The Microtox Acute Toxicity Test has different sensitivities to different pollutants, with generally more sensitivity to organic pollutants than metals (Qureshi and others, 1998). Microtox analysis quantifies differences (in percent effect) of the fluorescence values of bioluminescent bacteria exposed to sample water when compared to a laboratory control. Decreases in fluorescence (positive percent effect values) are attributed to the toxic effects of sample water on the viability of test bacteria, and the magnitude of the change in luminescence relates positively to the toxicity of the sample water. Toxicity results are generally reported in one of two ways, depending on the level of toxicity in the sample. For samples with low toxicity, values are reported as the percent effect observed on analyses run with undiluted sample water (Chang and others, 1981). For more highly toxic samples, toxicities are generally reported as the EC₅₀ or the concentration of sample water necessary to cause a 50-percent decrease in luminescence (when compared to laboratory controls) (Chang and others, 1981). The EC₅₀ is computed by running a series of Microtox tests with different dilutions of sample water; at least one of these dilutions should cause less than a 35-percent effect, and at least one of the dilutions should cause greater than a 65-percent effect (Rand and Petrocelli, 1985).

Samples submitted for Microtox analysis were collected from all 21 Phase II sites during summer-quarterly and event sampling, yielding a total of 4 samples per site over the 2-year period. Analyses were first run with undiluted sample water, and values were reported as percent effect. Values were consistently below 20-percent effect, with a small number of samples exhibiting a greater effect. All environmental samples showing greater than 20-percent effect were collected from harbor sites (table 24). The maximum percent effect observed (40-percent effect) was well below the 65-percent effect needed for the determination of an EC₅₀. Overall, results were not indicative of highly toxic conditions at any site.

Table 24. Percent effects in samples with readings at or above 20 percent for all Phase II sites in the Milwaukee Metropolitan Sewerage District planning area, Wis.

[All values in percent effect]

Microtox analyses were also performed on extracts from SPMDs deployed at selected sites and these data are discussed elsewhere in this report. Results from those analyses reflect toxicities of hydrophobic compounds accumulated from the water column over a month.

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