A radioactive cloud spread over large parts of Europe after the 1986 Chernobyl accident. The amount of radioactivity in the Baltic Sea’s food web is still higher today than it was before the accident.

Traces from Chernobyl still found in seals

New research suggests that marine mammals have stored radioactive caesium in their bodies after the Chernobyl accident to a greater extent than their prey. This has been demonstrated by a 30-year time series analysis recreated with the help of museum collections.

On the night of Saturday, 26 April 1986, a reactor accident occurred at the nuclear power plant in Chernobyl in the former Soviet Union. A radioactive cloud spread over large parts of Europe, reaching the Baltic Sea, among other places. Substances spread by the cloud included the radioactive isotope caesium-137.
‘Caesium is troublesome because it can accumulate in the body. It is chemically similar to potassium and is absorbed in the same way’, says Sadaf Saremi.

She has just completed her master’s thesis in which she and her supervisor, senior lecturer Karin Hårding at the Department of Biological and Environmental Sciences, have analysed muscle samples from 23 seals from the environmental databank of the Swedish Museum of Natural History. The samples were collected from 1985 onwards. They are part of the museum’s systematic programme to preserve tissue samples from seals captured as by-catch or shot to protect fishing gear.
‘We know that these kinds of fallout accumulate in fish, but we don’t know much about how they accumulate in mammals’, says Saremi. Seals are excellent subjects for studying environmental toxins, partly because they are at the top of the food chain and partly because they can live relatively long.’

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Sadaf Saremi has analysed muscle samples from 23 seals from the Swedish Museum of Natural History’s environmental databank.

WHEN THE RADIOACTIVE fallout from Chernobyl reached the Baltic Sea, it was absorbed by nature and stored in herring and other marine life. Herring is important prey for seals, and the caesium accumulated incrementally in the seals’ muscles. The highest levels of caesium were measured in the water and in herring relatively soon after the emission, while the caesium levels in seals peaked around 1990. After that there was a slow downward trend, with the amount of caesium in seals diminishing slightly year after year.
‘Caesium-137 has a half-life of 30 years, so it’s estimated that half of the radioactive caesium will be gone by 2020. In addition, caesium-137 is being transported out of food chains because of sedimentation. But the amount of radioactivity in organisms in the Baltic Sea ecosystem — that is, the food web — is still higher than it was before the Chernobyl accident.’

SAREMI EXPLAINS that there are two main ways in which humans have spread radioactivity in the environment in and around the Baltic Sea: fallout from atmospheric nuclear weapons tests and the Chernobyl accident. Of the two, Chernobyl accounts for an estimated 83 per cent of the radioactive caesium found in the Baltic Sea today.

Analyses of the seals’ muscle samples were done by Mats Isaksson, Professor of Radiation Physics at Sahlgrenska Academy, using a germanium detector. At present, the number of studies of this kind that have been done on marine mammals is extremely limited, and Mats Isaksson sees several opportunities for using the results in the future. He is among those involved in a Nordic project in which he and his colleagues have been working for several years with models to describe the movement of radioactive materials in the ocean.
‘For example, what happens if there is an accident involving a submarine armed with nuclear weapons? What would that mean for marine mammals? I would like to link with this model in that case and compare it with our experimental data.’

SCIENTISTS KNOW relatively little about the long-term effects the fallout is having on the seals. Saremi says that there are no physical signs that seals have been damaged by the levels measured.
‘But it’s important that we do this kind of research to see how radioactivity is accumulated in the food web — that is, how high the levels are in marine mammals in relation to the organisms they feed on. This knowledge will contribute a piece to the puzzle to help us understand what consequences a future accident might have.’