Scientists on a 24-day expedition to the Papahanaumokuakea Marine National Monument in the Northwestern Hawaiian Islands found most of the coral reefs thriving, but documented a distinct shift from a coral-dominated reef to an algae- dominated reef in an area where several hundred meters of coral bleaching was discovered three years ago.

Scientists aboard Hi‘ialakai, a National Oceanic and Atmospheric Administration ship, returned to Oahu Saturday after conducting monitoring operations to assess the health of the coral reef ecosystems, which house more than 7,000 marine species, one quarter of which are unique to the Hawaiian archipelago.

The process was physically demanding for the team, who dived up to five times a day, and spent most of their day doing intense fieldwork in small boats. They paid special attention to a several hundred-square- meter section on the shallow eastern side of the island where coral is more exposed to increasing water temperatures.

The monument’s 582,578-square-mile conservation area, which can only be accessed by few, is a living laboratory that is larger than all national parks combined. The Hawaiian archipelago includes the Kure Atoll, Pearl and Hermes Atoll, Laysan Island, Gardner Pinnacles, Mokumanamana, Nihoa, French Frigate Shoals, Maro Reef, Lisianski Island, and Midway Atoll.

“Monument monitoring gives us insight on how we should be managing reefs on the main Hawaiian Islands,” said Dr. John Burns, a NOAA ocean habitat researcher, who was part of the team that discovered the bleaching event in 2014 as well as this year’s team. “It’s like a looking glass. It’s a unique system where there are no fishing pressures and there is high biomass diversity. We do what we can to promote a similar environment elsewhere.”

Burns said there was a large amount of mortality at the bleaching site in 2015 and that there was evidence last year that algae had begun taking over.

The growing presence of algae occurs when a critical balance is upset, causing algae eaters to die, Burns said. When enough algae colonizes, reefs face death from disease-causing microorganisms. The good news is that “a large proportion of the monument is still filled with very vibrant, healthy coral reefs,” Burns said.

Conditions at the monument’s bleached site have taught scientists that reefs are important to protect and simply removing human stresses doesn’t eliminate global stresses.

“There is potential for that phase shift to occur anywhere and when it does, we’ll have a greater understanding of how it can impact fish, ” Burns said.

That’s why it’s important for people to do all they can to minimize global stresses and reduce their carbon footprint, he said.

According to NOAA, healthy coral reefs play an integral role in protecting Hawaii’s shores from storms, creating surfing-caliber waves, and providing sustenance for the monk seal, the green sea turtle, and over 14 million seabirds, including the Laysan duck. A 2002 economic study estimated the value of Hawaii’s coral reefs was at least $364 million annually.

Researchers from Papa­hanaumokuakea Marine National Monument, NOAA’s Pacific Islands Fisheries Science Center, Joint Institute for Marine and Atmospheric Research, University of Hawaii-Hilo, the Australian Institute of Marine Science (AIMS), and three University of Hawaii students participated in this year’s monitoring operation.