Stanford researchers have uncovered evidence that deep underwater earthquakes can spur the growth of massive phytoplankton blooms at the ocean surface.
Phytoplankton are microscopic, plant-like organisms that float in upper ocean layers and serve as the foundation of the oceanic food chain. They also store carbon dioxide pulled from the air and supply a large amount of the planet’s oxygen.
The new findings, published Dec. 9 in Nature Geoscience, point to a previously unknown relationship between the ocean floor and life at the surface.
Building on a 2019 discovery that iron 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 a particular bloom varied so dramatically in productivity year-to-year. Productivity refers to the rate at which algae convert light, carbon dioxide, and nutrients into biomass, with higher productivity leading to denser, more extensive blooms.
“When looking back over satellite observations of this bloom, we’ve seen it swell to the size of the state of California or down to the size of Delaware,” said lead study author Casey Schine, who conducted some of the research as a PhD student in the lab of Kevin Arrigo 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 this annual phytoplankton bloom was the amount of seismic activity in the preceding few months.”
The researchers reviewed earthquake records to test a theory that increased seismicity might cause the hydrothermal vents to emit extra iron and heated fluid that can bring that iron more easily to surface waters. They found that earthquake records did indeed strongly overlap with bloom patterns.
“This is the first ever study to document a direct relationship between earthquake activity at the bottom of the ocean and phytoplankton growth at the surface,” said Arrigo, senior study author and the Donald and Donald M. Steel Professor in the Doerr School of Sustainability.
A waxing and waning bloom
The study’s origins trace to a research cruise in 2014, when Schine, Arrigo, and colleagues sampled a large phytoplankton bloom along the Southern Ocean’s Australian Antarctic Ridge. This jagged rise is a little-explored part of the mid-ocean ridge system, a volcanically active underwater mountain chain that spans the globe.
Shortly after this expedition, other scientists discovered that hydrothermal vents dotted the area. The Stanford research team then reported in a 2021 study that their previously observed bloom happened to overlay a hydrothermal vent ridge some 1,800 meters below. After looking at satellite images going back to 1997, the researchers realized that this bloom always developed in the same place at the same time but was a notably different size each year.
That observation inspired the researchers to explore what could be driving the bloom’s reliable recurrence but fluctuating productivity. Other factors that influence nutrient availability, such as changes in sea ice and ocean surface temperature, did not fully explain the bloom’s year-over-year variability. “When we ruled out more obvious, possible drivers of this variation, we started thinking about the iron-nutrient sources themselves, the hydrothermal vents,” said Schine.
Previous research has demonstrated how earthquakes can boost vent activity. The shaking of the ground can alter vents’ internal plumbing, jarring open clogged conduits and cracking new paths for heated fluids to escape. Spikes in temperature from moving subterranean magma can also increase vent emissions and alter the chemistry of the dissolved minerals in the expelled fluid.
More earthquakes could therefore pump more iron into the Southern Ocean, Schine proposed. Because iron is known to be phytoplankton’s limiting nutrient in this region – that is, the essential nutrient in shortest supply – it followed that plumes rich in the metal would help the plant-like organisms thrive.
“Casey had what I thought was a crazy idea that maybe the number of earthquakes near the hydrothermal vent was controlling the release of trace metals into surface waters that could stimulate phytoplankton growth,” recalled Arrigo. “I figured that it was a long shot but told her to go for it. And it turns out that she was right!”
Correlations confirmed
To test her earthquake hypothesis, Schine connected with study co-author Jens-Erik Lund Snee, then a Stanford geophysics PhD student researching seismicity and tectonics. The team consulted records of earthquakes captured by multiple seismic monitoring stations in the region.
Those readings showed that when earthquakes of magnitude 5 or larger occurred in the few months before the Southern Hemispheric summer, the peak phytoplankton growth period, the eventual blooms grew far denser and more productive.
The study also found that the hydrothermal iron would have to ascend nearly 6,000 feet for uptake by plankton at the surface within a few weeks and no more than a few months to influence production on the observed timescales. The prevailing view has been that it would take upwards of a decade for hydrothermal iron to reach surface waters and likely thousands of miles from the original vent source. The transport process that causes the vent fluid to come to the surface so quickly and so close to the originating vents is the subject of continuing work. A recent expedition in December 2024 to the Australian Antarctic Ridge may lead to new insights.
Local and global effects
The new study paints a more complex ecological picture of the Southern Ocean: Earthquake activity could have a profound influence on the food web based on phytoplankton, which feeds the crustaceans and krill that support larger animals, including penguins, seals, and whales.
“We already know that marginal phytoplankton blooms beyond the sea ice around the Antarctic continent are an important feeding ground for whales; we’ve even documented humpback whales visiting the bloom in our new study,” Schine said. “So, there’s potentially 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 predicting how much carbon oceans may absorb in the future.
It’s not yet known, however, to what extent hydrothermal vents are impacted by earthquakes and may be powering blooms worldwide. “There are many other places across the world where hydrothermal vents spew trace metals into the ocean and that could support enhanced phytoplankton growth and carbon uptake. Unfortunately, these locations are difficult to sample and little is known about their global significance,” said Arrigo. “The more we learn about these systems, the better we will understand the capacity of the ocean 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.
END