Anthropogenic activities such as coal mining, oil and gas production, application of salts on roads for de-icing and agricultural practices can result in saline discharges to aquatic ecosystems. Salts are components of all natural waters. Salinity is a measure of inorganic ions or salts. It is mainly comprised of major cations (calcium-Ca, potassium-K, magnesium-Mg, sodium-Na) and major anions (chloride-Cl, sulfate-SO4, bicarbonateHCO3). The ionic composition of mine waters can have varying

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... portions of major cations and anions. Saline discharges to freshwater ecosystems can increase salinity as well as change the ionic composition. Major ions are also essential for the normal functioning of the organisms. Change in salinity or ionic composition of the surrounding water can have a detrimental effect on organisms. While increased calcium concentration is presumed to decrease the toxicity of saline solutions, the effect of calcium proportion of a mine water ionic composition is not well understood. The aim of this study was to investigate the influence of calcium proportion on the toxicity of a saline solution to Ephemeroptera, Austrophlebioides sp. AV11. The hypothesis tested was that increased calcium proportion decreases the toxicity of a saline solution to Austrophlebioides sp. AV11. The effect of calcium proportion was tested in presence of all the major cations, potassium, magnesium and sodium and the major anions, chloride, sulfate and bicarbonate. The aim of the study was evaluated using static nonrenewal 96 h acute toxicity tests. The saline solution used in the present study was based on the ionic composition of mine water from the study of and named as artificial mine water (AMW). The term calcium proportion used here refers to the proportion of calcium in relation to all the major ions measured in milliequivalents (% meq). Solutions with increased calcium proportion were prepared using calcium chloride and calcium sulfate salts together. Calcium proportion was increased above that of AMW two fold (Ca-Cl.SO4(2)), four fold (Ca-Cl.SO4(4)) and eight fold (Ca-Cl.SO4 (8) ). Additional tests were conducting using calcium chloride and calcium sulfate salts separately to assess the effect of calcium in isolation of the associated anions. Calcium proportion was increased as two fold and four fold designated Ca-Cl(2), Ca-Cl(4) and Ca-SO4(2) and Ca-SO4(4). For the experiments with calcium chloride and calcium sulfate together, there was a statistically significant reduction in toxicity for Ca-Cl.SO4(2) and Ca-Cl.SO4(4) solutions compared with AMW, but no difference between Ca-Cl.SO4(8) and AMW. The 96 h LC50 values were: AMW  6.0 mS/cm, Ca-Cl.SO4(2) -6.9 mS/cm, Ca-Cl.SO4(4) -8.1 mS/cm and Ca-Cl.SO4(8) -6.2 mS/cm. These results show that both two fold and four fold increase in calcium reduced toxicity by 15 % and 35 % respectively but that an eight fold increase in calcium did not affect toxicity. • AMW ■ Ca-Cl.SO 4 (2) ▲ Ca-Cl.SO 4 (4)  Ca-Cl.SO 4 (8) ▢ Ca-Cl (2) △ Ca-Cl(4) ◧ Ca-SO 4 (2) ◭ Ca-SO 4 (4) • AMW ▢ Ca-Cl(2) ■ Ca-Cl.SO 4 (2) △ Ca-Cl(4) ▲ Ca-Cl.SO 4 (4) ◧ Ca-SO 4 (2)  Ca-Cl.SO 4 (8) ◭ Ca-SO 4 (4) Present study Previous studies