An assessment of leeches (Hirudinea) as indicators of lake acidification

To assess their value as indicators of acidification, leeches (Hirudinea) were surveyed with baited traps in 40 small lakes covering a wide pH range near Sudbury, Ontario. Leeches were caught in all 20 lakes with pH >5.5, and represented nine species. Leeches, representing four species, were caught in only 4 of 20 lakes with pH <5.5, and none were caught in 8 of these in which pH was <4.9. Artificial surfaces made from Styrofoam® were provided on 20 lakes for cocoon deposition by Nephelopsis obscura. After 30 days, cocoons were deposited on surfaces on 10 of 13 lakes with pH ≥5.2, but were not deposited at lower pH.