Use of Non-linear Models Based on Saturation Kinetics to Determine Chronic Co, Se or Zn Toxicity for Either Exposure or Body Burden in Hyalella azteca
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Water chemistry can influence the bioavailability, and therefore the toxicity, of metals and other elements. Water chemistry measurements have been incorporated into many water guidelines for the concentration of metals in Canada. However, the application of these guidelines requires site-specific measurements of metal concentrations and can also require the measurement of water chemistry parameters. The amphipod Hyalella azteca has been used extensively in toxicity testing, and the whole-body concentration of an element in the organism can be related to toxic effects for some elements. The whole-body concentration is, therefore, assumed to be proportional to the concentration of the element at the site of toxic action. However, it is unknown if the water chemistry variables that influence element bioavailability and toxicity will also influence the whole-body concentration that is linked to a toxic effect. If the whole-body concentration of an element in H. azteca causing toxicity is not influenced by water chemistry conditions, then it could be a useful site assessment tool. Several trace metals, including cobalt (Co) and zinc (Zn), as well as the element selenium (Se), are essential to many organisms for metabolic processes. However, above a certain threshold these elements will have toxic effects. Mortality is often the most sensitive toxic endpoint in H. azteca, so the acute and chronic of mortality of H. azteca were determined in organisms exposed to a range of concentrations of cobalt, selenium, or zinc in soft water exposure media that varied in pH, alkalinity, or dissolved organic carbon. Non-linear regression models based on saturation kinetics were used to model mortality in both acute and chronic exposures, and to model bioaccumulation in chronic exposures in relation to exposure or body concentration of an element. From these models, lethal exposure concentrations and lethal body concentrations were determined for each element and each water chemistry scenario. Dissolved organic carbon (DOC) was protective against chronic exposure-based Co and Zn toxicity, but increased the toxicity of Se. The patterns of uptake of Se were also influenced by DOC, as well as pH and alkalinity. Concentrations of DOC greater than 5 mg L-1 influenced the uptake pattern of Co, but the lethal body concentrations of Co were not influenced by water chemistry. The lethal body concentrations of Zn in H. azteca were similarly not influenced by water chemistry, whereas concentrations of DOC greater than 5 mg L-1 decreased the Se body burden that caused mortality. The bioaccumulation models that were developed could predict observed body concentrations within two-fold of the actual value over 87% of the time for all elements. The resulting non-linear regression models and lethal concentrations were compared to studies conducted in hard water that had similar data. Increased hardness was protective against exposure-based toxicity of all elements tested. Lethal body concentrations for each metal exposure were consistent regardless of the water hardness. The existing bioaccumulation models were not appropriate to model soft water data. However, the existing mortality models for Co and Zn were robust enough to estimate the lethal exposure and lethal body concentrations. Even though the existing Se model could predict lethal concentrations from the soft water data, there did not appear to be a consistent Se body concentration associated with mortality. This research showed that non-linear models can be used to describe mortality and bioaccumulation of Co and Zn in many different water chemistry scenarios and predict both lethal water and lethal body concentrations. In addition, it was determined that whole-body concentration is a good predictor of mortality caused by Co or Zn exposure, regardless of water chemistry. The body concentration of Se causing mortality varies with water chemistry, so it is not advisable to use any Se model for toxicity prediction.
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Lesley Anne Steeds Wilkinson (2018). Use of Non-linear Models Based on Saturation Kinetics to Determine Chronic Co, Se or Zn Toxicity for Either Exposure or Body Burden in Hyalella azteca. UWSpace. http://hdl.handle.net/10012/13060