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

The combined toxicity of three common trace elements - aluminum, copper and manganese - on the endemic freshwater amphipod Paramelita nigroculus (Barn ard) was studied in acidic waters. In soft "artificial" water (distilled wa ter to which appropriate salts had been added), after exposure periods of 2 1 d and at a concentration of 1.388 mg/L AL + 0.018 mg/L Cu, survival was 5 5% 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 l east 10 times the AEV of Cu(0.0016 mg/L) and Mn (1.300 mg/L) from the offic ial South African guidelines for each metal. Risk and multiple regression a nalyses show that, using juveniles in natural water, when the concentration of each element increases by I ppb, the risk of death increases minimally (by 1.000086) while the addition of Al. Cu mixtures increases the risk of d eath 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 othe r 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 gr eater than that of a juvenile. The risk in artificial water (i.e. with virt ually no trace metals other than those specifically added) is 0.9 times tha t 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 concentratio n 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 = 469x (-0.194), while in river water with Mn added, y = 615x(-0.088). In "artific ial" water without Mn added, y = 434x(0.108) and with Mn added y = 226x(-0. 070). (C) 1998 Published by Elsevier Science Ltd. All rights reserved.