Abstract:
Lake Ziway is shallow freshwater located in Northern part of Ethiopian Rift Valley. Expansions of the flower industry, fisheries, intensive agricultural
activities, fast population growth lead to deterioration of water quality and depletion of aquatic biota. The objectives of the present study are to
evaluate the spatial and temporal variations in the external nutrient load and determine the trophic status of Lake Ziway in 2014 and 2015. The
nutrients and Chlorophyll-a were measured according to the standard procedures outlined in APHA, 1999. From the result Ketar and Meki Rivers
catchment showed the major sources of external nutrient loads to the lake ecosystem. The mean external soluble reactive phosphorus (SRP), total
phosphorus (TP), total inorganic nitrogen (TIN) and total nitrogen (TN) loads to Lake Ziway were 230, 2772, 4925 and 24016 kg day-1, respectively.
A general trend which was expected that the nutrient loads would be much higher in rainy season than in dry season. The mean concentrations of
trophic state variables for TN, TP and Chla were 6700, 212 and 42 μg L-1, respectively. The mean values of TSI-TP, TSI-Chl-a, TSI-TN and TSI-SD
were 79, 66, 81 and 83, respectively and the overall evaluation of Carlson Trophic State Index (CTSI) of Lake Ziway was 77. Therefore, the lake is
under eutrophic condition. The mean values of TN: TP ratio was 48 which were very high. The trophic state index determined with chlorophyll-a
showed lower value than those determined with all trophic state indices values of TN, TP, and SD which indicated that non-algal turbidity affected
light attenuation for algal growth. This suggested that phosphorus was the limiting nutrient in Lake Ziway. Due to its importance as being the lake is
an intensive agricultural site, management solutions must be urgently developed in order to avoid the destruction of the lake.
