Abstract
Marine mammals accumulate heavy metals in their tissues at different concentrations according to trophic levels and environmental conditions. The franciscana (Pontoporia blainvillei) is a small coastal species inhabiting the marine and estuarine areas of the Southwestern Atlantic Ocean. Its diet includes numerous species of small fish, squid and crustaceans. The aims of this study were to (i) assess the heavy metal concentration and burden distribution in different franciscana age classes and sex, and to (ii) evaluate both the accumulation processes and the transplacental transference of zinc, cadmium, copper and total mercury. Heavy metal concentrations (wet weight) were determined in eighteen dolphins by Atomic Absorption Spectrophotometry (AAS), by the cold vapour technique (mercury) or with air/acetylene flame (cadmium, zinc and copper). Liver showed the highest concentrations of mercury (max. 8.8 mg/g), zinc (max. 29.7 mg/g) and copper (max. 19.0 mg/g), whereas the highest cadmium concentrations (max. 6.7 mg/g) were found in kidney. Adults contained the highest concentrations for all heavy metals, followed by juveniles and calves in decreasing order, suggesting an age-related accumulation. No differences (p<0.05) were found between sexes within each age class. Organ burden distribution followed the same pattern for all metals and age classes: liver tissues contained maximum burdens. Mercury concentrations were higher than those of cadmium in both foetuses and newborns; and neither metal could be detected in the foetus. The analysed data suggested differences in the placental transference between metals, being significant for mercury and almost null in the case of cadmium. We can conclude that franciscana accumulates heavy metals and, due to its coastal distribution, it may be considered as a biomonitor of its environment.
Highlights
Heavy metals are natural elements whose biogeochemical cycles have been altered by man
In all the heavy metals studied, no significant differences in mean concentration were found between sexes within the same age class (Table 2)
Das et al (2002) reported high cadmium and mercury levels in liver and kidney of Lagenorhynchus acutus associated with the presence of metallothioneins
Summary
Heavy metals are natural elements whose biogeochemical cycles have been altered by man. The principal consequence of such alteration is the increased bioavailability of several metals (Förstner and Wittmann, 1983) This situation is closely related to physico-chemical environmental changes that have led to the alteration of the bioavailability of the chemical forms of heavy metals, known as speciation. Some metals, such as copper and zinc, are essential for metabolic functioning, whereas others, such as cadmium and mercury, are highly toxic to aquatic organisms and have no known biological activity. Zinc is principally a cofactor of several enzymes (e.g. carbonic anhydrase, alkaline phosphatase, DNA and RNA polymerase) Despite their biological function, both metals can produce toxic effects if their levels exceed those required for normal physiological functioning
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