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.