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.
