4.4.13 (HL)—Indicator species

Sensitive taxa such as many stoneflies and mayflies suggest cool, oxygenated water; tolerant bloodworms or Tubifex can dominate under organic pollution and low oxygen.

Biological communities integrate conditions over time, but absence can reflect season, habitat, predators or sampling failure. Compare abundance and diversity at matched sites and pair with chemical data.

A downstream rise in Tubifex plus falling DO is stronger pollution evidence than one missing mayfly sample.

A species may be absent for non-pollution reasons; abundance, site matching and chemistry improve inference.

Indicator species signal conditions; they do not identify one pollutant without supporting evidence.