The use of living organisms to assess the ecological health and water quality of aquatic environments. Biomonitoring provides an integrated measure of environmental conditions that chemical sampling alone cannot capture.
Biomonitoring (biological monitoring) uses living organisms—their presence, absence, abundance, diversity, and physiological condition—as indicators of environmental quality in aquatic ecosystems. The most commonly used organisms for freshwater biomonitoring include benthic macroinvertebrates (insect larvae, crustaceans, mollusks), diatoms (siliceous algae), fish assemblages, and increasingly, environmental DNA (eDNA). Macroinvertebrate-based indices such as the EPT index (number of Ephemeroptera, Plecoptera, and Trichoptera taxa), Hilsenhoff Biotic Index, and Index of Biotic Integrity (IBI) are widely used by regulatory agencies to classify stream health. Biomonitoring offers advantages over chemical sampling because organisms integrate exposure to all stressors over time, detect intermittent pollution events that grab samples might miss, and reflect the cumulative effects of multiple stressors including habitat degradation, altered flow regimes, and chemical contamination. The U.S. EPA's National Aquatic Resource Surveys and USGS NAWQA (National Water-Quality Assessment) program rely heavily on biological data for national water quality assessment. Reference condition approaches compare biological communities at test sites to those at minimally disturbed reference sites to quantify the degree of impairment. Biomonitoring data are increasingly used in Total Maximum Daily Load (TMDL) development and Clean Water Act Section 303(d) impaired waters listing.