stream and river waters, most industrial effluents do not contain significant quantities of particulate matter because specific treatment operations such as polymer-enhanced flocculation or filtration remove solids from the water.

Particulate matter can also alter the bioavailability or biological activity of some contaminants. Chlorine (measured as total residual chlorine), for example, is very toxic to daphnids (Taylor, 1993) but is rapidly detoxified when it reacts with algae, detritus, or chemically labile dissolved organic matter. Organic and inorganic particles also are important sinks for relatively insoluble contaminants such as polychlorinated biphenyls, hydrophobic hydrocarbons, and various metals. In general, naturally occurring particulate matter lowers the concentrations of dissolved pollutants, thereby lowering the water's toxicity.

Monotonic response to an increase in the signal of interest is an important consideration in any detector system. The dose-response concept in toxicity testing embodies this consideration and has been very influential in the development of effluent toxicity tests. Dose-response patterns, where organism responses are a function of toxin concentration, are fundamental to effluent and pure-chemical toxicity testing. It is through adherence to an expected dose-response relationship that effluent toxicity testing gains predictive value. Therefore, much effort in the development of toxicity tests has gone into the selection of test procedures that generate smooth dose-response curves. Procedures for establishing regulatory limits on effluent toxicity are based on the premise that a monotonic dose-response relationship can be determined (Figure 1). This premise dominates every aspect of effluent toxicity testing: Adherence to a linear dose-response pattern allows extraction of the toxicity signal. The statistical procedures for estimating toxicity of effluents that yield smooth dose-response curves are clearly outlined in EPA manuals (Kooijman, 1996; Weber et al., 1989) (Figure 1).

A weak toxicity signal and relatively high background noise are typical of ambient toxicity test conditions. Low signal-to-noise ratios prevent effective quantification of ambient toxicity using the statistical framework of the dose-response model that works so well for effluent toxicity tests. The difference in appropriate statistical procedures for analysis of test results is the crucial distinction between the analytical procedures for ambient and effluent toxicity tests. This difference is central to the formulation of a cost-effective strategy for ambient toxicity testing.

Ambient toxicity tests using Ceriodaphnia dubia (a freshwater microcrustacean) and Pimephales promelas (fathead minnow) larvae have been used to support biological monitoring programs for 12 receiving streams at DOE facilities in Oak Ridge, Tenn. and Paducah, Ky. (Stewart and Loar, 1994). The tests used EPA-approved procedures for estimating chronic toxicity (Mount and Norberg, 1984; Norberg and Mount, 1985; Weber et al., 1989), specifically, rear-