The study finds that deep-ocean earthquakes lead to massive phytoplankton blooms in the Southern Ocean

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Using a device to collect water samples from various ocean depths in the northern Russian Sea to determine the concentration of iron in them.

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Image credit: Geert van Dijken

Researchers at Stanford University have discovered evidence that deep underwater earthquakes can stimulate the growth of massive phytoplankton on the ocean's surface.

Phytoplankton are microscopic plant-like organisms that float in the upper ocean layers and serve as the basis of the ocean food chain. They also store carbon dioxide extracted from the air and provide a large amount of oxygen to the planet.

The new findings, published December 9 in the journal Nature Geoscience, point to a previously unknown relationship between the ocean floor and life on the surface.

Building on the 2019 discovery that iron extracted from underwater hot springs, called hydrothermal vents, fuels phytoplankton blooms in the vast Southern Ocean around Antarctica, the study authors set out to find out why the productivity of certain blooms varies so dramatically from year to year. Productivity refers to the rate at which algae convert light, carbon dioxide and nutrients into biomass, with increased productivity leading to denser and more extensive blooms.

“When looking at satellite observations of this bloom, we saw it swell to the size of California or to the size of Delaware,” said lead study author Casey Shen, who did some research as a doctoral student in Kevin Arrigo's lab at the Stanford Doerr School of Sustainability and is now a postdoctoral research associate at Middlebury College. “Our study ultimately showed that the main factor controlling the size of the annual phytoplankton bloom is the amount of seismic activity in the previous few months.”

The researchers reviewed earthquake records to test a theory that increased seismic activity might cause hydrothermal vents to release excess iron and hot fluids that could more easily bring that iron into surface waters. They found that earthquake records strongly overlap with bloom patterns.

“This is the first study ever to document a direct relationship between seismic activity on the ocean floor and phytoplankton growth at the surface,” said Arrigo, the study's lead author and the Donald and Donald M. Steele Professor in the Doerr School of Sustainability.

Increasing sunshine and declining flowering

The origins of the study go back to a research expedition in 2014, when Shine, Arrigo and colleagues sampled large phytoplankton along the Australian Antarctic Ridge in the Southern Ocean. This jagged rise is a little-explored part of the mid-ocean ridge system, a volcanically active underwater mountain range that stretches across the globe.

Shortly after this expedition, other scientists discovered that hydrothermal vents were common in the area. The Stanford research team then reported in a 2021 study that their previously observed bloom covered a hydrothermal vent at a depth of about 1,800 metres. After looking at satellite images dating back to 1997, the researchers realized that this bloom always develops in the same place at the same time but at a noticeably different size each year.

This observation inspired researchers to explore what could be the reason behind reliable repeat flowering but fluctuating productivity. Other factors that affect nutrient availability, such as changes in sea ice and ocean surface temperature, do not fully explain the year-over-year fluctuation of blooms. “When we ruled out the most obvious potential drivers of this difference, we started to consider the sources of the iron nutrients themselves: hydrothermal vents,” Shen said.

Previous research has shown how earthquakes can enhance venting activity. Ground shaking can alter the internal plumbing of the vents, opening clogged ducts and creating new paths for hot fluids to escape. Temperature rises resulting from stirring subterranean magma can also increase vent emissions and change the chemistry of dissolved minerals in the expelled fluid.

Shen suggested that more earthquakes could pump more iron into the Southern Ocean. Because iron is known to be a limiting nutrient for phytoplankton in this region—that is, an essential nutrient that is in short supply—it followed that plumes rich in the metal would help plant-like organisms to thrive.

“Casey had what I thought was a crazy idea that the number of earthquakes near the hydrothermal vent might be controlling the release of trace metals into surface waters that could stimulate phytoplankton growth,” Arrigo recalls. “I thought it was too far-fetched, but I asked her to do it. And it turns out she was right!”

Links confirmed

To test her earthquake hypothesis, Shen reached out to study co-author Jens-Erik Lund-Snee, who was then a geophysics doctoral student at Stanford University who was researching earthquakes and tectonics. The team reviewed earthquake records taken by multiple seismic stations in the area.

Those readings showed that when earthquakes of magnitude 5 or greater occurred in the few months before Southern Hemisphere summer, the peak period for phytoplankton growth, the terminal blooms became denser and more productive.

The study also found that hydrothermal iron would have to ascend nearly 6,000 feet for plankton to absorb it at the surface within a few weeks and no more than a few months to affect production on the observed timelines. The prevailing view was that it would take well over a decade for hydrothermal iron to reach surface waters, likely thousands of miles from the original vent source. The transport process that causes the venting fluid to reach the surface so quickly and so close to the original vents is the subject of ongoing work. A recent expedition in December 2024 to Australia's Antarctic mountain range may lead to new insights.

Local and global influences

The new study paints a more complex ecological picture of the Southern Ocean: Earthquake activity could have a profound impact on the phytoplankton-based food web, which feeds the crustaceans and krill that support larger animals, including penguins, seals and whales.

“We already know that marginal phytoplankton that thrive off the sea ice around Antarctica represent important feeding grounds for whales; we even documented humpback whales visiting to thrive in our new study,” Shen said. “So, there's probably more to the story now that we suspect seismic activity plays a role in bloom productivity.”

Because phytoplankton blooms pull carbon dioxide from the atmosphere, understanding the factors that drive their growth can help scientists improve models that predict how much carbon the oceans may absorb in the future.

However, it is not yet known to what extent hydrothermal vents are affected by earthquakes, and may be the reason behind flower blooms around the world. “There are many other places around the world where hydrothermal vents spew trace metals into the ocean, and this could support enhanced phytoplankton growth and carbon uptake. Unfortunately, these sites are difficult to sample and little is known about their global importance,” Arrigo said. “The more we learn about these systems, the better we understand the oceans’ ability to remove atmospheric carbon dioxide.”

Arrigo is also a senior fellow at the Stanford Woods Institute for the Environment. This research was supported by the National Science Foundation and NASA.

magazine

Natural Earth Sciences

Article title

The net primary production of the Southern Ocean is influenced by seismically modified hydrothermal iron

Date the article was published

9-December-2025

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