This study was conducted between April 2015 and February 2016 for four round monitoring periods on Lake Ziway. The study focused on assessing impact of flower farm effluent on lake Ziway biodiversity using macro-invertebrates as indicator of water quality. Sweeping with a pond net was done for three minutes per sample in all the sampling sites and points for macro-invertebrate. A single composite sample of sediment and water was collected from one site. Individual samples of sediment and water were collected some 5 â€“ 10 meters apart using a scoop and a small plastic bucket respectively, and then composited together in a big bucket to represent a single sample of the specific site. Physico-chemical parameters were recorded including PH, conductivity, dissolved oxygen total dissolved solid (TDS). Nitrogen and phosphorus content of the water were also recorded in the second, third and fourth sampling rounds. Residues of dieldrine, heptachlor, chlordane, endosulfan, dBHC and DDD at concentrations ranging from 0.01-0.21mg/l were recorded in effluent drainage area. DDT and its metabolites were found mostly in effluent sediments at concentrations ranging from 0.28-7.76mg/kg The result shows that, the diversity of macro-invertebrate at the outlet (effluent drainage area) is less than the inlet and control. Water quality was best at Site 3 >Site 1 >Site 2. Both sites 1 and 2 were at the low end of their FBI scales so in terms of water quality there was little difference between all 3 sites. Nevertheless, the poorest score computed was for Site 2 (5.29), which does tie with the siteâ€™s water chemistry, where the highest mean pH, conductivity, TDS, PO4-P, NO3-N, and variety and concentrations of pesticide residues in the water and sediment were recorded. Therefore, action is needed to protect the lake from further pollution by applying waste water treatment and protecting untreated water from reaching the lake. Key words: bio-monitoring, macro-invertebrate, flower farm, Lake Ziway, pesticides