The acute and chronic toxicity of lanthanum to Daphnia carinata

The rare earth elements (REEs) are increasingly being used as trace supplem ents in agriculture. This study measured the acute and chronic toxicity of one REE, lanthanum (La), to Daphnia carinata. The 48-h EC50 of La to Daphni a was measured in three media of differing composition and hardness. Lantha num was most toxic to Daphnia ih soft tap water (TW) with an acute 48-h EC5 0 Of 43 mu g/l compared with 1180 mu g/l in ASTM hard water (ASTM). In the third daphnid growth medium (DW), based on diluted sea water, the acute 48- h EC50 was 49 mu g La/l, however, there was significant precipitation of La in this media. The chronic toxicity of La to Daphnia was measured in the D W and ASTM media. Nominal exposure concentrations were 100, 200, 400, 600, 800, and 1000 mu g La/l. Mortality was a more sensitive endpoint than growt h or reproduction in both chronic experiments. Very little La was detected in either media after 24 h and the measured concentrations below were estim ated by logarithmic mean of nominal and measured values. There was 100% mor tality at concentrations greater than or equal to 80 mu g La/l (400 mu g/l nominal) by day six of the experiment using DW media, but no effect on surv ival growth or reproduction at lower concentrations. In the ASTM media, La caused significant mortality to Daphnia at concentrations greater than or e qual to 39 mu g/l (208 mu g/l nominal), however, at least one animal surviv ed to the end of the study at each of the tested concentrations. There was no effect of La on growth of surviving daphnids at concentrations less than or equal to 57 mu g/l (400 g/l), however, second brood clutch sizes were s ignificantly increased at 30; 39, and 57 mu g/l (100, 200, 400 g/l nominal) compared with controls. Lanthanum also caused a delayed maturation in Daph nia. (C) 2000 Elsevier Science Ltd. All rights reserved.