On a routine summer expedition to do an annual survey of the reef at French Frigate Shoals, marine scientists made two unexpected discoveries: a demolished reef and an invasive alga.

By Susan Enright.

The first discovery on what should have been a routine annual research expedition to the Papahānaumokuākea Marine National Monument this past summer shocked scientists aboard the survey vessel Rainier. The south side of French Frigate Shoals, where there had been a flourishing, stunningly beautiful reef teeming with life, now looked like a bulldozed parking lot covered in mud and debris.

The damage was caused by Hurricane Walaka, a Category 3 event that hit French Frigate Shoals in October 2018. It was one of the most intense hurricanes on record for the Central Pacific Basin. But marine scientists had not yet surveyed the damage before the research team arrived in July.

“We were expecting some hurricane swell damage, but we were shocked to find reefs demolished,” says Kailey Pascoe, a research technician at the Multi-scale Environmental Graphical Analysis (MEGA) Lab at the University of Hawai‘i at Hilo. Pascoe was the lead for the MEGA Lab activities on the expedition and managed all diving operations and data analyses.

The MEGA Lab

The MEGA Lab, developed by John Burns, an assistant professor of marine science and an alumnus of UH Hilo, conducts long-term monitoring of coral health and 3D habitat structure in the Papahānaumokuākea Marine National Monument.

“We have been participating in expeditions for the last eight years,” Burns says. “For this project we received specific funding from the National Fish and Wildlife Foundation to investigate how climate change is affecting coral reef habitats at French Frigate Shoals.”

Pascoe says the work of the MEGA Lab is essential to understanding ecological changes in coral reefs. Hawaiian coral reefs are biologically unique and one of the most valuable ecosystems on earth. Corals are constantly facing global and local stressors that have led to large amounts of mortality.

“Our study sites are long-term sites that have been studied for over 10 years,” Pascoe says. “It’s important for us to go back and survey these sites year after year, we are learning so much about changes in coral health and reef complexity.” The surveys involve in-situ coral health surveys, in which each coral colony located along the transect is given a physical examination. The researchers measure demographics and coral health disease metrics.

In addition to the coral health surveys, the MEGA Lab also completes structure from motion (SfM) photogrammetry surveys. “With these methods we can make 3D models of each long-term monitoring site,” Pascoe explains. “We have SfM surveys tracking back to 2015.”

She further explains, “Coral reefs provide structure and habitat for many different species. It is critically important for us to quantify how subsequent changes in coral reefs will alter ecological processes. The 3D models allow us to measure a number of 3D characteristics and we are able to model these variables with coral species and fish species to determine trends within 3D characteristics that drive a healthy ecosystem.”

The summer expedition

The research expedition this past summer was called the Papahānaumokuākea Marine National Monument Biogeo (Biogeography) Cruise. Pascoe, an alumna of UH Hilo’s undergraduate program in marine science and graduate program in conservation biology and environmental science, says the purpose of the trip was to conduct monitoring surveys in shallow and mesophotic (low light) coral reefs.

“The different types of surveys that were completed were long-term monitoring of coral health, photogrammetry, fish surveys, seaweed surveys and different types of water quality surveys,” she explains. Collections for new species at mesophotic depths were also one of the goals of this research expedition.

The expedition, with a team of partner scientists from the National Oceanic and Atmospheric Administration, UH Hilo, UH Mānoa, and others, lasted 22 days from July 16 to Aug. 13 on the NOAA ship, Rainier. The team visited French Frigate Shoals, Lisianski Island, and Pearl and Hermes Atoll.

A demolished reef

There were many discoveries on the expedition, but two stand out.

The first was the shocking discovery of the damage from Hurricane Walaka on the south side of French Frigate Shoals. All three sites of French Frigate Shoals, Lisianski Island, and Pearl and Hermes Atoll showed significant amounts of damage from the hurricane.

“These sites were completely unrecognizable, [with] large amounts of sand and dead coral pieces basically the size of your hand,” Pascoe describes. “There was no longer a thriving reef. It was as if a bulldozer came in and created a new parking lot.”

Previous surveys show the reef with many large table corals of four to six feet, huge schools of fish and predators such as monk seals, ulua, and sharks. Pascoe says one of the sites named Rapture Reef was a site studied by many different scientists and was a monumental site due to its beautiful and diverse reefs. “Most divers very much looked forward to diving Rapture reefs,” she says.

But now the area is leveled. Pascoe says the aftermath of Walaka is a wake-up call for climate change.

“These islands and atolls are at high latitudes and should have cooler water in general,” she explains. “Walaka was a very powerful hurricane happening at high latitudes. This just shows that water temperatures are rising and the potential for very strong hurricanes is also increasing. This hurricane was able to wipe out an eleven-acre sand island and it also was able to wipe out reefs found in about 85 feet of water. This hurricane really put it into perspective exactly what kind of damage can be found with a Category 3 hurricane.”

Despite the obliteration of the reef area, Pascoe says the research team observed some very tiny recruits of corals at these sites. “The MEGA Lab is excited to see how reefs will grow over the years,” she says.

An invasive alga

After surveying the hurricane damage, the trip was to continue beyond these three areas but was cut short with the discovery of an invasive alga at Pearl and Hermes Atoll. The ship was put in quarantine and was not allowed to visit other sites.

“Our phycologist Heather Spalding is an expert in limu [seaweed],” Pascoe explains. “She happened to notice just a patch of this invasive algae. But then we started to dive other areas and we found areas the size of football fields completely covered in this invasive alga. This alga creates thick mats roughly six to seven inches in height that just smother corals and native algae underneath it.”

The researchers quickly realized the alga could be a huge problem. This instigated a rapid response to determine the spatial extent, depth distribution, and habitat preference of this invasive alga.

“After 177 snorkel and dive surveys we were able to establish baseline data for this invasive alga,” explains Pascoe. “This species can be very cryptic at certain areas and hard to miss while it can also be very invasive especially in water of twenty to fifty feet. Currently, this invasive alga can be found on the north, east, west and southwest reefs of Pearl and Hermes Atoll.”

Currently, a group of scientists is trying to genetically identify the species. “Once we establish the species, the management board can move forward with addressing how to deal with this invasive alga,” says Pascoe.

Story by Susan Enright, a public information specialist for the Office of the Chancellor and editor of UH Hilo Stories. She received her bachelor of arts in English and certificate in women’s studies from UH Hilo.