Internal and External Agrochemical Loads, Dynamics and Impacts on the Freshwater Ecosystem of Lake Ziway, Ethiopia

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Addis Ababa Universty


Excess agrochemicals input from agricultural activities and industrial effluent around Lake Zi-way catchment can pose a serious threat on the lake ecosystem. This study was undertaken to investigate the external and internal agrochemicals load, their dynamics, and impacts on Lake Ziway ecosystem and suggest possible management options for sustainable use of the lake. Different environmental samples such as water, sediment and fish were collected from nine rep-resentative sampling sites of the lake for the measurement of physicochemical parameters, nutri-ents and pesticides in 2014 and 2015 in both dry and wet seasonal basis. The physicochemical parameters were measured in-situ with portable multimeter and nutrients and chlorophyll a were determined by following the standard procedures outlined in the American Public Health Asso-ciation (APHA) using UV/Visible spectrophotometer. The trophic status of the lake was deter-mined using the Carlson and Vollenweider models. The external and internal nutrient loads were computed using the methods of Huai-en and Steinman, respectively. Multivariate techniques of cluster analysis (CA), principal component analysis (PCA)/factor analysis (FA) were applied to evaluate water quality of the lake. The lake water quality level was also evaluated using compre-hensive evaluation index model. There were spatio-temporal variations in the physico-chemical parameters and nutrients in the lake ecosystem during the study period. Higher concentrations of nutrients, electrical conductivi-ty (EC) and total dissolved solid (TDS) were recorded in sampling sites of effluents of the flori-culture industry (Fb) and around the floriculture industries (Fa) in all seasons. Results of PCA analysis of the four data sets which were explained more than 88 % and 91 % of the total vari-ance in wet and dry seasons, respectively. The pollutant sources were mainly from sampling sites around effluent of floriculture industry (Fb) during dry season and Meki (Mb) and Ketar (Kb) Rivers during wet season, respectively. CA grouped the nine sampling stations into three clusters of similar water quality features and hence the whole lake was categorized into low, moderate and high pollution status. The values of the comprehensive pollution index ranged from 0.69 to 1.80 and 0.38 to 0.68 during dry and wet seasons, respectively. According to the values of com-prehensive pollution index, the lake is moderately and slightly polluted in dry and wet seasons, respectively. The vertical nutrient profiles of the lake water showed no significant variability, making it diffi-cult to rely on the observed nutrient concentration profiles to understand the nutrient dynamics in the lake. Ketar and Meki Rivers catchments showed the major sources of external nutrient loads to the lake ecosystem. The study showed a general trend of higher external nutrient load in the wet than in the dry seasons. The nutrient budgets for the lake clearly show the amount of nu-trients that enter the lake exceeds the output, indicating that nutrients are being retained in the lake which is important for algal growth. These high nutrient loads indicate the susceptibility of Lake Ziway to eutrophication. Person correlation indicated that precipitation, water level, dis-charge flow and air temperature had weak to strong positive correlations with SRP, TP, TIN and TN while DO, pH, EC, total alkalinity (TA) and soluble reactive silica (SiO2-Si) were nega-tively correlated with water level and discharge flow. The results of sediment depth profile analyses showed that the mean concentrations of SRP, TP, NO3-N, NO2-N and TN were 27.7, 62, 5.28, 8.51 and 1733 mg/kg, respectively in dry season, and 21.2, 73, 7.99, 28.4, 24.2 and 1750 mg/kg, respectively in wet season. The values for SRP, TP, NO3-N, NO2-N and TN distributions were higher at sediment top surface and decline with depth of the sediment profiles in most of the sampling sites and seasons. Carlson trophic state and Vol-lenweider models showed Lake Ziway is eutrophic. In order to stop further deterioration of the lake water quality and to eventually restore the beneficial uses of the lake, management of agro-chemicals (fertilizers and pesticides) in the lake catchments should be given urgent priority. It is mandatory to prepare guidelines for the trophic status of the Ethiopian freshwater bodies in the country by making bottom-up-down discussion. Keywords: Multivariate analysis, water quality, agrochemical, external and internal nutrient load, trophic state index, eutrophication, Lake Ziway



Multivariate analysis, water quality, agrochemical; external and internal nutrient load; trophic state index; eutrophication; Lake Ziway