New Species of Mat-Forming Algae Smothers Coral on Subtropical Atoll

[By Marissa Garcia]

In August 2019, researchers took a dive at Pearl and Hermes Atoll (Manawai) in Papahanaumokuakea Marine National Monument. At about 40 feet below the surface, they expected to see brilliant pinks and greens, characteristic of native stony corals. Instead, they saw mats of a red alga they did not recognize, stretching for the length of football fields and smothering the native corals. Not only were colorful corals not in sight; none of the usual grazing fishes, such as parrotfish, were nearby.

A NOAA diver swam closer to this unexpected landscape and began to tug at the mat-forming algae. Pulling it upward, the research diver discovered that the once vibrant corals were now only skeletons. No native algae other than cyanobacteria were growing under the mats. The research task became clear: they needed to identify this unfamiliar algal species that was outcompeting the native species in the monument.

Nearly a year later, after extensive molecular and morphological analyses, researchers have determined that this mat-forming algae was an undescribed species. Newly named as Chondria tumulosa, the alga’s origin is unknown. Though it is possible that the alga is native, it nevertheless exhibits alarming invasive-like qualities. When NOAA divers first detected the alga in 2016, it grew in low abundance, not yet widespread. In three years, the alga had grown into abundant mats of over 100,000 square feet each at Pearl and Hermes Atoll, outcompeting the species typically living in these ecosystems.

Researchers have not yet determined if Chondria tumulosa was introduced from another region. Randall Kosaki, NOAA research coordinator at Papahanaumokuakea Marine National Monument, says, "We still don’t know. Is it a native species that was just completely overlooked until it went berserk at one atoll, or was it an accidental human introduction from somewhere else?" Accordingly, researchers are not yet labeling the alga as "invasive," instead opting to call it a "nuisance" species for its invasive-like qualities.

Grazers such as parrotfish usually feed upon native species of algae to keep them from growing out of control; no native grazers were observed feasting on the Chondria tumulosa. During his dive, Kosaki noted that the native species only reappeared again at the end of the Chondria mats. He remembers, "Some of the less desirable native species were grazed down, and native grazers were avoiding the Chondria and were being forced to graze more heavily on things they don’t really care for." This behavior by native grazers suggested that this alga could be an alien invasive, though further evidence is needed to confirm this. For now, scientists consider Chondria tumulosa to be a nuisance species.

Native coral species prior to being smothered by the nuisance algal species Chondria tumulosa. (Heather Spalding/College of Charleston)

Oval damselfish, moorish idol, milletseed butterflyfish, and Thompson’s anthias swim in Pearl and Hermes Atoll, not yet affected by the overgrowth of the nuisance species Chondria tumulosa. Photo: Andrew Gray/NOAA.

Research diver pulls back a Chondria tumulosa algal mat to reveal a coral’s skeleton. Photo: Taylor Williams/University of Hawai‘i

Taylor Williams, undergraduate intern at the University of Hawai’i, holds up a mat formation of the nuisance alga Chondria tumulosa. Photo: Heather Spalding/College of Charleston.

Close-up view of a Chondria mat in the lab. Photo: Heather Spalding/College of Charleston.

Such extensive presence of a nuisance species is a new phenomenon in the monument. "The main Hawaiian Islands are impacted by several well-known invasive seaweeds, but reports of nuisance algae in Papahanaumokuakea Marine National Monument are far fewer, and none have been present at the level of abundance seen in this new alga," describes Alison Sherwood, the lead researcher on the project.

Minimizing its spread

Chondria tumulosa can easily break down into fragments, with spinuous branches that could easily snag onto dive gear and equipment, causing them to unintentionally spread to other regions beyond the atoll. Accordingly, the research divers diligently ensured all of their diving gear and equipment was free of any algal fragments by soaking it in six percent bleach.

"Until we understand whether it is native or introduced, and until we better understand what is driving this outbreak, it is critically important that research divers and research ships do not inadvertently transport this species to other islands," says Kosaki. "Thus, all of our dive gear was soaked in bleach, and all of our dive boats were sprayed down with bleach prior to returning to Honolulu."

The nuisance algae also has a tumbleweed-like growth that raises concern. This growth pattern allows it to more easily travel from one location to another. "Just the fact that you can have this clump of biomass that can be moved by oceanographic conditions is concerning," says Sherwood.

How can an algae like this spread?

Nuisance species may be at risk of becoming more prevalent with rising temperatures. Kosaki notes. "As sea waters warm, we may have more truly tropical species dispersing further north to cooler subtropical Hawaiian waters. Some of those may be invasive, some of them not—it’s something we need to be on the look-out for."

Researchers are still determining what may have caused its overgrowth. Possibilities include a removal of grazing pressure or an excess of nutrients from upwelling. Kosaki already has plans for next research directions. "During our field season next year, we are going to be taking water samples—looking at nutrients and looking at stable isotopes in the algae to see whether there’s evidence of nutrient enrichment. That’s something that could very well be driving this: nutrient input from deeper water due to upwelling."

The need for conservation is urgent

Observing the overgrowth of Chondria tumulosa in Papahanaumokuakea National Marine Monument is particularly alarming, given the pristine condition of these reefs and their remote location. "This is in the middle of one of the biggest marine protected areas in the world. It’s very remote," Kosaki notes. "There’s virtually no vessel traffic. None of the usual means of introducing alien invasive species exist there. The few vessels that go to the monument all have mandatory hull inspections." Scuba divers inspect the hulls to ensure that no colonies or fragments from alien species are on the ship, so that they do not colonize an ecosystem that is not a part of their native range.

"If this can happen here, then this can happen anywhere, and the chance of this happening in populated areas such as Honolulu is 1,000 times greater than at Pearl and Hermes Atoll," says Kosaki. This could put many industries (such as tourism) that are dependent on the biodiversity of the coral reefs at risk.

When an invasive species invades a terrestrial ecosystem, researchers can design effective strategies for restoring ecosystems back to their original state. These cannot be so easily applied to removing invasive species from marine ecosystems. Kosaki notes, "In the ocean, it’s virtually impossible. Once it’s in, you will never get rid of it. And so, the only tool at our disposal is not eradication; it’s prevention. The stakes are so high… prevention really is the only thing that we have to work with that will be successful."

The importance of collaboration in science

The nuisance algal species Chondria tumulosa was newly named in a PLOS ONE article, "Taxonomic determination of the cryptogenic red alga, Chondria tumulosa sp. nov., (Rhodomelaceae, Rhodophyta) from Papahanaumokuakea Marine National Monument, Hawai‘i, USA: a new species displaying invasive characteristics."

Identifying this new algal species was a collaboration that brought together researchers from the University of Hawaii, Western Australian Herbarium, College of Charleston and National Oceanic and Atmospheric Administration (NOAA). Collaboration will also be essential in learning whether Chondria tumulosa is invasive to northwestern Hawaiian waters.

"This is a highly destructive seaweed with the potential to overgrow entire reefs," College of Charleston Assistant Professor Heather Spalding says. "We need to figure out where it’s currently found, and what we can do to manage it. This type of research needs trained divers in the water as quickly as possible. The sooner we can get back to Papahanaumokuakea Marine National Monument, the better."

Marissa Garcia is a student at Harvard College and a Virtual Student Federal Service intern for NOAA’s Office of National Marine Sanctuaries.

This article appears courtesy of NOAA's National Ocean Service and may be found in its original form here.