ing replicate groups of fathead minnow larvae, or individual Ceriodaphnia , in full-strength (i.e., nondiluted) samples of water from the site(s) being evaluated. During the 7-day tests, fresh samples of water were collected daily from each site being evaluated. The water was then warmed to the test temperature (25°C) and used to replace the previous day's water in the test chambers. This procedure is referred to as static renewal of test cultures. In almost every case, we evaluated the sites for ambient toxicity by testing both species concurrently. The ambient toxicity tests include negative controls (i.e., tests with mineral water, diluted to an acceptable ionic strength with distilled water) and water samples from reference sites, located upstream of known point- or area-source inputs of pollutants. We estimated toxicity using survival and growth of minnow larvae and survival and reproduction of Ceriodaphnia. We also measured the pH, conductivity, alkalinity, hardness, and total residual chlorine of all freshly collected water samples.

Ambient toxicity tests were run on water from as many as 10 sites per stream, but in some cases one site was sufficient for effective monitoring. At one of the DOE facilities near Oak Ridge (the Oak Ridge National Laboratory), 15 sites on 5 receiving streams have been tested 42 times (concurrently with both species) since 1986. A total of 630 site and test-period combinations were represented by the sampling and testing strategy used for the five receiving streams (i.e., 15 sites in each of 42 test periods). For each stream or suite of streams that is monitored for ambient toxicity, the primary unit of statistical analysis is the mean response for each site-date combination. The response parameters include Ceriodaphnia survival (percentage, based on 10 animals per site-date combination) and reproduction (number of offspring per female, for females that survive all 7 days), and fathead minnow survival (percentage, based on 4 replicates, each containing 10 fish) and growth (mean milligrams dry mass per surviving fish, per replicate, corrected for initial weight). Survival data for the minnows are generally arc-sine square-root transformed before analysis; growth data for the minnow larvae can be corrected for growth of larvae in the controls or reference sites, depending on the objective of the analysis. We do not normally transform Ceriodaphnia survival values because each test generates only a single value (e.g., 100 percent, 90 percent, 80 percent) derived from 10 individual animals, each of which constitutes a replicate.

We have explored various methods for analyzing the results of ambient toxicity tests. The case-study examples below summarize these methods, the key findings revealed through their use, and their major advantages and disadvantages. Most of these examples are derived from studies published elsewhere (e.g., Kszos et al., 1992; Stewart, 1996; Stewart et al. ,1990, 1996). In general, the results of ambient tests are used either to reveal differences among sites (e.g., a longitudinal pattern within a stream or differences among streams) or to demonstrate the occurrence of water-quality changes over time.

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