Pollutants in polar regions have long been an important environmental issue. This also applies to mercury, an element that actually occurs naturally, but which has entered the environment in large quantities as a result of industrialisation. The dangerous neurotoxin is also slowly accumulating in the Southern Ocean. A study by an international research team focused on humpback whales as a bioindicator species and made a surprising discovery.
The team led by lead author and principal investigator Dr. Susan M. Bengtson Nash concluded that the humpback whales studied deposited only small amounts of mercury in their adipose tissue. At the same time, however, the amounts in the samples from muscle tissue were the second highest in the study and were more than a hundredfold higher than the values from blubber or skin. Only Antarctic fur seals as top predators had shown higher values in comparison. This is very surprising, since mercury is actually lipophilic and tends to accumulate most in the correspondingly fatty tissues. “The low level of mercury found in the whales’ skin and blubber, and the apparent lack of bioaccumulation between Antarctic krill and these tissues in humpback whales, likely masks preferential storage of mercury in other body tissue like the muscle,” explains lead study author Dr. Bengtson Nash, who works at Griffith University in Australia.
Mercury is actually a naturally occurring heavy metal, but even in small amounts it can cause severe damage to the nervous system. As a result of industrialisation, large quantities have been and are still being released into the environment via the atmosphere, especially into the waters and thus also into the Southern Ocean. The depletion of ozone and the stagnation of large air masses play an important role in how mercury reaches the Southern Ocean. There it is converted by bacteria into a form that makes it fat-loving and thus enters the food chain via krill.
The new study now examined for the first time how mercury accumulates in different stages of the food web and whether humpback whales are a suitable indicator species, a so-called bioindicator, for monitoring mercury levels in the Southern Ocean. This is because the UN, following the Minamata Protocol on the Monitoring of Mercury, is calling for a global monitoring network that also includes the Southern Ocean. “We need systems in place for long-term monitoring of mercury levels in the environment, but these are challenging to effectively implement in the remote Antarctic region,” explains Dr Bengtson Nash. “How this neurotoxin cycles between the living organisms and the environment in the remote Antarctic region is poorly understood, particularly in the context of a rapidly changing Antarctic climate.” Humpback whales are a possible indicator species because of their migratory movements from different regions, and they are also easy to sample. Skin and fatty tissue can be removed relatively easily and with little harm to the animal and examined for mercury. At least that’s the theory. But the results of the study show that the matter is not as simple as this. Because instead of the blubber, the muscle tissue of beached whales showed the higher levels of mercury. “That means, before we can use non-invasive skin and blubber samples from humpbacks as to evaluate mercury accumulation, we need an accurate understanding of how mercury distributes throughout the whales’ various body tissues, in order to relate these back to levels in the blubber,” explains Dr Bengtson Nash.
The international team’s study compared the humpback whales with other Antarctic species. The highest amounts were found in Antarctic fur seals, followed by Weddell seals. Adélie penguins and other krill-eating seabirds also showed higher levels than humpback whales, although both directly feed on krill and are thus on the same food web level. Overall, however, it was found that the amounts of mercury detected in the animals were still below the limits of concern for humans and animals. But this could change with new climatic conditions, warns Dr Bengtson Nash. For humpback whales, the findings mean more research is needed. The whale species is used already as a indicator for the Antarctic marine ecosystem, in the Humpback Whale Sentinel Program. The aim is to draw conclusions from the condition of the humpback whales to the entire Antarctic marine ecosystem. The accumulation of pollutants in the animals is definitely part of this, because Antarctica is not as isolated as we think.
Dr Michael Wenger, PolarJournal
Link to the study: Bengtson Nash et al. (2021) Mar Poll Bul 172, Mercury levels in humpback whales, and other Southern Ocean marine megafauna; https://doi.org/10.1016/j.marpolbul.2021.112774
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