Sunday, September 15, 2019

How a volcanic eruption set off a phytoplankton bloom

14 September, 2019

HAWAII — The eruption last year of the Kilauea volcano in Hawaii produced the equivalent of 320,000 Olympic-size swimming pools of lava. Much of it ended up flowing into the Pacific Ocean, creating plumes of acidic, glassy steam in the process.

The eruption also unexpectedly coincided with an explosion in the population of phytoplankton, a diverse array of sea surface-dwelling, sunlight-drinking microscopic organisms. This massive bloom began just three days after lava from Kilauea first touched the sea.

It expanded rapidly, stretching nearly 100 miles offshore in just two weeks. When the eruption dwindled and the lava stopped flowing seaward, the bloom quickly disappeared.

Scientists were initially baffled as to how 2,100-degree Fahrenheit, life-annihilating lava could trigger a biological bloom. A study published Thursday in Science reveals that it came down to a volcanic sleight of hand: As the lava tumbled to the ocean’s depths, it heated the deeper, nutrient-rich waters, allowing them to bubble up to the nutrient-starved surface. This provided a grand banquet for the phytoplankton, leading to their rapid proliferation.

Understanding how phytoplankton respond to their environment is a vital undertaking, as they are critical to the planet’s health. Phytoplankton form the foundation of marine food chains, and their seasonal blooms are responsible for more than half the photosynthesis, and the resulting production of oxygen, that occurs on Earth.

The event is unlikely to be an anomaly. Kilauea is just one of several volcanoes around the world capable of dumping fresh lava into the ocean. Lava-driven nutrient fountains “could be a pretty important driver of phytoplankton ecology in the broader ocean,” said Dr Harriet Alexander, a biological oceanographer at the Woods Hole Oceanographic Institution who was not involved in the latest study.

The bloom was tracked by satellite, betrayed by the appearance of a teal-green film on the sea surface. Its growth strongly tracked with the lava’s entry into the water. It was clear that the two were connected, but not how.

Scientists scrambled, hoping to get there in time to take as many samples as possible. “As we were planning the expedition, we were sort of praying that the lava flow wouldn’t stop,” said Dr Sam Wilson, a microbial biogeochemist at the University of Hawaii at Manoa and a lead author of the study.

In mid-July 2018, at the height of the seaward lava discharge, a rapid-response scientific expedition shipped out to the region. A team of 18 scientists took as many chemical and biological samples as possible, both from the bloom’s surface and below it.

They found plenty of nitrates, nutrients that drive phytoplankton blooms. But the lava itself did not contain any nitrogen, so the origin of the nutrients was unclear.

Back on land, the team wondered how lava mixing with seawater might somehow produce nitrates, but by then the lava flow had ceased.

“We had to get creative, and in this instance we made synthetic lava,” Dr Wilson said. The staff of the University of Hawaii’s art department gleefully melted hardened lava rock from Hawaii’s Big Island and poured it into buckets of seawater.

“I hadn’t appreciated how much artists love to melt things,” Dr Wilson said.

Sadly, perhaps because of the small scale and limited verisimilitude of these experiments, no nitrates were generated in these buckets, so the team turned to another kind of chemical sleuthing. Nitrates contain oxygen and nitrogen, which come in different forms, or isotopes.

These isotopes can be used to work out where the nitrates came from, said Dr Nick Hawco, a chemical oceanographer at the University of Southern California and the other lead author of the study. In this case, the nitrates from the mid-eruption phytoplankton bloom had a deepwater provenance.

In the tropics, nitrates are rapidly used up by surface-dwelling phytoplankton, but the nutrients begin to accumulate at around 1,000 feet underwater, where light-drinking, nitrate-hungry organisms do not thrive.

As Kilauea’s lava entered those depths, it pushed the deeper, nitrate-rich layer of water upward, much as the particles of a dissolved bouillon cube rise from the bottom of a cooking pot.

There is some evidence that volcanic ash can trigger a phytoplankton bloom, but only in nitrate-rich surface waters. The study of the Kilauea-adjacent phytoplankton bloom is believed to be the first time researchers have shown that volcanic activity can drive a bloom in waters lacking that vital nutrient.

“When the eruption was happening, I was consumed by the destructive aspect of it,” Dr Hawco said. As the rapid appearance of this bloom demonstrates, life has a habit of prevailing even in the face of nature’s most ferocious conflagrations. THE NEW YORK TIMES

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