Investigation of spatio-temporal variations of selected water quality parameters and trophic status of Lake Tana for sustainable management, Ethiopia

Abstract

of aquatic ecosystems is dependent on right proportions and required succession of major nutrients in the water and sediment. Surface water pollution by agrochemicals has become a growing threat to the human society and natural ecosystems in the recent decades. Lake Tana is the largest fresh water body in Ethiopia, located at an altitude of 1786m above sea level. The lake has an average depth of 9m and maximum depth of 14m with surface area of 3150 km2. The lake is an economically important shallow lake in the country and is threatened by anthropogenic and climatic factors. Excess agrochemicals input from agricultural activities and industrial effluent around Lake Tana watershed can pose a serious threat on the lake ecosystem by expansion of water hyacinth. The study was made to investigate the spatio-temporal variations of selected water quality parameters and trophic status of Lake Tana for sustainable management options for sustainable use of the lake. Different environmental samples such as water and sediment were collected from fourteen representative sampling sites of the lake for the measurement of physicochemical parameters and nutrients in both dry and wet seasonal basis. The physicochemical parameters were measured in-situ with portable multimeter and nutrients and chlorophyll a were determined by following the standard procedures outlined in the American Public Health Association (APHA) using UV/Visible spectrophotometer. The trophic status of the lake was determined using trophic state variable and Carlson model. The lake water quality level was also evaluated using comprehensive evaluation index model. There were spatio-temporal variations in the physico-chemical parameters and nutrients in the lake ecosystem during the study period. Higher concentrations of nutrients, electrical conductivity (EC) and total dissolved solid (TDS) were recorded in sampling sites of effluents of the some resort and the major feeding rivers in all seasons. The values of the comprehensive pollution index ranged from 0.68 to 1.01 and 0.91 to 1.69 during dry and wet seasons respectively. According to the values of comprehensive pollution index, the lake is moderately and slightly polluted in dry and wet seasons, respectively. The values for SRP, and TP distributions were higher in the wet season in most of the sampling sites. Carlson trophic state model showed the average values of Carlson Trophic State Index (CTSI) of Lake Tana were found to be 69.77 and clearly indicated Lake Tana TSI value lies well above the minimum value for eutrophic condition. In order to stop further deterioration of lake water quality and to eventually restore the beneficial uses of the lake, management of agrochemicals in the lake watershed should be given urgent priority. It is mandatory to prepare guidelines for the trophic status of the Ethiopian freshwater bodies in the country by making bottom-up-down discussion.