Effects of water-borne iron and calcium on the toxicity of diethylenetriamine pentaacetic acid (DTPA) to Daphnia carinata

First instar D. carinata neonates were exposed to 0, 10 or 50 mg l^-1 diethylenetriamine pentaacetic acid (DTPA) and 0.5 (normal), 1.5 (high) or 3.9 (very high) mg l^-1 Fe, or 0, 1 or 10 mg l^-1 DTPA and < 5 (low), 20 (normal) or 100 (high) mg l^-1 Ca, in a two and one brood experiment, respectively. Daphnids exposed to 0 mg l^-1 DTPA/3.9 mg l^-1 Fe died within three days, while those exposed to 50 mg l^-1 DTPA/0.5 mg l^-1 Fe died after 6-9 days. The former was attributed to Fe toxicity, and the latter to DTPA toxicity. Daphnids exposed to 10 mg l^-1 DTPA/3.9 mg l^-1 Fe produced 56.3 ± 8.0 offspring per adult, which was similar to control daphnids (0 mg l^-1 DTPA/0.5 mg l^-1 Fe) at 63.5 ± 5.6 offspring per adult, but significantly greater than daphnids exposed to 10 mg l^-1 DTPA at both 0.5 and 1.5 mg l^-1 Fe. Chemical equilibrium modelling indicated that the decrease in DTPA toxicity at high Fe concentrations was potentially due to an increase in the amount of available Fe, not to the total binding-out of DTPA with Fe, as was initially suspected. When exposed to Ca concentrations of < 5 mg l^-1, growth rate of D. carinata was severely reduced, and reproductive maturity delayed. However, when also exposed to 1 mg l^-1 DTPA, there was a further reduction in growth rate and associated parameters, while at 10 mg l^-1 DTPA/< 5 mg l^-1 Ca, all daphnids died. Chemical equilibrium modelling indicated that the increase in DTPA toxicity at very low Ca concentrations was potentially due to a further decrease in the amount of available Ca, as a result of chelation by DTPA. The major effects observed at 1 mg l^-1 DTPA/< 5 mg l^-1 Ca would most likely result in adverse population level consequences, and may be of environmental concern.