Water quality of the River Nile. 3. Toxicity assessment of six industrial effluents polluting the River Nile

Six different industrial effluents were chemically and physically characterized. Their toxicity was evaluated using biotests with Selenastrum capricornutum strain NIVA-CHL1. The growth of the test alga showed either inhibition or stimulation. Two of the effluents were highly inhibitory with EC sub(50) values ranging between 1 and 10% (v/v) effluents concentrations. Others showed how growth inhibitory effects (50% < EC sub(50) < 100%. EC sub(50) values derived from the growth parameters, algal dry weight, cell count, growth rate, and area under the growth curve were almost similar for those two effluents exhibiting highly inhibitory effects. The toxicity values, calculated from algal dry weight and cell count showed, a strong positive linear relationship (r = 0.97 at p < 0.0001). The potential toxicity of some effluents was partly related to their high content heavy metals. Changes in mean cell volume of the test alga may be used as an indicator for the potential effects of toxic effluents on the overall cellular activities. The results emphasized the indispensable role of algal bioassays among the basis of relevant and quantitative information about the biological impacts of the investigated effluents on the receiving waters.