Scientists and resource managers recognize that ecosystems are dynamic. One way to track the status and changes of these dynamic systems is through the use of indicators. An indicator represents the state or trend of certain environmental conditions over a given area and a specified period of time. 

For the National Aquatic Resource Surveys (NARS), a suite of biological, chemical, and physical indicators were chosen to assess biological integrity, trophic state, recreational suitability, and key stressors impacting quality. Each survey uses some of the indicators listed below for assessment purposes. Although there are many more indicators and/or stressors, these are among the most representative at a national scale. The descriptions for these indicators are meant to be general. For more specific information on how the NARS use these indicators to characterize the different water body types, please visit information on individual surveys pages for the National Coastal Condition Assessment, National Lakes Assessment, National Rivers and Streams Assessment, and National Wetland Condition Assessment.