No radiation from the crippled Fukushima nuclear reactors was detected in a high-tech analysis of kelp samples collected along the West Coast this spring, including two sites in Mendocino and Marin counties.

But scientists participating in the Kelp Watch 2014 program said the absence of the telltale isotope — Cesium-134 — was not surprising, and results reported this week will establish a “baseline” for radiation already present in coastal waters.

A radioactive plume stretching 5,000 miles across the Pacific Ocean from Japan is expected to reach North America sometime this year and follow-up collections of kelp samples in July and October are intended to measure it.

“It’s out there, but it’s very diluted and it hasn’t rubbed up against our shoreline yet,” said Steven Manley, a marine biologist at Cal State Long Beach and co-leader of Kelp Watch 2014.

The program’s first batch of 26 kelp samples, collected from Kodiak Island, Alaska to Baja California and beyond, included two from the North Coast: at Van Damme Beach in Mendocino County and off the Point Reyes Peninsula in Marin County.

Kelp, a type of marine algae found in abundance along the coast, is considered a biological sentinel because it absorbs and concentrates the elements in seawater, including radiation.

All the West Coast samples included low levels of Cesium-137, which has a 30-year half-life and has lingered in the Pacific Ocean since the nuclear weapons tests of the 1950s and ’60s.

But Cesium-134, with a half-life of only two years, can be traced directly to the Fukushima reactor meltdown in March 2011.

The absence of Cesium-134 in the kelp is “pretty much the key” to a conclusion that contamination from Fukushima is still on its way, Manley said.

Seaweed samples from Hawaii and Guam also came up free of Fukushima radioactivity, he said.

“Science can answer our questions,” said Laura Rogers-Bennett, a California Department of Fish & Wildlife environmental scientist based at the Bodega Marine Laboratory. “Instead of worrying about it (contamination from Japan), we can go out and test it and find out if it is a concern or not,” she said.

Estimates of the concentration of Cesium-137 that might reach the West Coast range from a level equal to what’s in the water now to a level 10 to 30 times higher, Manley said.

By comparison, the concentration of naturally occurring Potassium-40 in seawater is 10,000 times higher than the baseline level of Cesium-137, he said.

Scientists refer to the potassium isotope as “primordial,” meaning it has been around since the Earth formed and infuses all living things, including humans.

Rogers-Bennett and a colleague collected 14 pounds of giant kelp, a perennial species, from a cove at Van Damme Beach in February, kicking off Kelp Watch 2014.

A sample of bull kelp was collected by a National Park Service team in early March near the northern tip of the Point Reyes National Seashore.

All of the program’s kelp samples were dried and ground and sent to the Lawrence Berkeley National Laboratory in Berkeley for analysis by $80,000 gamma-ray spectrometers to determine their radioactive content.

“The results should assure the public that our coastline is safe and that we are monitoring for these materials,” Manley said.

Monthly samples will be taken from Vancouver Island, Canada, in the area where the radioactive plume from Fukushima is expected to hit North America, he said.