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Estimating the risks of multiple, covarying stressors in the National Lakes Assessment
Citation:
VAN SICKLE, J. Estimating the risks of multiple, covarying stressors in the National Lakes Assessment. Freshwater Science. The Society for Freshwater Science, Springfield, IL, 32(1):204-216, (2013).
Impact/Purpose:
The National Lakes Assessment (NLA) used relative and attributable risks to measure the apparent nationwide effects of excess nitrogen, reduced lakeshore habitat, and other stressors, on planktonic assemblages in lakes.
Description:
The National Lakes Assessment (NLA) used relative and attributable risks to measure the apparent nationwide effects of excess nitrogen, reduced lakeshore habitat, and other stressors, on planktonic assemblages in lakes. The risk measures, borrowed from human health research, use a nontechnical language to compare the apparent effects of disparate lake stressors, thus helping the public and policymakers identify the stressors of greatest national concern. However, the NLA and similar, prior surveys of streams did not adjust their risk estimates of each stressor for possible confounding by other, covarying stressors. As a result, the NLA point estimates overstate the risks of individual stressors and are hence unreliable for assessing their importance relative to that of closely-related stressors. Using NLA data from 966 lakes, I illustrate existing statistical methods of risk adjustment. Point estimates of adjusted relative risk for 7 stressor variables were 15% to 64% lower than unadjusted estimates. Adjusted attributable risks were also lower than unadjusted values, however, for some stressors they were widely inconsistent across 3 adjustment methods. In addition, adjusted risk estimates used only part of the available data, because each estimate was adjusted for many (6) covarying stressors. Closely-related stressor variables (for example, nitrogen, phosphorous and turbidity) can be combined into a "bundle" representing a broader type of stress (reduced water quality). For the NLA data, adjusted risk estimates for the stressor bundles (water quality and habitat) were more consistent across estimation methods and had lower relative uncertainty, as compared with estimates for their component stressors. In addition, closely-related stressors with similar sources and modes of impact are more likely to be managed together rather than individually. For these reasons, I suggest evaluating stressor bundles in future aquatic surveys.
