Cesium levels in water, plankton baffle scientists

by Mizuho Aoki

Staff Writer

Plankton and seawater sampled at 10 points less than a year after the Fukushima meltdowns found concentrations of radioactive cesium were highest at different locations in the Pacific, puzzling scientists.

The group collected zooplankton and surface seawater at 10 points between Hokkaido and Guam, 500 to 2,100 km from the crippled power plant, between Jan. 14 and Feb. 5, 2012.

Cesium 134, with a half-life of two years, and cesium 137, with a half-life of 30 years, were detected in plankton and seawater at all 10 locations, according to their report. Plankton with the highest concentrations of cesium 134, at 10.5 becquerels per kilogram, and cesium 137, at 14.9 becquerels, were found around 25 degrees north latitude and 150 degrees east longitude, the report said. The samples were taken from the surface to a depth of 200 meters.

“Plankton are thought to play a key role in the dispersion of the cesium because they are eaten by bigger fish. We want to study further what is influencing the accumulation of radioactive cesium,” said Minoru Kitamura, a marine ecologist and senior researcher at the Japan Agency for Marine Earth Science and Technology, who led the group.

The plankton could have been contaminated by eating even smaller plankton and through seawater, Kitamura said.

Meanwhile, the concentration of radioactive cesium in surface seawater was highest — at 41.5 becquerels per kilogram — from around latitude 36 to 39 degrees north, where the Oyashio Current meets the Kuroshio Current from the south, the report said.

The amount of radioactive cesium in seawater at other locations was low, it said.

Kitamura said they haven’t figured out why the concentrations of contamination differed in plankton and seawater.

“Our concern is the high level of (radioactive cesium found in plankton) taken from waters around latitude 25 degrees north, and we don’t know why the level got high around that area,” Kitamura said. “We need to study whether the concentration will decline, or stay the same.”

The finding was released at the Japan Geoscience Union at the Makuhari Messe international convention center in Chiba Prefecture on Tuesday.

Kitamura said he and his fellow members plan to conduct a followup study in July in the Pacific.

He said they have no plans to study contamination of bigger ocean fish because they lack the equipment to catch them in statistically significant amounts.

The survey was conducted alongside a study on ocean circulation by another group on board.

Information from Kyodo added

  • Peter Nichols

    The article says ” they have no plans to study contamination of bigger ocean fish because they lack the equipment to catch them in statistically significant amounts”. But surely the annual catch of squid that migrate north on the Kuroshio current and caught between Hokkaido and Honshu would provide ample volume to be statistically significant.

    • Starviking

      But the problem then would then be: where on their migration did the Squid pick up the most contamination?

      • Peter Nichols

        The migration path is known. It runs the length of the east coast of Japan. If the concentrations of cesium in the squid exceeds the health standard and the radiation signature matches Fukushima then the economic impact and the public concern from the finding would surely justify more detailed research funding. Conversely, if little cesium is found then the squid have helped to survey the length of migration area.

      • Starviking

        A survey would involve taking samples along the length of the migration area – that certainly is not possible with the method suggested. As for caesium levels in squid, it would be a good idea to keep an eye on them. There was an earlier report on cesium in plankton on JT which gave the caesium levels as around a max of 10 bq per kg, which is well under the strict Japanese limit.

      • Peter Nichols

        “A survey would involve taking samples along the length of the migration area” – that would not be necessary. The normal catch around the Tsugaru Strait would provide the sample. The squid have travelled the length of the survey area. If the levels of radiation is safe, then no one point of the survey area is bad enough to be a problem for at least squid. If certain squid have levels that exceed safe human consumption, then none of the squid should be sent to market until more research is done to isolate the problem.

      • Starviking

        We may be thinking of different types of survey – I’m thinking of a traditional geographic one where data is gather from different points.

        Your squid idea is nice, but I think it would only hold if there was massive contamination, as different sea animals absorb and expel contaminants at different rates.

  • Masa Chekov

    They don’t seem “baffled” – it seems quite clear they haven’t studied the reasons thoroughly yet. I’m sure the further studies will clarify this more.

  • Daniel Qualkinbush

    “We need to study whether the concentration will decline, or stay the same.”

    Seems Kitamura left out “or increase”

  • Chernobyl Children Fukushima C

    How do Radionuclides travel?

    As every other substance travels! FOOD CHAIN!

    Plankton takes in 200,000th of a radioactive substance. This is know for decades, what are the scientists doing? Sleeping?

    Total release of Fukushima: 40,000 trilltion becquerel – 70% went into sea. 2013: 47 quadrillion becquerels of cesium-137 released into Pacific from Fukushima — Nearly 50 times original Tepco estimate.

    The specific activity of 1 g of Cesium 137 (or 0.035oz) is 3,200,000,000,000 Becquerel.

    2013: Tokyo University of Marine Science: 8,000,000,000 ~ 93,000,000,000 Bq of Cs-137 can leak to the sea every single day.

    up tp 0,03 g Cesium 137 PER DAY into sea fro Fukushima.

    f the cesium in the fresh water happens to be radioactive cesium-137,
    from a nuclear reactor or other source, then the fish will contain 1000
    times more cesium-137 than the fresh water itself, on a
    weight-for-weight basis.