NITRATE (NO3-N) TOXICITY TO AQUATIC LIFE - A PROPOSAL OF SAFE CONCENTRATIONS FOR 2 SPECIES OF NEARCTIC FRESH-WATER INVERTEBRATES

Safe concentrations(SCs) of nitrate (NO3-N) for early and last instar larvae of two species of Nearctic net-spinning caddisflies, Cheumatops yche pettiti and Hydropsyche occidentalis, are estimated from short-te rm toxicity bioassays using an innovative methodology, the multifactor probit analysis (MPA) software. Toxicity bioassays were conducted in soft water (average hardness value of 42.7 ppm CaCO3). Larvae were exp osed to five different concentrations of sodium nitrate(NaNO3) for 120 hours. SCs were estimated on the basis of mortality data. SCs (or 876 0 hour LC0.01 values expressed in ppm NO3-N) and their 95% confidence limits were 1.4 (0.4-3.0) for the early instar of H. occidentalis, 2.4 (0.7-5.4) for the early instar of C. pettiti, 2.2 (0.8-4.7) for the l ast instar of H. occidentalis, and 3.5 (1.0-8.1) for the last instar o f C. pettiti. These results suggest that larvae of C. pettiti and H. o ccidentalis may be much more sensitive to nitrate pollution than fishe s during long exposures. Yet, further investigations would be required to improve nitrate safe criteria for aquatic life.