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Do Static and Flow-Through Toxicity Tests Yield the Same Toxicity Information?
Roger Bergstedt, William Swink, and Karen Slaght1
1USGS, Great Lakes Science Center, Hammond Bay Biological Station, 11188 Ray Road
Millersburg, Michigan 49759
Six pairs of static and flow-through toxicity tests were conducted between January 18 and February 27, 2007. Tests were run in Lake Huron Water (overall average alkalinity of 81) and at 12ēC. Measures of pH were taken hourly and the measures averaged. Alkalinity was measured at the start and end of the test. Tests were conducted in accordance with ASTM E-729-88a, with the exception that the static-test aquaria were gently aerated. The Litchfield and Wilcoxon method was used to calculate the 24 hour LC99.9 (for larval sea lamprey).
The static tests yielded estimates of the LC99.9 that were consistently higher by about 0.4 ppm TFM (Table 1). However, the average pH values in the static tests were 0.09 units higher. Based on the pH-Alkalinity treatment charts, this difference in pH should result in an increase of about 0.4 ppm of TFM to achieve the LC99.9. Within static and flow-through tests, the average LC99.9 and the expected LC99.9 from the pH-Alkalinity treatment chart were identical. The observed difference was clearly consistent with the expected effect of the observed pH difference.
Because only one set of tests were conducted, there can be no formal comparison of the standard deviation of pH values observed in static and flow through tests. But the difference between SDs of 0.06 and 0.05 are not likely to be significantly different other than over a large number of sets of tests. Interestingly the discussion that led to this project speculated that respiration of CO2 in the static tests would lower the pH, when in fact aeration appears to have stripped a small amount of CO2 and raised the pH. This would not appear to be an issue if pH is monitored and used to arrive at the expected toxicity.