Limnological features and water-quality changes of two Ethiopian Rift Valley lakes, Lake Abaya and Lake Chamo

We conducted a study in Lake Abaya and Lake Chamo, the two largest Ethiopian Rift Valley lakes, between 2016 and 2019, to evaluate their limnological features and infer longer-term changes in some water-quality parameters. Data generated through in situ measurements and laboratory analyses of physico-chemical parameters and the plankton communities were compared with results from the literature. Turbidity, salinity, nitrates and soluble reactive phosphorous showed an increasing trend in Lake Abaya, while in Lake Chamo conductivity, salinity, nitrates and soluble reactive phosphorous showed an increasing trend. In both lakes the water transparency and chlorophyll a concentration showed a declining trend. Despite higher available nutrients in Lake Abaya compared with Lake Chamo, higher turbidity with suspended solids contributed to lower chlorophyll-a concentrations and lower zooplankton abundance. The abundances of rotifers and crustaceans were lower in Lake Abaya compared with Lake Chamo. The mean trophic state index using total phosphorus, Secchi depth and chlorophyll-a shows that both lakes are in a eutrophic state. Our findings are consistent with previous studies and indicate that changes in major limnological features are a signal to take intervention measures to prevent further ecological degradation and to sustain the ecological services and livelihoods of fishers who are directly dependent on the lakes.