1 CSVTU International Journal of Biotechnology, Bioinformatics and Biomedical. 2018, Vol. 3, No. 2 doi.org/10.30732/ijbbb.20180302001 © CSVTU International Journal of Biotechnology, Bioinformatics and Biomedical: Swami Vivekanand Technical University All Rights Reserved. CSVTU International Journal of Biotechnology, Bioinformatics and Biomedical Vol. 3(2), 01–16, 2018 ISSN: 2455-5762 Research Article External Nutrient Load and Determination of the Trophic Status of Lake Ziway Dessie Tibebe *1 , Feleke Zewge 2 , Brook Lemma 3 , Yezbie Kassa 4 , Ashok N. Bhaskarwar 5 1* Department of Chemistry, College of Natural and Computational Sciences, University of Gondar, P. O. box 196, Gondar, Ethiopia. 2 Department of Chemistry, College of Natural and Computational Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia. 3 Department of Zoological Sciences, College of Natural and Computational Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia. 4 Department of Biology, College of Natural and Computational Sciences, University of Gondar, P. O. box 196, Gondar, Ethiopia. 5 Department of Chemical Engineering, Institute of Indian Technology, New Delhi, P.O. Box 110016, New Delhi, India. * Corresponding author, Email: dessie.1977@gmail.com, HIGHLIGHTS • This study aims to evaluate external nutrient loads and determine trophic status of Lake Ziway • Katar and Meki Rivers are the major sources of the external nutrient load of the lake ecosystem • Total phosphorus, Chlorophyll-a and secchi depth are important trophic state variables • Currently Lake Ziway is under eutrophic condition G R A P H I C A L A B S T R A C T 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. Key words: Lake Ziway, external nutrient load, eutrophication, trophic status, chlorophyll-a