Identifying riparian vegetation as indicator of stream water quality in the Gilgel Gibe catchment, southwestern Ethiopia

Abstract

Riparian land use has substantial effects on aquatic habitats and biological communities resulting in a dramatic loss of natural riparian vegetation and affecting the physicochemical properties of streams. The study investigates the relationships among indigenous riparian plants and water quality in the upper Gilgel Gibe catchment in southwestern Ethiopia. The floristic composition of the riparian vegetation and the water quality of streams were studied at selected sites, ranging from first to third order streams. We quantified relationships between disturbance level and both physicochemical characters and traits of riparian plant species during two sampling periods (December 2013 and April 2014). Data were collected from a priori designated three land use types (forest, plantation and agriculture) and ranked along nine streams. Ranks were based on surrounding land use characteristics and deforestation categories. We used analysis of variance (ANOVA) and the Tukey’s post-hoc test to conduct pair-wise comparisons among different land use types. Both species richness and diversity values of forest sites were significantly (p<0.001) higher than agricultural sites. Whereas, stream water quality deterioration indicator gradient such as total suspended solid (TSS), water turbidity, and orthophosphate were significantly (p<0.001) higher in agricultural sites than forest sites. We identified species such as Croton macrostachyus, Ficus sur, Maytenus arbutifolia, and Millettia ferruginea as indicator species of water quality (p<0.05). Our study is the first assessment of the role of indigenous plant species as indicator of highland stream water quality in the tropical area. The study contributes to the on-going discussion on the assessment and monitoring of stream ecosystems and for following stream restoration projects in tropical regions around the globe.