The concentration of potentially toxic metals is increasing in the population of the Franciscan dolphin, a small cetacean endemic to the Rio de la Plata and in serious danger of extinction. This is confirmed by a work led by a team from the Faculty of Biology and the UB Biodiversity Research Institute (IRBio) which has been published in the journal Science of The Total Environment .
The impact of human activity in the region could be the cause of the increase in trace elements such as chromium, copper, iron and nickel in dolphins ’biological tissues, the study notes. The work, in which members of the National Museum of Natural History of Uruguay have also participated, is subsidized through a project of the research and conservation program of the Barcelona Zoo Foundation. The principal investigator is Massimiliano Drago (UB-IRBio).
One of the smallest and most endangered species of dolphin in the world
The Franciscan dolphin ( Pontoporia blainvillei ) is an endemic species of the marine regions of Brazil, Uruguay and Argentina and is considered vulnerable according to the International Union for Conservation of Nature (IUCN). It is currently considered the most endangered cetacean in the southwestern Atlantic Ocean, and its population has declined due to bycatch that accelerated in the mid-20th century by artisanal shark fishing to extract it. Today, the future of the species is still in danger due to accidental fishing - which causes the death of between 1,200 and 1,800 dolphins each year, especially juveniles - and the progressive degradation of the natural environment due to impact of maritime traffic, tourism and environmental pollution.
The Rio de la Plata: biological productivity and anthropogenic pollution
The Rio de la Plata estuary, on the west coast of the southwest Atlantic, is one of the richest and most productive ecosystems on the planet. It is a marine region heavily affected by anthropogenic activity (maritime traffic, industries, expansion of urban areas, untreated wastewater, etc.), which favors the accumulation of pollutants. In addition, the estuary also discharges pollutants transported by the hydrographic network of effluents from the Paraná and Uruguay rivers and other secondary rivers. With more than three million square kilometers of extension, this immense hydrographic system transports a large volume of highly polluted bodies of water due to its passage through large cities and highly urbanized regions of the South American continent.
In the inner area of the estuary, the most polluted, there is an abundance of mainly fresh water from effluents and waste from large cities nearby (Buenos Aires, Montevideo, La Plata, etc.). The intermediate zone has fresh water with marine influence and is less polluted, while the outer zone has brackish water with a salinity gradient. The sea currents generated by the tidal regime in the estuary drive the inflow of seawater to the intermediate zone and the outflow of fresh water to the outermost section.
Analysis of trace elements in the bone remains of dolphins
Contaminants such as polychlorinated biphenyls, pesticides, hydrocarbons, plasticizers or some metals can be endocrine disruptors, carcinogens, or cause adverse reproductive effects or osteoporosis, among other problems. Of the pollutants dumped in the estuary, "trace elements are of particular concern, such as certain heavy metals that can be highly toxic to marine life and, indirectly, to humans," says Odei Garcia-Garin, first author of the article and member of the GRC of Great Marine Vertebrates, led by Professor Àlex Aguilar.
The work analyzes the concentration of trace elements in Franciscan dolphin bone remains of the Rio de la Plata during the period 1953-2015. According to the results, the concentration of chromium, copper, iron and nickel has increased in these sixty-two years, while the levels of lead have been reduced in the bone remains of the dolphins.
Anthropogenic activities could be the source of the increasing concentration of metals in marine mammals, the study notes. Trace elements from the waste of the leather industries, oil refineries or ship paints would gradually accumulate in the sediments of the estuary of the Rio de la Plata, and finally in the tissues of the Franciscan dolphins. In contrast, the ban introduced in the 1990s to use lead as an additive in fossil fuels has led to a gradual reduction in the concentration of this metal in dolphin bones.
The study also finds a temporary increase in aluminum and manganese concentrations, and in parallel, a decrease in arsenic and strontium concentrations. These temporal trends are difficult to relate to anthropogenic pollution and will require further studies in order to draw conclusions. The results also indicate a higher concentration of aluminum, iron and chromium in the case of females, although the differences do not seem very significant.
The work published in Science of The Total Environment confirms the suitability of trace element studies on bone remains preserved in museums or private collections for large-scale temporary studies. Using this methodology, both the impact of pollutants on a species in time series and the evolution of compounds in the environment can be analyzed.
Protect the Franciscan dolphin to conserve the marine ecosystem
The Franciscan dolphin is an apical marine predator and plays an essential role in the marine ecosystem. It modulates the abundance of various species — fish, octopuses, shrimp, etc. — that occupy an intermediate trophic level in the ocean. Therefore, if the Franciscan dolphin population decreases, the entire marine food web in the estuary would be completely altered. “In addition, Franciscan dolphins act as an umbrella species. In other words, the fact of protecting their populations would benefit many other species whose viability depends on the presence of the Franciscan dolphin in the marine ecosystem ", underlines Odei Garcia-Garin, member of the Department of Evolutionary Biology, Ecology and Sciences Environmental and IRBio.
"To improve the survival of the species, bycatch should be significantly reduced as a first urgent measure," Garcia-Garin continues. "Because juvenile individuals are most affected, it would be important to implement closed periods during the breeding months, the most critical for the species." Empowering fish farms would also help reduce bycatch, although this measure could have other negative effects on the marine environment (eutrophication and pollution from waste produced by fish farms, for example). “It would be convenient to create or expand the marine reserves where the species lives.