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ABSTRACT
The purpose of this study was to investigate the influence of temperature and dissolved oxygen levels on the acute toxicity of profenofos to fathead minnows (Pimephales promelas). Exposure conditions were as follows: normal temperature and normal dissolved oxygen (NTNO; 20 ± 2°C and 6.0–9.0 mg/L, respectively); normal temperature and low dissolved oxygen (NTLO; 20 ± 2°C and 1.7–2.6 mg/L, respectively); high temperature and normal dissolved oxygen (HTNO; 30 ± 2°C and 6.6–6.9 mg/L, respectively); high temperature and low dissolved oxygen (HTLO; 30 ± 2°C and 1.5–3.0 mg/L, respectively). Initial 96-h acute toxicity studies with profenofos were conducted at NTNO and HTLO exposure conditions. The 96-h LC50 at NTNO was 333 µg/L with 95% confidence limits ranging from 244 to 558 µg/L. However, the 96-h LC50 at HTLO was significantly lower at 21.5 µg/L with 95% confidence limits ranging from 17.4 to 28.8 µg/L. Acetylcholinesterase (AChE) activity was measured in the head and torso of surviving fish at 96-h. A weak dose-related decrease in AChE was observed at NTNO but no dose–response relationship was observed at HTLO exposure condition. Additional experiments were conducted using 50 µg/L profenofos at NTNO, NTLO, HTNO, and HTLO exposure conditions. Mortality, sublethal effects (erratic and hyperactive swimming), and AChE activity in the head and torso were measured at 2, 4, and 12-h following exposure to profenofos. No mortality or significant sublethal effects were observed in controls or profenofos-treated groups in NTNO and NTLO exposure conditions. However, significant mortality and sublethal effects were observed in profenofos-treated fish in HTNO at 12 h and at all time points in HTLO. Both high temperature and low dissolved oxygen, as well as combinations of high temperature and low dissolved oxygen significantly decreased AChE activity in control fish. Exposure to 50 µg/L profenofos in all exposure conditions further decreased AChE activity, but no apparent correlations between mortality and AChE activity were observed. These results suggest that the acute toxicity of profenofos to fathead minnows may be exacerbated during summer conditions in southern U.S. aquatic ecosystems.