How can something as variable as an aquatic ecosystem be measured and compared?
Aquatic ecosystems are dynamic. Measured conditions will steadily change from season to season. But if the right set of indicators are chosen, the comparison to the reference site should yield similar results in similar seasons and conditions unless there are new watershed stressors. If the comparison changes dramatically over time, it is safe to assume that watershed conditions have changed. Chronic pollution levels may have increased, there may have been a short-term but severe pollution event, or habitat quality may have declined. If the comparison yields dramatically different results at two different points in a stream, it is reasonable to assume that there is a stressor upstream of one but downstream of the other, or that the effects of a problem upstream of both diminish between them.
