Dolphins, whales and human pollution

The dolphins of the Everglades National Park are less impacted by organic pollutants than their fellow creatures on the south coast of Florida. However, they are far more contaminated by mercury!

The Everglades National Park , inscribed on the UNESCO World Heritage List in 1979, was also designated a wetland of international importance in 1987. It is home to a large number of species including the common bottlenose dolphin, Tursiops truncatus (Flipper!). As a top predator, this mammal is high up in the food webs. Consequently, it has the misfortune to accumulate high concentrations of toxic pollutants in its tissues (e.g. mercury and persistent organic pollutants - POPs- such as PCBs). So what about the dolphins in Florida?

This question was the subject of international research involving the collaboration of several researchers from the University of Liège, including Krishna Das, F.R.S-FNRS senior research associate (Laboratory of Oceanology, University of Liège). These results have just been published in the journal Environmental Pollution (1) in collaboration with Florida International University (Prof. J. Kiszka) and the University of Groningen (Prof. M. Fontaine), among others. The first author of the article is France Damseaux, FNRS research fellow at the Laboratory of Oceanology, who did her master's thesis in biology on the subject. This research aimed to determine concentrations in total mercury (T-Hg) and in certain organic pollutants in the common bottlenose dolphin in South Florida. Skin and fat biopsies were performed on dolphins from the Everglades National Park and Key West (in the Keys - small islands in South Florida) in order to understand the influence of their habitat on their levels and profiles of contamination. 

As expected, the dolphins tested in the Keys have PCB concentrations that are three times greater than the dolphins tested in the Everglades National Park. The impact of pollution on the environment of the Keys is far from negligible: there is a great deal of industrial activity, atmospheric pollution, tourism, etc. The good news is, though, that these concentrations are nevertheless lower than those observed in other populations of common bottlenose dolphins in the south-east of the United States, closer to large industrial hubs. It is worth noting that various clean-up measures were also implemented in 2000 in order to protect the ecosystems of the Everglades and the Keys.

However, contrary to organic pollutants, the concentrations in total mercury in the skin were three times higher in the dolphins from the Everglades compared with the dolphins from the Keys. As far as we know, these concentrations are among the highest ever recorded in common bottlenose dolphins, and can be explained by the biogeochemistry of the Everglades' mangroves and the historical contamination of this now protected site (the mangrove contains a lot of organic matter whose bacteria absorbs and retains the mercury discharged from coal plants in particular).

Humpback whales from the warm waters of the Indian Ocean reveal pollution from... Antarctica! 

Humpback whales are known for travelling far and wide. They can be found in all the planet's seas, including the Indian Ocean. They come to the warm waters of Reunion Island between the months of June and November to give birth and nurse their young. For the rest of the year, the whales are present in the nearshore waters of Antarctica, more than 8000 km away from Reunion Island. Here, the whales feed in the cold waters, mainly on krill, a small species of crustacean that is highly prized by these great mammals. Krishna Das, in collaboration with the Groupe Local d'Observation et d'Identification des Cétacés (GLOBICE, Dr. V. Dulau) and the Toxicological Centre at the University of Antwerp (Prof. A. Covaci), among others, focused on the exposure to pollutants of the tested humpback whales around Reunion Island (2). 

As humpback whales mainly feed in the Southern Ocean (and not in the Indian Ocean), they accumulate the pollutants present in Antarctica in their tissues. Despite Antarctica's distance from any human activity, we can nevertheless detect industrial pollutants and pesticides used many thousands of kilometres away in many marine species. 

And the humpback whales of the Indian Ocean are sadly no exception. The researchers found detectable concentrations of pesticides in their blubber, as well as molecules used by industry (polychlorinated biphenyls, PCBs) and flame retardants (polybrominated diphenyl ethers, PBDEs). The use of these pollutants is forbidden but these organic molecules are highly resistant, don't disintegrate and accumulate in living organisms. 

However, the humpback whales of the Indian Ocean are less contaminated than their fellow creatures of the northern hemisphere because they mainly consume krill and consequently occupy a lower position in the food web (whales from the north also eat fish). Furthermore, the humpback whales of the southern hemisphere are further away from human activity, mainly based in the northern hemisphere. 

Another interesting fact is the variability of pollutant concentrations in whales from the southern hemisphere. When comparing the results with several earlier studies, the researchers noticed that despite lower contamination levels in the Antarctic ecosystems, some whales showed relatively high levels. There are a number of reasons: age, sex, diet, and the redistribution of organic pollutants during their migration and when giving birth and nursing. The humpback whales of the southern hemisphere don't feed at all, or very little, during their stay in the waters of Reunion Island and during their return journey. A true physiological challenge and a rather unique toxicological model! 

(1) Spatial variation in the accumulation of POPs and mercury in bottlenose dolphins of the Lower Florida Keys and the coastal Everglades (South Florida), Damseaux F. et al. Environmental Pollution, Jan. 2017.

(2) Linking pollutant exposure of humpback whales breeding in the Indian Ocean to their feeding habits and feeding areas off Antarctica, Das K. et al. Environmental Pollution, Jan. 2017.