WASHINGTON — As one of the final acts of his presidency, President Obama added roughly 6,000 acres to the California Coastal National Monument on Thursday, bringing the total acreage of public land he has protected to an astonishing 550 million acres, double the amount protected under famously conservation-minded President Teddy Roosevelt.

The California Coastal National Monument includes the state’s near-shore waters, rocks, reefs and other areas along its 1,110-mile coastline, adding to the 1,665 acres in the Point Arena-Stornetta area north of San Francisco that Obama established as a monument in 2014.

The newly protected areas include five onshore sites: the Lost Coast Headlands (440 acres), Trinidad Head (13 acres) and the Waluplh-Lighthouse Ranch (8 acres) in Humboldt County; the Cotoni-Coast Dairies in Santa Cruz County (5,780 acres); and the Piedras Blancas lighthouse in San Luis Obispo County (20 acres).

The expansion also includes a group of small rocks and islands off the coast of Orange County that the Coast Guard once considered using for lighthouses.

Obama made the designations under the Antiquities Act, a 1906 law that gives the president power to protect public lands of scientific, cultural or scenic importance. Republicans have strongly opposed his use of the law, which included a designation of 1.8 million acres of desert lands in California’s Mojave Desert that had been stymied in Congress for years.

Obama’s recent designation of the 1.35 million-acre Bears Ears National Monument in Utah has particularly angered Rep. Rob Bishop, R-Utah, chairman of the House Natural Resources Committee, who has threatened to attempt to reverse several Obama monument designations. The Antiquities Act does not have a provision for reversal, and no president has ever revoked a previously designated monument but Congress has the power to do so.

The California Coastal Monument was first established in 2000 by President Bill Clinton. California Sen. Dianne Feinstein and recently retired Sen. Barbara Boxer, along with Rep. Jared Huffman of San Rafael, all Democrats, had sponsored legislation to protect the area, but were blocked by the GOP-controlled Congress.

The additions Thursday also included a monument in Oregon and, elsewhere, several monuments preserving cultural landmarks, from the Civil War to the civil rights movement, as part of his effort to include minorities in the public lands designations.

“These monuments preserve the vibrant history of the Reconstruction Era and its role in redefining freedom,” Obama said in a statement. “They tell the important stories of the citizens who helped launch the civil rights movement in Birmingham and the Freedom Riders whose bravery raised national awareness of segregation and violence. These stories are part of our shared history.”

Obama earlier had designated the nation’s first monument honoring the LGBT movement with the Stonewall National Monument in New York City, and the Cesar Chavez National Monument in Keene (Kern County).

“I have sought to build a more inclusive National Park System and ensure that our national parks, monuments and public lands are fully reflective of our nation’s diverse history and culture,” Obama said.

In a statement, Feinstein applauded the inclusion of the California coastal and Cascade-Siskiyou national monuments. “The Antiquities Act is a vital conservation tool, and I’ll do all I can to preserve it for future administrations,” she said.

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. Email: clochhead@sfchronicle.com Twitter: @carolynlochhead