SAN ONOFRE -- On the surface, it just does not make much sense. A yellow bulldozer pushes boulder after boulder into the Pacific Ocean like a bored kid chucking rocks into a lake.

But, for the San Onofre artificial reef project, magic happens as the rocks pile up on the sandy bottom at a depth of 30 to 40 feet.

Soon, kelp spores will drift onto the submerged rock piles from the nearby San Mateo kelp beds.

They will use the submerged structures as a foothold for a new 150-acre underwater forest that marine biologists hope will soon house a complete ecosystem of sea creatures, from lobster to kelp bass.

Kelp is the key.

"This is a good kelp year, we're seeing a lot of spores, and we think it should take about two years to grow once we're finished building," said Pat Tennant, a marine biologist who works for Southern California Edison, the power producer building the $40 million kelp project.

A total of 100 tons of rock, all mined on nearby Catalina Island and hauled to the reef site on barges, will be placed by summer's end. Tennant said he expects to see kelp and algae blooms on the submerged rock piles by the end of the year, and full-grown giant kelp on the surface within two years.

Edison, the majority owner and operator of the nearby San Onofre Nuclear Generating Station, is building the reef as state-mandated penance for using seawater to cool the plant's twin nuclear reactors. Every day, San Onofre pulls 2.5 billion gallons of saltwater from the ocean through its enormous intake pipes. The water cools San Onofre's innards before being discharged into the ocean through a second set of pipes placed farther out on the ocean floor.

Killing kelp

The outlet pipes tend to stir up sand and other material on the ocean bottom. A 15-year study that measured how the plant's cooling system would affect the ocean environment concluded that stirring up the bottom can cloud the seawater, block sunlight and stunt kelp growth.

Originally, Edison was to build a 300-acre artificial kelp reef, but the California Coastal Commission later cut the size of the reef to only 150 acres, after subsequent studies found that the plant was not killing as much kelp as expected.

In addition to kelp, Edison also kills fish and fish eggs. To compensate for that, the Coastal Commission forced Edison to create a white sea bass hatchery in Carlsbad and restore 150 acres of wetland habitat at the mouth of the San Dieguito River in Del Mar. San Onofre also was required to add "fish ladders" that separate live fish from incoming cooling water and dump the fish back into the ocean before they can be killed in the intake pipes.

In 1999, Edison created a 22-acre test reef that consisted of 56 rock piles scattered on a section of ocean bottom south of the San Clemente Pier and north of San Mateo Point near the San Diego and Orange County border.

After dumping tons of rock on the ocean bottom, Edison hired marine biologists to regularly dive on the reefs and monitor the plants and animals that appeared. A study published by Edison in 2005 found that kelp spores took hold in the test plot quickly and attracted tons of fish and other bottom-dwelling invertebrates like spiny brittle stars and lobster.

The new project will simply enlarge Edison's test plot.

The 150 acres of artificial reef will not be one large circle of rocks. Instead, smaller reefs will be scattered inside an 862-acre parcel of ocean bottom that Edison has leased from the California State Lands Commission. Edison officials say there are only certain locations inside the large parcel that are ideal for kelp gardening.

Water depth should range from 30 to 45 feet. The bottom should be sandy, but not too sandy.

"We are looking for areas with a sandy bottom that is only about 3 meters deep, because we don't want the rocks to get covered when a storm comes along," Tennant said.

Precision counts

Edison divers mapped out sections of the parcel that meet depth and sand criteria using global positioning equipment, which pinpoints specific spots on the planet by measuring distance from orbiting satellites.

A large barge that can be seen sitting about a half-mile offshore uses the digital map, and a complex six-point anchoring system, to position itself directly over target reef plots on the ocean bottom.

"When you look at it from up here on the surface, it looks like they're just dumping a bunch of boulders into the ocean," Tennant said. "But it's actually very precise. When they built the test reef, they found that they could hit the targets within a few inches."

The resulting kelp forest will be open to divers and fishermen for recreation year round.

"We want to create a self-sustaining reef and have it available to the public," Tennant said.

Long-term benefits questioned

It remains unclear whether the new kelp forest will have staying power.

Paul Dayton, a professor of marine ecology at Scripps Institution of Oceanography in La Jolla, said artificial reefs that support kelp forests often have short life spans. He noted that, in addition to water depth and the makeup of the ocean bottom, giant kelp needs nitrogen-rich water in order to thrive.

"We have a few artificial reefs down here in La Jolla that had kelp at one time, but now it's all gone. What happens is, the kelp ends up getting eaten by sea urchins and there is not enough nitrogen in the water to keep it going," Dayton said. "My guess is that 10 years down the road, there won't be a lot of giant kelp out there."

But Dayton added that the reefs will still create an ocean ecosystem that will be home to lobsters, sea fans and many other bottom-dwelling fish that do not depend on kelp.

"There are actually not that many species that really depend on giant kelp for survival," Dayton said. "They're going to create a bottom topography that will be just great for a whole lot of critters, even if the kelp doesn't stay."

Contact staff writer Paul Sisson at (760) 901-4087 or psisson@nctimes.com.