A close look at the temperature-dependent chemical toxicity to aquatic organisms and its implication on derivation of water quality guidelines for protecting aquatic life

Abstract

The anticipated anthropogenically-driven climate change not only can increase the average air and water temperatures and prolong the hottest period, but also result in increased incidents of temperature extremes that will have profound implications on the toxicity of chemical contaminants and hence their ecological risks to aquatic organisms. In this talk, I will comprehensively examine and discuss the mechanisms of temperature-dependent chemical toxicities to aquatic ecototherms based on both literature review and empirical laboratory studies. Examples will be drawn from an array of marine organisms including amphipod, copepod, diatom, and fish after exposure to a chemical (e.g., chlorothalonil, copper, copper pyrithione, DDT or nano-zinc oxide (nZnO)). In general, we observed that chemical toxicity often increases with increasing temperature over the thermal tolerance range (TTR) of a species, and is further exacerbated at extreme temperatures (i.e., lower or higher than the TTR). Most of the aquatic organisms showed the highest tolerance to chemicals at their optimal temperature(s) where they displayed the highest value of median lethal or effect concentration. For aquatic animals, such universal phenomena can be well explained by the oxygen limited thermal tolerance theory established by Hans Portner. Moreover, the overall temperature-dependent toxicity profiles vary considerably amongst different chemicals. Such differences may be partially attributable to the differences in temperature-mediated modifications of their physicochemical properties, toxicokinetics and bioavailability. For instance, we observed that ion dissolution of nZnO in seawater significantly increases with decreasing water temperatures. The diatoms exposed to nZnO had a significantly reduced growth rate at the lowest experimental temperature in contrast to the control; this response was probably due to the increased availability of toxic Zn ions at low temperatures. Recently, we have been conducting a novel meta-analysis to address whether an assessment factor of 10 (AF10) applied to fresh water quality guidelines (WQGs) would be sufficient to account for variation in chemical toxicity brought by thermal extremes. Our preliminary results suggest that AF10 seems adequately protective. Implications of our results will be discussed in relation to ecological risk assessments of chemical contaminants and derivation of WQGs for protecting aquatic life.