Nitrate and phosphate additive toxicity to the Mexican salamander Ambystoma dumerilii and implications for water quality management.

Salamanders are among the most threatened vertebrates due to chemical and habitat stressors. The Mexican salamander, Ambystoma dumerilii is endemic to Lake Pátzcuaro, Michoacán, Mexico and is exposed to nutrient runoff and discharges from surrounding agriculture and municipal waste. This study evaluated the toxicity of nitrate and phosphate singly and in mixtures to embryonic and the first-stage larval development of A. dumerilii to determine whether measured surface water concentrations comparable to those found in Lake Pátzcuaro pose a risk and whether Mexico's water quality standards are adequate to protect the species. Eggs and larvae of A. dumerilii were exposed for 26 days to nitrate and phosphate individually as well as to dilutions of a mixture based on their LC₅₀ concentrations. The Probit determined LC₅₀ for NO₃-N was 887 mg/L and for PO₄-P was 2015 mg/L. The LC₅₀ for the mixture was 1.0 toxic unit (TU), while the EC₅₀ for the hatching inhibition mixture was 0.26 TU. Effect thresholds were 0.1 and 0.04 TU respectively. Concentrations of nitrate and phosphate mixtures in selected Mexican surface waters were above the hatching inhibition threshold and the equivalent Mexican standards expressed as mixtures also exceeded this threshold. The toxicity responses to the mixtures suggest that nitrate and phosphate anions compete for the same sites and act in an additive manner. Nitrate and phosphate need to be managed as mixtures rather than singly to protect species with similar sensitivities to A. dumerilii. However, further research is needed to confirm these effects across other aquatic species.