By Roberta Attanasio
Bottlenose dolphins – the playful, intelligent and sleek swimmers frequently seen in warm and shallow waters along coastlines – are important biological indicators or sentinels. As coastal dwellers, they are exposed to pollutants deriving from human activities and, as predators at the top of the food web, they can help evaluate the overall health status of their ecosystems.
In 2011, two teams of researchers published results from their studies on bottlenose dolphins (Tursiops truncates) as indicators of persistent organic pollutants in coastal ecosystems.
Persistent organic pollutants are toxic chemicals recognized as a global threat to human health and the environment. Because they can be transported by wind and water, most persistent organic pollutants generated in one location affect humans and wildlife far from where they are used and released. They persist for long periods of time in the environment and accumulate and transfer from one species to another through the food chain.
One of the two teams of investigators studied dolphins from the Western North Atlantic Ocean and Northern Gulf of Mexico. Results from this study indicated widespread food web contamination. The other team studied dolphins from coastal Georgia, USA, and found that higher concentrations of persistent organic pollutants were present in dolphins closer to the contaminated site. In addition, male bottlenose dolphins had higher concentrations of persistent organic pollutants than any other marine mammal.
Mercury is also a toxic chemical recognized as a global threat to human health and the environment. Mercury exists in various forms, and people are exposed to each of these forms in different ways. Dietary exposure is very common, and it occurs mostly by eating fish and shellfish containing methylmercury. How does methylmercury get into fish? Through a process called “bioaccumulation“.
Microscopic organisms convert inorganic mercury into methylmercury, which is taken up by tiny aquatic plants and animals. Fish that eat these organisms build up methylmercury in their bodies and, as ever-bigger fish eat smaller ones, the methylmercury is concentrated further up the food chain. Concentrations of methylmercury in large fish can be over a million-fold higher than in the surrounding water. As mentioned above, bottlenose dolphins are top level marine predators and, therefore, potentially at risk of exposure to high concentrations of methylmercury.
Now, results from a recent study carried out by researchers at the University of Alaska Fairbanks show that free-ranging bottlenose dolphins from Sarasota Bay, Florida, USA, contain blood amounts of total mercury (methylmercury and its demethylation product) 10- to 100-fold higher than relatively conservative benchmarks established for potential adverse effects in humans. The study, entitled “Distribution of mercury and selenium in blood compartments of bottlenose dolphins (Tursiops truncatus) from Sarasota Bay, Florida” and published in the journal Environmental Toxicology and Chemistry (November 2013), focuses not only on mercury, but also on selenium.
Selenium, an essential element present in marine diets, protects marine predators from mercury toxicity, probably by binding mercury and forming a highly stable, relatively nontoxic complex. Therefore, the researchers determined the amounts of total mercury and selenium present in different blood components (whole blood, serum, plasma, packed cells). Although total mercury was similarly represented in all blood components, selenium was not. The investigators conclude that differential compartmentalization could affect the potential for interaction between the two elements, thus affecting the ability to form nontoxic complexes. The results provide the basis for a newly established system to routinely determine concentrations of total mercury and selenium in the various blood components of bottlenose dolphins.
Randall Wells, Program Manager of the Sarasota Dolphin Research Program — the world’s longest-running study of a dolphin population — is a collaborator for the three studies on dolphins as sentinels for toxic chemicals (persistent organic pollutants and mercury).
In a seminal paper published in 2004 (Bottlenose Dolphins as Marine Ecosystem Sentinels: Developing a Health Monitoring System), Wells and colleagues clearly summarize all concepts discussed above “Bottlenose dolphins are long-lived, long-term coastal residents in tropical and temperate regions throughout the world. Long-term research on such a species allows one to document the history of exposure to ecosystem perturbations and their effects. They are top-level predators on a wide variety of fishes and squids, and thus concentrate contaminants through bioaccumulation and integrate broadly across the ecosystem in terms of exposure to environmental impacts. Dolphin health and population status not only reflect the effects of natural and anthropogenic stressors on the species, but they serve as sentinels of the health and status of lower trophic levels in the marine ecosystem.”