Can bioluminescent ‘milky seas’ be predicted?

BURLINGTON, Vt. — For the first time, a researcher has found a “milky sea” without relying on happenstance.

For centuries, sailors have been amazed and mystified by a rare phenomenon: the water around their ship glowing as far as the eye can see. Scientists have struggled to study such milky seas because they had no way of knowing when and where one would occur.

But now, using weather and ocean temperature data, atmospheric scientist Justin Hudson of Colorado State University in Fort Collins has successfully predicted — or rather, postdicted — an occurrence of the phenomenon.

Relying on historical reports of milky seas along with intel about local conditions at the time, Hudson developed an algorithm that indicates when and where ocean conditions are ripe for milky seas. That algorithm led him to satellite images taken in 2017 of the Indian Ocean south of the island of Java, Indonesia.

A milky sea — one never noticed before — showed up right where his algorithm said it would, Hudson reported June 24 at a meeting of the American Meteorological Society.

When further developed, this method could make forecasting a milky sea possible, Hudson says, so that a research vessel would have a better chance to visit one. Closeup study of milky seas — thought to be caused by the blooming of algae and light-emitting bacteria — could help explain how exactly they form and persist, he says.

Milky seas have been reported from mostly tropical waters all over the world. Based on sailors’ reports from the last two centuries and satellite images made since 2012, Hudson estimates they occur only two to three times a year, most frequently in the Arabian Sea and south of Java. But this is only a rough estimate, he says. Ships aren’t always around, and weather satellites can see milky seas only on moonless nights.

So far, a milky sea has been investigated by scientists just once. In 1985, a U.S. Navy research vessel lucked into one in the Arabian Sea. Water samples taken from that sea contained high concentrations of an alga called Phaeocystis and a bacterium called Vibrio harveyi. When cultured in a flask in a laboratory, these bacteria will emit light when their concentration goes up.

Milky seas, Hudson says, are hypothesized to be caused when ocean conditions create a “natural flask,” a body of water where temperature and the availability of nutrients allow an explosive increase of algae and of the bacteria that use the algae as a substrate to grow on. That can result in densities of more than 100 million microbes per milliliter across an expanse sometimes exceeding 100,000 square kilometers. Normally, ocean water contains about 10 of these bacteria per milliliter.

“There are still so many things we don’t know about [a milky sea] beyond that it must be caused by bacteria,” says Steven Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., who collaborates with Hudson on milky seas research. “What are the substrates that fuel the growth of these massive numbers of bacteria, and what are the environmental factors that keep them from getting diluted into the water column?”

To find out when a milky sea would be most likely, Hudson gathered reports of milky seas from 1960 to 2023 south of Java. He also acquired concurrent environmental data such as the status of the El Niño Southern Oscillation, which causes warming of waters in the Pacific but also increases air pressure over Indonesia, and the Australian Monsoon, which every southern summer brings rain to the region. After making the connection between those two sets of data, his algorithm could point him to other possible previous occurrences.

The approach used to look back in time for a milky sea should also work with forecasted ocean conditions, Hudson says, although those are more uncertain than recorded ones. “Based on what I currently have and know, I wouldn’t feel comfortable making a prediction for the future except that the period between June and October would be slightly more [likely] or less likely to have a milky sea.” He hopes to develop the system enough to be able to make predictions for a particular month.

Such predictions could become even better once more is known about how the bacteria and the surrounding plankton interact, says Youri Timsit, a biologist at the Mediterranean Institute of Oceanography in Marseille, France, who was not involved with the study. “All these organisms could multiply on a very large scale on the sea surface in a collective and mutually reinforcing manner.”

Another important factor, Timsit says, could be the influence of nearby land, “such as the seeding of sea surfaces by winds carrying sand or sediment. These phenomena are common in seas near deserts, such as the Arabian Sea.”