4.4.14 (HL)—Biotic index

The Trent biotic index is a named indirect water-quality method that converts indicator-taxon presence and pollution tolerance into a score; biotic indices more generally may also use relative abundance and diversity to summarize community response over time.

Sensitive-group loss and tolerant-group dominance usually lower quality in the named protocol. Compare the score with BOD or DO, which measure more immediate conditions.

In a Trent-style comparison, loss of sensitive nymphs and dominance by tolerant bloodworms or rat-tailed maggots indicate greater organic pollution and lower oxygen, even after a short-lived sewage pulse has diluted.

The index integrates biological history; BOD samples current biodegradable oxygen pressure.

Do not assume every biotic index has the same scoring direction or taxa weights; use the specified protocol.