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Relative Risk from Stressors to Stream Biota

John Van Sickle, John L. Stoddard, and Steven G. Paulsen

U.S. EPA, ORD, NHEERL, Western Ecology Division, 200 SW 35th St, Corvallis, OR 97330

Past EMAP-Surface Waters assessments of the importance of stressors have focused on ranking the "extent" of stressors. For example, in the Mid-Atlantic Region, 28% of the stream length in the region has severe excess sedimentation problems. In comparison, only 1% of the stream length has a signature of serious acid mine drainage problems. This type of analysis is important and yet does not address the "severity" of the stressor effects. To address this concern, we borrow the concept of "relative risk" from medical epidemiology. Relative risk, in this context, can be defined as the risk of finding poor biology when the stressor score (e.g. excess sedimentation, high nitrogen) is poor, relative to the risk of finding poor biology when the stressor score is good. Three primary biological assemblages (fish, macroinvertebrates, algae) were evaluated in concert with nine primary stressors incorporating a rank of chemical, physical and biological stresses to streams. As expected the relative risk signatures varied for the three biological assemblages. For fish, nitrogen and phosphorus showed the highest relative risk values (between 1.7 and 2) while excess sedimentation and acidification (both mine drainage and deposition) were the stressors with highest relative risk (just over 1.5) for macroinvertebrates. In addition, the ranking of stressors by "extent" and "severity" was evaluated to provide recommendations on the stressors that should be early focus for restoration and remediation strategies.

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