Zinc toxicity in the fish Cnesterodon decemmaculatus in the Paraná River and Río de La Plata Estuary

Acute static tests were performed to assess Zn toxicity on adults of Cnesterodon decemmaculatus (not sexed), a neotropical fish of wide distribution in the Río de la Plata basin. Tests were carried out with ZnCl2 dissolved in natural waters from the Lower Paraná River, the Río de la Plata Estuary, reconstituted waters of similar composition, tap and distilled waters. Median lethal concentration at 24 h was positively correlated to conductivity and the concentrations of Ca2+, Mg2+, and HCO−3 (r = 0.99, 0.96, 0.97, 0.99, p < 0.05, respectively), being higher at the Río de la Plata Estuary (61.2 mg l−1) than at the Lower Paraná River (35.3 mg l−1). Contaminant load in the natural waters tested was similar at both sites, being Zn concentration 40 and 44 μg l−1, respectively. An assay with reconstituted water of similar alkalinity to the estuary showed a simultaneous decrease in HCO−3 and dissolved Zn, while the Zn speciation, predicted with a computer program, suggested that Zn2+ precipitated as ZnCO3. The effect of Ca2+ on Zn toxicity is also discussed. Zn toxicity was higher in the Río de la Plata Estuary water than in synthetic water of similar ionic composition (24-h LC50 = 93.2 mg l−1), suggesting the synergistic effect of other toxic pollutants such as metals, present in the former which were absent in the later.