SAVANNAH, Ga. – As government engineers work to keep the nation’s fourth-busiest seaport from losing its competitive edge, they are also planning what amounts to a massive science project to ensure fish in the harbor can still breathe.

When the Savannah harbor is deepened to allow for supersized cargo ships, the U.S. Army Corps of Engineers wants to install a dozen 20-foot-tall steel cones that suck water from the river, swirl it with oxygen and then pump it back to compensate for an expected drop in dissolved oxygen.

Buying and installing the machines costs a hefty $70 million, plus yearly operating costs of $1.2 million. And the manufacturer says they’ve never been used for a project of this scale.

The oxygen machines are a piece of the $653 million proposal to dredge the Savannah River shipping channel to the Port of Savannah, a project that officials hope will win final approval later this year.

East Coast seaports are scrambling to deepen their harbors to accommodate supersized cargo ships expected through an expanded Panama Canal in 2014.

Deeper water will mean less oxygen toward the river bottom for bacteria, worms, shrimp, crabs and fish. But some scientists aren’t sure the machines will be able to boost low oxygen levels as planned along 27 miles of the river.

“It’s like putting the river on a respirator,” said Chris DeScherer, an attorney for the Southern Environmental Law Center, which has sued on behalf of environmentalists who say the project would do irreparable harm.