Another growing threat to Hawaii’s coral reefs: Invasive algae

[USA] Several years ago, the state stopped the infestation of leather mudweed by pulling up 3 million pounds of the algae. Now it’s back.

Hawaii’s corals appear to have been spared this summer from another mass bleaching, a stress response caused by warmer waters that has ravaged reefs in recent years.

But they haven’t been so lucky with another emerging threat.

An invasive algae called leather mudweed is rapidly spreading in places where it had been mostly removed and has been found in new areas around Oahu, according to a site survey last month by the state Department of Land and Natural Resources Division of Aquatic Resources.

“It’s really kind of scary,” said Bruce Anderson, who heads the agency.

It’s unclear what the plan of attack will be at this point but the options are limited.

Managing the roughly 410,000 acres of living reef in the Main Hawaiian Islands is a perpetual challenge. There are technical hurdles in some cases, such as removing the mudweed, and political barriers in others, like establishing marine protected areas.

Many reefs in the state — especially those around Oahu, by far the most populous island with 1 million residents and more than 5 million visitors annually — are severely degraded from overfishing, polluted runoff and global factors that are causing a warmer, more acidic ocean.

The reefs play a critical role, such as protecting coastal communities from storm surges and providing homes for fish that people depend on for food and tourism.

Hawaii Department of Land and Natural Resources Director Suzanne Case, left, looks over her notes before a January press conference about monk seals as DLNR Division of Aquatic Resources Administrator Bruce Anderson speaks at Waikiki Aquarium.

DLNR, for a time, was able to get a handle on a mudweed infestation in Maunalua Bay on the eastern side of the island with the help of nonprofits and a $3.4 million grant from the National Oceanic and Atmospheric Administration.

“It’s really kind of scary.” — State Aquatics Resources Administrator Bruce Anderson, on the leather mudweed infestation.

Known as “The Great Huki,” volunteers pulled 3 million pounds of leather mudweed in 2010. Removing it by hand was incredibly labor intensive, Anderson said.

Unfortunately it has grown back. Dozens of volunteers returned to the bay’s shallow waters this summer, pulling thousands of pounds of the leather mudweed and another invasive, a prickly seaweed called gorilla ogo.

Now the aggressive mudweed is also at Bellows on Oahu’s windward side, Barbers Point on the south shore and in the northern portion of Kaneohe Bay. It’s also at depths of up to 100 feet, which had not been seen before. The algae thrives in degraded marine environments, Anderson said.

The invasive algae blankets reefs, depriving the corals from the sunlight they need to survive. It also outcompetes native algae and seagrasses while trapping sediment to form a muddy layer on the sand, according to the University of Hawaii’s Botany Department.

Scientists aren’t sure how the mudweed got to Hawaii. It was first discovered in 1981 on the leeward shore of Oahu.

It was University of Hawaii physiologist Celia Smith who found it this summer near Mokolii Island in Kaneohe Bay during a study sponsored by the Smithsonian Institute, called the MarineGeo Hawaii survey. The study will identify and document all aquatic life in Kaneohe Bay for research and management purposes, Anderson said.

 

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