Some indicators are selected as surrogates for multiple associated statistics, e.g., as the dominant vegetation might indicate a satisfactory habitat for other plant and animal species, plus adequate underlying physical-chemical conditions. Indicators, in turn, can sometimes be aggregated into indices. An index aggregates quantities that are not necessarily commensurate into a dimensionless quantity, e.g., an air quality index. Because of their nature, indices have practical shortcomings, such as how to clearly articulate the underlying rationale, the tendency to obscure real changes in the component indicators, and how to assign weights to component indicators. Stresses A logical and understandable way to achieve desired outcomes is to deal with the stresses that impact the system.
Stresses can take numerous forms.
They can be living or nonliving and operate at the ecosystem, community, population, or individual level. Nutrient contamination, in which nutrients lead to eutrophication of a water body, resulting in loss of beneficial uses. Toxic substance contamination, in which persistent toxics are associated with a variety of problems in biota. Physical alterations, whereby changes in the physical landscape affect the aquatic system. Human activities and values, including economic, societal, and technological decisions become manifest in physical, chemical, and biological changes.