LONG BEACH — Fish along the Orange County coast may have been affected by radioactivity that fell on California in the days after Japan’s 2011 nuclear disaster, as small levels of radioactive isotopes accumulated in local seaweed, researchers have reported.

The study poses the possibility that small amounts of Fukushima radioactivity has entered the California coast’s food web, but the Long Beach State marine sciences professor who co-wrote a new study said he does not know if there is a measurable detrimental effect.

The radioactive forms of cesium and iodine “get dispersed over a variety of organisms,” including fish, said marine biology professor Steven L. Manley. “I would assume it’s there” in the biomass of plants and animals off California’s coast.

“It’s not a good thing, but whether it actually has a measurable detrimental effect is beyond my expertise,” he said in a news statement issued by the Long Beach State public affairs office.

Writing in the scientific journal “Environmental Science & Technology,” Manley and co-author Christopher G. Lowe said researchers measured a radioactive isotope of iodine in kelp within a month after massive radiation leaks were caused by the Japanese earthquake and tsunami.

Acting on a hunch, Manley began monitoring the kelp samples brought in for a different project last year by graduate student Danielle Burnett from Corona del Mar, Laguna Beach and Crystal Cove. Long Beach State’s old radiation counter was not capable of identifying which particular isotopes were giving off electrons, Manley said.

“You have to look for the decay profile,” Manley said. “I’d count them every few days and then you’d see a decay.”

Kelp accumulate iodine 131, a radioactive form of iodine that has a very short half life — less than 10 days. A half life is the time period in which half of a sample of a radioactive element loses its radioactive electrons, which can present a danger to human and animal health.

“I thought, ‘I’ve picked up something. I wonder what it is?’ ” he said. “We let it decay away and calculated the half life, and it was eight days,” he said, “and it was iodine 131.”

The level of radioactivity in the Southern California seaweed was “probably not harmful because it was relatively low levels,” said Manley. But ”it may have affected certain fish that graze on the (seaweed) tissue, because fish have a thyroid system that utilizes iodine.”

Cesium 137 has a half life of 30 years, as opposed to iodine 131’s half life of below 10 days, so it may be present in California kelp to this day, said Manley.

“We were limited in what our instrumentation allows us to do,” he said. “The big question was, `is another major isotope that came over in the cloud, cesium 137, present in the kelp, too?’ ”

Manley and Lowe theorize that the radioactive elements blew across the Pacific in winter storm fronts that lashed the California coast shortly after the Fukushima Daiichi reactors blew apart, in the weeks following the massive March 11, 2011 earthquake and tsunami.

One particular strain of seaweed, Macrocystis pyrifera, is present in large canopies along shallow areas of the California coast. Radioactive rainfall was absorbed by the seaweed before the seawater had a chance to dilute it, the scientists said in their article.

Follow-up work showed varying amounts of low levels of radioactive cesium in seaweed from samples near UC Santa Barbara and UC Santa Cruz, the scientists said. No radioactivity was found in seaweed from Alaska.

Manley noted that future research into cesium accumulation in kelp is needed, but the graduate programs that support such efforts are in jeopardy due to state budget cutbacks.

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