Scientific Approach 
As a first step, key combinations of stressors for which interactions are expected to occur will be 
identified based on mechanisms of action outlined in conceptual models and review of 303(d) 
listings for common combinations of stressors. Individual stressor dose^•esponse relationships 
and models developed under project 3 will provide a starting point for examining the interactions 
of multiple stressors in freshwater and marine ecosystems. The importance of interactive effects 
will be evaluated by including the documented pathways of stressor action and interaction in 
deterministic dynamic models calibrated with field studies and/or historic data, and then 
simulated over many runs to discover the sensitivity of measurement endpoints to changes in one 
or more of the stressors (e.g., Bartell et al. 1984, Mitsch and Reeder 1991, Hanratty and Stay 
1994, EPA 2000c). A generic model including the impact and interaction paths for the dominant 
stressors of interest will be applied for each of three resource classes: streams, lakes and 
reservoirs, and estuaries. These generic model frameworks will be developed as a joint product 
among the four Ecology Divisions. Sensitivity analysis of these models will serve as a first order 
estimator for allocating observed ecological effects among two or more interacting stressors, as 
well as a means for evaluating the relative importance of indirect and interactive effects. This 
approach is independent of scale requiring only that the stressor- response relationships and 
interaction pathways be documented on the scale of interest. Once expected interactive effects 
and ranges of interactions are identified, the results of existing case studies and ongoing regional 
case studies will be reviewed for evidence of interaction effects. Pending outcomes of simulation 
exercises, additional field studies will be performed, combined with carefully crafted laboratory 
experiments and physical models calibrated to match loadings and functional properties observed 
in the field system. 
Products 
FY03 Identification of key combinations of stressors expected to interact within conceptual 
model (all Ecology Divisions). 
APM 3B FY06 Simulation of key stressor interactions with generic ecosystem models using 
sensitivity analysis to define the range of stressors and stressor combinations under which 
nonadditive interactive effects will occur (MED, AED). 
Benefit of Products 
Part of the TMDL process involves allocation of the cause of impairment among multiple 
stressors. The simplest case possible involves additive effects, which can be predicted from 
single stressor models. If effects are synergistic or antagonistic, then the results of reduced 
loadings will be more difficult to predict. The proposed products would identify the extent to 
which stressor interactions are expected to occur in natural ecosystems and those combinations of 
factors which favor their occurrence. This knowledge is critical to allow the States and Tribes to 
develop viable restoration and remediation plans for water bodies and watersheds affected by 
multiple stressors. 
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