Comparative acute and chronic toxicity of diethylenetriamine pentaacetic acid (DTPA) and ferric-complexed DTPA to Daphnia carinata

The acute and chronic toxicity of diethylenetriamine pentaacetic acid (DTPA) and ferric complexed DTPA (Fe[III]-DTPA) to Daphnia carinata were compared, while the effects of DTPA exposure prior to and/or during 1st brood embryogenesis were also assessed. For chronic exposures, daphnids were exposed to DTPA at high or low food levels, or to Fe(III)-DTPA at high food level until the 6th reproductive instar. The 48 h LC₅₀s of DTPA and Fe(III)-DTPA to D. carinata were 245 mg/L and > 1,000 mg/L, respectively. Chronic exposure to 10 mg/L DTPA resulted in a significant reduction in all individual brood sizes, while it increased the age at each reproductive instar. Ten mg/L DTPA also significantly decreased the cumulative number of offspring per adult at high and low food level from 161.3 ± 14.6 to 11.3 ± 4.9 offspring, and 56.4 ± 1.8 to 0 ± 0 offspring, respectively, while a similar effect was observed for the number of offspring per adult per day. Both the 3rd and 5th brood sizes were also significantly reduced at 1 mg/L DTPA, but only at high food level, from 39.0 ± 2.9 to 27.6 ± 3.8 offspring, and 49.3 ± 5.0 to 39.9 ± 4.2 offspring, respectively. Chronic exposure to Fe(III)-DTPA had little effect on D. carinata, but there was a significant negative relationship between Fe(III)-DTPA and the number of offspring per adult per day (y = -0.024x + 14.048, r² = 0.20, n = 26, P < 0.02). This was due to a 25% reduction in reproduction at 134 mg/L Fe(III)-DTPA, the highest test concentration, compared to controls. The no-observed-effect concentrations (NOEC) and lowest-observed effect concentrations (LOEC) for DTPA and Fe(III)-DTPA following chronic exposure to D. carinata were 1.0 and 10 mg/L, and 67 and 134 mg/L, respectively, although the possibility of effects occurring below 10 mg/L DTPA could not be discounted. Exposure to DTPA prior to 1st brood embryogenesis significantly decreased the 1st brood size but did not affect the 2nd brood size, while exposure during 1st brood embryogenesis significantly decreased the 2nd brood size, but did not affect the 1st brood size, indicating the reproductive impairment was due to maternally-mediated factors and not direct toxicity to the eggs. The decrease in DTPA toxicity when complexed with Fe(III) was attributed to preferential binding with that metal, thereby limiting any further chelating ability. Concentrations of DTPA in receiving waters are unlikely to be toxic to D. carinata.