Nitrate (NO3---N) toxicity to aquatic life: A proposal of safe concentrations for two species of nearctic freshwater invertebrates

Safe concentrations (SCs) of nitrate (NO3---N) for early and last instar larvae of two species of Nearctic net-spinning caddisflies, Cheumatopsyche pettiti and Hydropsyche occidentalis, are estimated from short-term 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 exposed to five different concentrations of sodium nitrate (NaNO3) for 120 hours. SCs were estimated on the basis of mortality data. SCs (or 8760 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. occiderualis, 2.4 (0.7 – 5.4) for the early instar of C. pettiti, 2.2 (0.8 – 4.7) for the last instar of H. occidentalis, and 3.5 (1.0–8.1) for the last instar of C. pettiti. These results suggest that larvae of C. pettiti and H. occidentalis may be much more sensitive to nitrate pollution than fishes during long exposures. Yet, further investigations would be required to improve nitrate safe criteria for aquatic life.