Impact/Purpose:

The objective of this research is to assess the likelihood that synergistic and/or antagonistic ecological effects will occur from the interactions of multiple stressors. To set priorities for the development of TMDLs and the restoration of impaired water bodies, it is necessary to understand how the potential interactions among stressors will affect system recovery once one of the stressors is reduced. 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.

Description:

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. 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. 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. If models exist that meet our needs, they may be used in lieu of developing new models. 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. 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 may be performed, combined with carefully crafted laboratory experiments and physical models calibrated to match loadings and functional properties observed in the field system.

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