What caused parts of the ocean along the Outer Banks to appear yellowish?

The shorebreak area of the Atlantic Ocean appears yellowish the afternoon of Saturday, June 21, 2025, in South Nags Head. (Photo by Corinne Saunders)

By Corinne Saunders

SOUTH NAGS HEAD — Outer Banks beachgoers from South Nags Head to Corolla reported sections of the Atlantic Ocean appearing neon greenish to yellow-hued over a span of several days in late June.

Scientists identified the cause as a bloom of nontoxic algae, a spokesperson for the Coastal Studies Institute (CSI) in Wanchese told Outer Banks Insider.

“I have forwarded your question to some of our faculty and after corresponding with others from UNC-IMS, NOAA and DEQ about this, they have identified it as a bloom of a prasinophyte, which is nontoxic,” CSI Associate Director for Education, Outreach & Communications John McCord said in a Sunday email.

The other personnel represent the University of North Carolina at Chapel Hill Institute of Marine Sciences (UNC-IMS), National Oceanic and Atmospheric Administration (NOAA) and North Carolina Department of Environmental Quality (NCDEQ).

Prasinophyte green algae are widespread and are related to plants, according to information published on the National Institutes of Health’s National Library of Medicine website.

Algae are microscopic organisms that use photosynthesis to produce energy. Some varieties of algae produce toxins, and rapid increases of those varieties lead to harmful algal blooms, which can be toxic to humans and animals.

Prasinophyte green algae, by contrast, are nontoxic and are considered to be an important part of the marine habitat, according to scientific articles published on the National Library of Medicine website.

Outer Banks Insider received reports of visible water discoloration from beachgoers in South Nags Head, Kill Devil Hills and Corolla from Saturday, June 21, through Wednesday, June 25, with different areas affected on different days.

The shorebreak area of the Atlantic Ocean appears yellowish the afternoon of Saturday, June 21, 2025, in South Nags Head. (Video by Corinne Saunders)

Over two decades since last local bloom

The last time this type of bloom was documented along the Outer Banks coast was almost exactly 24 years ago, on June 27, 2001; but this recent bloom was more intense, according to Nathan S. Hall, a research assistant professor with UNC-IMS.

NCDEQ documented cell densities of 77,000 cells per milliliter in 2001, while the sample Hall viewed this time had densities of about 500,000 cells per milliliter, he said in a Tuesday night email.

Hall and a NCDEQ Division of Water Resources staffer “identified it as the genus of prasinophytes called Tetraselmis back then too,” he said. “We didn’t try to figure out the species, but all the prasinophytes are considered harmless and part of the ‘good’ phytoplankton of coastal waters.” 

The algae are always present, but generally in much lower densities. They are a quality food source for zooplankton, which small fish eat, and which in turn are eaten by larger fish, he explained.

“Blooms of prasinophytes are rare; probably because they’re good food for zooplankton and thus get eaten before they can bloom to high cell densities,” Hall said.

Blooms are usually caused by an injection of fertilizing, mineral nutrients—nitrate and phosphate—into the water, he said, guessing that an upwelling event led to this bloom.

Upwellings are when wind and currents bring the deep water to the surface. All deep ocean waters are nutrient rich, he said.

“Prasinophytes can swim, too, and their swimming motion can concentrate cells in areas where water masses collide,” Hall added. “That might also have helped create the high cell densities that were observed.”

Historically, such a bloom is unique.

“They’re rare, weird, and most likely 100% harmless,” Hall said. “If people see the bloom again, get in touch with NCDEQ and UNC-IMS. We’re interested and if it becomes common, we’d like to know because it could signal a change in the ecosystem.”

Hall encourages anyone observing a bloom to log their observations into NCDEQ’s online “Fish Kill & Algal Bloom Report Dashboard” through this link and to also email him at nshall@email.unc.edu.

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This article was updated at 9:45 a.m. on Wednesday, July 2, 2025, to include the research assistant professor’s responses received after initial publication.

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