Long-term monitoring of Water Hyacinth in Lake Nokoué, Benin, West Africa

Abstract

Water hyacinth is a globally recognized invasive aquatic plant known for its significant environmental impact and the substantial costs associated with its management. Its proliferation has caused widespread damage across Lake Nokoué in southern Benin, home to fishing communities that practice traditional fishing techniques called Acadja. Although these fishing structures increase fishing yields, they also exacerbate the water hyacinth infestation rate. This study, therefore, models the extent of water hyacinth, Acadja with attached water hyacinth, other land, and other vegetation in the Lake Nokoué area using Landsat Collection 2 Tier 1 and Sentinel-1 C-band Synthetic Aperture Radar imagery from 2015 to 2022 with Random Forest machine learning. Seventeen predictors were selected to model each land cover, including five spectral indices, seven spectral bands, two radar bands, and three terrain predictors. The model mapped a total of 17,413.4 ha for water, 2907.5 ha for water hyacinth, 1780.6 ha for Acadja, 2128.6 ha for other land, and 8289 ha for other vegetation areas by the end of 2022. The rate of change in the region since 2015 was −6.8 % (water), +149.7 % (water hyacinth), +726.1 % (Acadja), −20.6 % (other land), and −15 % (other vegetation) for each class. A separate method was also tested to compare the supervised modeling to an unsupervised method. Otsu segmentation was used for the same study period. Otsu detected 614 ha of vegetation for 2015 and increased to 1133 ha by 2022, but results indicate this method is unreliable. Vegetation found in Lake Nokoué was also assessed monthly from 2000 to 2022 using manual segmentation of a harmonic Landsat time series. By 2022, results indicated that infestations consistently maxed out during December and January and exponentially expanded. Although infestations traditionally peaked during November and December, the study found that lake vegetation increased by 941 % and 304 % for the high- (September to December) and low-water (January to May) seasons.