Heavy metal pollution and nutrient levels in Lake Muhazi, Rwanda

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Most tropical African lakes are facing problems of rapid population growth in the riparian communities, which normally discharges pollution loads into the lakes. This has led to the deterioration of water quality in receiving lakes. Some lakes are getting eutrophic whilst others are facing problems of siltation and heavy metal discharges, thereby reducing their economic and aesthetic values. Some lakes are experiencing a decrease in fish production. In Rwanda, generally the surface waters are full of sediments caused by soil erosion. The proliferation of water hyacinth and other aquatic weeds is now a common phenomenon. The reduction in lake water levels and low fish production are also main problems of lakes in Rwanda. In particular, Lake Muhazi has experienced a dramatic decrease in fish production since the eighties, typified by low water transparencies and high turbidities. The lakeshores are now being developed for ecotourism and this requires sound environmental management to make the planned activities viable. The purpose of this study was to investigate the nutrient status of Lake Muhazi by assessing vertical and horizontal distributions of nutrients in the Lake. The parameters studied are nitrogen, phosphorous, chlorophyll a, and Secchi disc measurements. The nutrient levels were determined over a period of four months from July 2007 to October 2007. Four sampling stations were located within the lake and the sampling was done fortnightly. The samples were collected at depths of 0.5 m, 2 m, 5 m, and 1 m from the bottom of the lake. Samples were taken using a Van dorn Bottle water sampler and were analyzed for heavy metals, TP, TN, NO2-N, NO3-N and chlorophyll-a using the Standard Methods. Field measurements of temperature, transparency (Secchi disc), and pH were also measured. The results indicate that the nutrient levels in the Lake are higher than previously reported figures and urgent intervention is required. It was observed that the high nutrient levels in the lake are related to landuse activities in the catchment. It is recommended that farming practices and erosion be controlled on the catchment to contain pollutant discharges into the Lake.



Lake water quality, Lake Muhazi, Landuse practice, Nutrient loading, Trophic status