly than the other polarisations. The integration of these two methods could provide an enhanced approach to monitoring marine plastic debris.