Toxicity of dtpa to daphnia carinata as modified by oxygen stress and food limitation

First-instar Daphnia carinata were exposed to one of four or five sublethal concentrations of the industrial chelating agent diethylenetriamine pentaacetic acid (DTPA) either alone, or in conjunction with, high (90-100%) or low (10-25%) oxygen saturation and high (2 x 10⁵ cells/ml) or low (2 × 10⁴ cells/ml) food conditions for 6 to 7 days, in a series of three experiments. Survival, growth, reproduction, and hemoglobin (Hb) content were assessed. Mortality increased significantly from 6.5 ± 4.2 to 38.9 ± 5.2%, and mean length was significantly reduced from 2.73 ± 0.02 to 1.37 ± 0.01 mm at 100 mg/liter DTPA in experiment 1. Mean length was also significantly reduced from 2.64 ± 0.12 to 1.9 ± 0.1 mm at 50 mg/liter DTPA in experiment 3. This was attributed to an indirect effect via the food supply in the third experiment. There was a significant decrease in the mean number of first- brood eggs at 10 mg/liter DTPA in all three experiments. Hemoglobin concentration was significantly increased under low oxygen conditions from 27.6 ± 1.7 to 65.5 ± 4.6 mg Hb/g Daphnia dry wt, and 23.0 ± 1.8 to 49.4 ± 3.5 mg Hb/g Daphnia dry wt in experiments 2 and 3, respectively. However, DTPA had no effect on hemoglobin concentration in any experiment. DTPA toxicity to D. carinata was not significantly altered by oxygen stress or food limitation and could not be attributed to an inhibition of Hb synthesis. Increased exposure times may result in further reproductive effects and also an indirect effect on hemoglobin concentration via the gradual depletion of iron stores. The no-observed effect concentration and the lowest observed effect concentration for D. carinata in this study were 1.0 and 10 mg/liter DTPA, respectively, based on reproduction, giving an estimated threshold concentration of 3.2 mg/liter DTPA.