Ecosystem assessments employ two kinds of indicators, descriptive indicators and normative indicators. “Indicators can be used descriptively for a scientific purpose or normatively for a political purpose.”[31]
Used descriptively, high chlorophyll-a is an indicator of eutrophication, but it may also be used as an ecosystem health indicator. When used as a normative (health) indicator, it indicates a rank on a health scale, a rank that can vary widely depending on societal preferences as to what is desirable. A high chlorophyll-a level in a natural successional wetland might be viewed as healthy whereas a human-impacted wetland with the same indicator value may be judged unhealthy.[32]
Estimation of ecosystem health has been criticized for intermingling the two types of environmental indicators.[31][33] A health indicator is a normative indicator, and if conflated with descriptive indicators “implies that normative values can be measured objectively, which is certainly not true. Thus, implicit values are insinuated to the reader, a situation which has to be avoided.”[3