Assessment of pollution and trophic state of a water hyacinth infested tropical highland lake: Lake Tana in Ethiopia

Freshwater quality is declining globally due to changes in natural and human-driven land use. Lake Tana, Ethiopia’s largest lake, exemplifies this change. Human activities within its watershed have pushed it toward a trophic state, evident in the expansion of water hyacinths since 2011. This research aimed to evaluate the degree of pollution and trophic state of Lake Tana and identify the most important factors for effective management strategies. Eleven water quality parameters were collected from 20 sampling stations in Lake Tana from July 2018 to June 2019. Spatial distribution maps for water quality, trophic state and pollution index were generated using ArcMap. Water quality indicators were explored through factor analysis (FA) and principal component analysis (PCA). Lake Tana’s trophic level index ranges from 37.1 (oligotrophic) to 87.9 (hypereutrophic) annually. The pollution index classification indicated that the lake fell within categories three and four, changing from slightly polluted to polluted. According to the PCA and FA methods, oxygen depletion, nutrients, eutrophication, and suspended solids are identified as major factors influencing deteriorating water quality. Shores exhibit higher eutrophication and pollution levels than pelagic zones. Thus, all stakeholders must actively participate in management strategies prioritizing coastal areas and targeting the identified pollution factors.