a b s t r a c t
Reactive nitrogen compounds such as nitrite (NO2 ) are highly toxic to aquatic animals and are partly
responsible for the global decline of amphibians. On some fish and Caudata amphibian species low levels
of sodium chloride significantly reduce the toxicity of nitrite. However, the nitrite–salinity interaction
has not been properly studied in anuran amphibians. To verify if chloride (Cl) attenuates NO2 toxicity,
eggs and larvae of three anuran species were subjected to a series of NO2 solutions combined with three
salt concentrations (0, 0.4 and 2 or 0, 0.052 and 0.2 g L1 NaCl). One of the species tested originated from
two different populations inhabiting highly contrasted nutrient richness environments: lowland Doñana
Natural Park and Sierra de Gredos Mountain. In general, the presence of Clincreased survival and
growth of lowland Pelophylax perezi and activity of mountain P. perezi larvae exposed to NO2 , thus attenuating
the toxicity of NO2 to developing amphibians. Mountain amphibian populations appeared to be
much more sensitive to the concentrations of NO2 and Clused in this experiment than coastal conspecifics,
suggesting possible adaptation of populations to local conditions. Nitrogen pollution in coastal
wetlands poses a serious threat to aquatic organisms, causing direct toxicity or indirect effects via ecosystem
eutrophication. The presence of low to medium levels of salinity that would be common in coastal
wetlands may attenuate the direct effects of increasing concentrations of nitrogenous compounds in
water bodies. Furthermore, treating cultures of endangered anurans with small amounts of NaCl may
provide an additional protective measure.