Results of our previous research suggested that small exceedences of the four-day or monthly average permit limit may not result in chronic toxicity effects if the magnitude is not acutely toxic and the exceedence is limited in duration and frequency. Our current research, testing larval
fathead minnows and several different exposure regimes of copper and ammonia, reinforced these results. Exposures consisted of 3-4 concentrations (magnitude), ranging between the LC10 and LC25, each of which was tested at one of 3-4 pulse durations (3, 6, 12, and 24h),
and the pulse repeated at one of 3-4 time intervals (0, 12, 24, 96, 144h). All tests were 14d in length and exposures were static with daily renewals of solutions. A fairly consistent survival pattern was observed in relation to the combination of magnitude and duration; high magnitude and
longer durations had the greatest effect. Growth effects were not apparent in any test. Nearly all mortalities occurred within 24h of a pulse and lag effects were not observed, contrary to expectations based on the supposedly high acute-to-chronic ratios for both chemicals and fathead minnows.
The second pulse of copper or ammonia had a greater effect on fish survival if it occurred ≥96h after the first pulse as opposed to shorter intervals, suggesting short-term physiological adaptation after the first pulse that diminished as the recovery period increased. The 4d average concentration
corresponding to toxic effects was typically an order of magnitude less than the continuous exposure larval fathead minnow IC25 for both chemicals. Our results indicate that both copper and ammonia are relatively fast acting, such that chronic or sublethal effects are unlikely given
pulse durations ≤ 24h.
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