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(R) 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 greater-than-or-equal-to 5.2, but were not deposited at lower pH.