Abstract
Lakes and water reservoirs encircled by a catchment with a clear influence of anthropogenic pressure are generally characterized by low water quality and high trophic status. In this study, 20 samples from major inflow rivers into Lake Tana were taken to estimate the phosphate and nitrate loading using a Vollendier Model for the four seasons. 67 samples were taken from three different portions of the Lake, and physicochemical water quality parameters in these samples were measured by the standard procedure in APHA, 2005 to estimate the trophic status using the Carlson trophic state model for the dry and wet seasons. The result showed that 0.369 to 106, 1.882 to 2.91, 1.564 to 2.51, and 0.67 to 2.16 kg of nitrate from Megech and Ribb watershed and 0.321 (Megech) to 0.370 (Gumara), 0.578 (Megech) to 0.728 (Gilgel Abbay), 0.418 (Megech) to 0.476 (Ribb) and 0.339(Megech) to 0.378 kg of phosphate from Ribb is discharged in second into the Lake in season 1, 2, 3 and 4 respectively. Ribb River which lies in the most extended flood plain of the Fogera Werda has a relatively high concentration of nitrate and phosphate; as a result, it discharges a higher amount of these nutrients in kg per second than the other tributary rivers in all seasons into the lake. Moreover, the rivers carried more nutrients in the wet season than in the dry season. As a result, the river mouth portions are more prone to nutrient enrichment than the central and Bahir Dar Gulf portions of the lake. Due to these, these parts of the lake have relatively average low Secchi Depth Transparency (0.54,0.36,0.50& 0.47m; 0.61,0.42&0.58m) and high average total phosphorus concentration (1.26,1.43,1.21& 1.23 mg/l; 0.80, 1.09,0.67 & 0.69mg/l) in the wet and dry season respectively. So, 50% of the river mouth portions have an STI value of 38 < TSI < 49 which falls in a mesotrophic status index compared to the central and Bahir Dar Gulf portions with an STI value < 38 which falls in an oligotrophic status index especially in the wet compared to the dry season. Preventive measures have to be practiced in the Lake Tana basin to reduce nutrient loading and improve the trophic status of the lake.