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Evaluation of toxicity and bioavailability of metal mixtures to two freshwater invertebrates
Traudt, Elizabeth Marie
Traudt, Elizabeth Marie
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2016
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Abstract
Multiple metals are often present in natural water systems, leading to mixture toxicity that currently is difficult to predict. To help develop predictive toxicity models, the toxicity of Cd, Cu, Ni and Zn was measured in individual-metal, binary, and ternary mixtures in acute toxicity tests following USEPA protocol using Daphnia magna neonates (< 24 hour old). Toxicity tests in which D. magna were exposed to binary mixtures of Ni combined with Cd, Cu, or Zn demonstrated a protective effect (Cd-Ni), a greater-than-additive effect (Cu-Ni), and a slightly less-than-additive toxic effect (Ni-Zn). Following these results, I tested ternary mixtures of Cd-Ni with either Cu or Zn in an attempt to observe multiple interactions occurring concurrently (i.e. – mixing less-than-additive interaction with a more-than-additive interaction). In Cd-Cu-Ni mixtures, the toxicity was less-than-additive, additive, or greater-than-additive, depending on the concentration of the varied metal. In Cd-Ni-Zn mixtures, the toxicity was always less-than-additive or approximately additive. These results demonstrate that complex interactions of potentially competing toxic mechanisms can occur in metal mixtures but should be predictable by mechanistic models of metal-mixture toxicity. In these studies, variability was high among replicate acute Cd lethality tests (e.g., >10-fold range of median effects concentrations [EC50s]). I hypothesized that age-related differences in sensitivity to metals might occur even within that relatively narrow age range. Daphnia magna neonates collected during three 4-h age windows were used to start acute toxicity tests. In repeated sets of tests, the Cd EC50 of the youngest neonates was approximately 10-fold greater than the EC50 of the oldest neonates. These results demonstrate that decreasing the age range of D. magna used in toxicity tests could help to improve the accuracy and precision of toxicity models, particularly for metal mixtures. To continue my work with metal-mixture toxicity, I traced the flux of Cu in isotopically-labeled freshwater snails (Lymnaea stagnalis) that were fed flocculent material rich in Fe and other metals (e.g., Cu, Zn) collected from sediment in the North Fork of Clear Creek (NFCC). The uptake and depuration of Cu were measured over a 48-h exposure in which the organisms consumed control or contaminated food. The assimilation efficiency, which describes the extent to which the particle-associated Cu is taken up by an organism, can be used to quantify the bioavailability of Cu in the particulate-metal mixture. This study evaluated the biodynamic parameters of three types of flocculent contamination found at NFCC sites. The assimilation efficiency remained elevated around 40-50% in all three flocculent types. These results will have direct implications to understanding and predicting potential in-stream toxicity of contaminated sediments before, during, and after remediation of AMD.
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