Aggressive seaweed smothers remote reefs

HONOLULU – Researchers say a recently discovered species of seaweed is killing large patches of coral on once-pristine reefs and is rapidly spreading across one of the most remote and protected ocean environments on Earth.

A study from the University of Hawaii and others says the seaweed is spreading more rapidly than anything they’ve seen in the Northwestern Hawaiian Islands, a nature reserve that stretches more than 1,300 miles north of the main Hawaiian Islands.

The study was published in the journal PLOS ONE on Tuesday.

The algae easily break off and roll across the ocean floor like tumbleweed, scientists say, covering nearby reefs in thick vegetation that out-competes coral for space, sunlight and nutrients.

“This is a highly destructive seaweed with the potential to overgrow entire reefs,” said biologist Heather Spalding, a study co-author and longtime Hawaii algae researcher. “We need to figure out where it’s currently found, and what we can do to manage it.”

In 2016, government researchers were on a routine survey of Pearl and Hermes Atoll when they found small clumps of seaweed they’d never seen before.

Last summer, they returned to find algae had taken over huge areas of the reef – in some areas covering “everything, as far as the eye could see” – with seaweed nearly 8 inches thick, said Spalding, who was among the divers.

“Everything underneath of it was dead,” she said.

The area was mostly devoid of large schools of tropical fish and other marine life that usually cruise the vibrant reef, and fish that typically eat algae were not grazing on the new seaweed, researchers said. Dives along the outer reef of the 15-mile atoll revealed the seaweed in varying densities and depths.

Scientists say the actual coverage area is likely much larger than documented because they couldn’t survey many sites during their brief visit.

Close to Midway Atoll, site of a pivotal World War II air and sea battle, Pearl and Hermes Atoll is mid-pacific about 2,000 miles from Asia and North America.

The uninhabited atoll is in the 600,000-square-mile Papahanaumokuakea Marine National Monument.

HEATHER SPALDING/COLLEGE OF CHARLESTON VIA AP A thick mat of a new and destructive species of seaweed is held by a diver at Pearl and Hermes Atoll in the remote Northwestern Hawaiian Islands.