Water Quality and Trophic Status of Lake Ziway, Ethiopia, determined with contrasting methods and models

Abstract

The published literature on Lake Ziway report disparate data on its water quality and trophic status. This study aimed to determine the current water quality and trophic status of Lake Ziway using recent approaches and extensive sampling regimes. Water and bacteriological samples were collected from seven sampling sites for five months from April to August 2017. Physico-chemical parameters such as DO, EC, pH, and temperature were measured in situ with a portable multimeter probe. Nutrients were determined following the standard procedures of APHA. Total coliform was determined by spread plate technique and fecal coliform with membrane filter method. Water quality was computed with weighted arithmetic Water Quality Index method. Trophic status was calculated with Carlson index, and other two indices developed for tropical reservoirs (Lamparelli and Cunha et al.). Results indicated significant spatial difference of physicochemical parameters. Nutrients such as NO­₃ and TP varied between 0.37-0.18 mgL^-1, and  0.24- 1.32 mgL^-1, respectively.  Chl a  values ranged from 24.3 to 88.1 µgL^-1 and  turbidity  ^from 71 – 550 NTU.. WQI value was above 100 and indicated poor WQ status. Based on different TSI models, the trophic status of Lake Ziway was determined as hypereutrophic (TSI > 59) from all seven sites. The present study concludes that Lake Ziway is hypereutrophic and not oligo- to eutrophic status as reported earlier and promotes the use of recent TSI models which consider only TP and Chl a data to determine TS. Urgent integrated water management is recommended for the sustainable use of this important freshwater lake in the rift valley.