water Article Satellite Imageries and Field Data of Macrophytes Reveal a Regime Shift of a Tropical Lake (Lake Ziway, Ethiopia) Yohannes Tefera Damtew 1,2, * , Boud Verbeiren 3 , Aymere Awoke 1 and Ludwig Triest 1   Citation: Damtew, Y.T.; Verbeiren, B.; Awoke, A.; Triest, L. Satellite Imageries and Field Data of Macrophytes Reveal a Regime Shift of a Tropical Lake (Lake Ziway, Ethiopia). Water 2021, 13, 396. https://doi.org/10.3390/w13040396 Academic Editor: Chang Huang Received: 12 January 2021 Accepted: 31 January 2021 Published: 3 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Laboratory of Ecology and Biodiversity, Department of Biology, Vrije Universiteit Brussel (VUB), 1050 Brussels, Belgium; aymerea@gmail.com (A.A.); ltriest@vub.be (L.T.) 2 Department of Environmental Health Science, Haramaya University, 235 Harar, Ethiopia 3 Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, 1050 Brussels, Belgium; Boud.Verbeiren@vub.be * Correspondence: yhnnstefera@gmail.com Abstract: Lake Ziway is one of the largest freshwater lakes located in the central Ethiopian rift valley. The lake shoreline is dominated by macrophytes which play an important role in immobilizing run-off pollution, stabilize sediments and support biodiversity. Monitoring the spatio-temporal changes of great lakes requires standardized methods. The aim of this study was to assess the current and long-term trends of macrophyte distribution, surface water area and the water level of Lake Ziway using remote sensing images from 1986 to 2016 with additional hydro-meteorological data. A supervised image classification with classification enhancement using Normalized Difference Aquatic Vegetation Index (NDAVI) and Normalized Difference Vegetation Index (NDVI) was applied. The classification based on NDAVI revealed eight target classes which were identified with an overall producer’s accuracy of 79.6%. Contemporary open water and macrophyte fringes occupied most of the study area with a total area of 407.4 km 2 and 60.1 km 2 , respectively. The findings also revealed a regime shift in the mean water level of the lake and a decline in macrophyte distribution. The long-term water surface area of Lake Ziway also decreased between 1986 and 2016. The changes in water level could be explained by climate variability in the region and strong anthropogenic disturbance. A decline in water level was also associated with lowered surface water area, lakeward retreated macrophyte fringes and enhanced landward encroachment of mudflats, and resulted in a succession of macrophytes with semi-terrestrial vegetations. Keywords: Lake Ziway; macrophytes; water level change; image classification; land cover change 1. Introduction The impact of anthropogenic stressors and climate change on water-level and aquatic vegetation has been evident in recent decades [1–3] ENREF_1. The interaction between anthropogenic stressors, land-use change, and water-level variability affects aquatic macro- phytes to a great extent [4–7]. Aquatic macrophytes are a diverse group of photosynthetic organisms that grow permanently or periodically in wetlands, the shoreline of lakes and along streams [8–11]. They can be classified into four functional groups as emergent, floating-leaved and rooted, free-floating, and submerged, based on their growth form, morphology, and adaptation to the environment [12,13]. Macrophytes are an integral part of aquatic ecosystems and play an important role in immobilizing pollutants, regulating oxygen production and global carbon cycle, stabilizing sediments and protecting shore erosion [5,11,14]. Distribution and composition of macrophyte communities are influenced by nutrient load, and climatic and hydrological conditions such as spatial and temporal variations of water level [7,11,15–18]. Lake water level changes provide an opening for seedling recruitment for perennial emergent aquatic plants [19–21]. On the contrary, a low water level causes succession and terrestrialization of aquatic ecosystems [22–25]. Climatic Water 2021, 13, 396. https://doi.org/10.3390/w13040396 https://www.mdpi.com/journal/water