The effect of Mn on mortality and growth in the freshwater amphipod Paramelita nigroculus (Barnard) exposed to a mixture of Al and Cu in acidic waters

Abstract The combined toxicity of three common trace elements — aluminum, copper and manganese — on the endemic freshwater amphipod Paramelita nigroculus (Barnard) was studied in acidic waters. In soft “artificial” water (distilled water to which appropriate salts had been added), after exposure periods of 21 d and at a concentration of 1.388 mg/L Al+0.018 mg/L Cu, survival was 55% for juveniles and 60% for adults. At a concentration of 1.388 mg/L Al+0.018 mg/L Cu+13.993 mg/L Mn, survival was 61.5% for juveniles and 62% for adults. These concentrations are 139 times the AEV for Al(0.01 mg/L), at least 10 times the AEV of Cu(0.0016 mg/L) and Mn (1.300 mg/L) from the official South African guidelines for each metal. Risk and multiple regression analyses show that, using juveniles in natural water, when the concentration of each element increases by 1 ppb, the risk of death increases minimally (by 1.000086) while the addition of Al+Cu mixtures increases the risk of death by 2.6 times. The addition of Al+Cu+Mn increases the risk of death by 1.5 times, suggesting that Mn reduces the risk associated with the other two elements. The risk of death for an adult is 0.8 times the risk for a juvenile, while the risk of death for a moulting individual is 5.3 times greater than that of a juvenile. The risk in artificial water (i.e. with virtually no trace metals other than those specifically added) is 0.9 times that of the risk in stream water. The relationship between log growth and log concentration is: log( y )= a + b *log( x ) and between growth and concentration is: y=cx b where y =growth; x =concentration of each element, and c =log( a ) to base e . Thus growth in river water without Mn added is y =469 x −0.194 , while in river water with Mn added, y =615 x −0.088 . In “artificial” water without Mn added, y =434 x 0.108 and with Mn added y =226 x −0.070 .