Phytoplankton are microscopic, plant-like organisms that drift near the ocean surface, underpin marine food webs, store carbon taken from the atmosphere, and release a substantial portion of Earth's oxygen. The study, published Dec 9 in Nature Geoscience, shows that seismic activity on the seafloor can influence how much iron reaches surface waters, in turn controlling phytoplankton productivity in a major bloom system near Antarctica.
The work builds on a 2019 finding that iron from hydrothermal vents fuels large phytoplankton blooms in the Southern Ocean. The new research focuses on a bloom along the Australian Antarctic Ridge, where iron-rich vent fluids rise from a volcanically active section of the global mid-ocean ridge.
Lead author Casey Schine, who conducted part of the research as a PhD student in the lab of Stanford's Kevin Arrigo, examined why this bloom's productivity varies so much from year to year despite forming in the same place and season. Satellite records show the bloom can expand to an area comparable to California or contract to a size similar to Delaware, indicating large swings in net primary production.
Schine and colleagues hypothesized that changes in earthquake activity near the hydrothermal vents might modulate iron release into the overlying water column. After ruling out influences such as sea ice cover and surface temperature, they turned to the vent system itself as the likely driver of the observed variability.
Previous studies have shown that earthquakes can alter hydrothermal circulation by shaking and cracking the crust, reopening blocked pathways and creating new channels for heated fluids. Temperature increases driven by moving magma can further boost vent discharge and change the chemistry of dissolved minerals in the fluids.
Because iron is the limiting nutrient for phytoplankton growth in much of the Southern Ocean, enhanced vent-derived iron should stimulate larger 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 Schine. "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."
To test the idea, Schine collaborated with co-author Jens-Erik Lund Snee, then a Stanford geophysics PhD student, to analyze records from multiple seismic monitoring stations. They found that when earthquakes of magnitude 5 or greater occurred in the months before the Southern Hemisphere summer, the subsequent blooms became denser and more productive.
The study also constrains how quickly vent-sourced iron must move upward to influence surface production on these timescales. Model and observational analysis indicate that hydrothermal iron has to rise nearly 6,000 feet and reach the surface within weeks to a few months, challenging the prevailing view that such iron typically takes around a decade to reach surface waters and does so thousands of miles from its source.
The rapid transport mechanism that carries vent fluids to the surface so efficiently and so near the vents remains under investigation. A dedicated research cruise to the Australian Antarctic Ridge in December 2024 collected new data that may clarify the physical processes involved.
Ecologically, the findings suggest that seabed seismicity can shape food-web dynamics across the Southern Ocean. Marginal phytoplankton blooms along the sea-ice edge provide key feeding grounds for crustaceans and krill, which support penguins, seals, whales, and other higher predators.
"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 remove carbon dioxide from the air, understanding what controls their size and frequency is important for improving ocean carbon uptake projections. The study indicates that seafloor processes, including tectonic shaking and hydrothermal venting, may be significant but underrepresented components in global carbon cycle models.
It remains unclear how widespread earthquake-modulated vent fertilization is beyond the Australian Antarctic Ridge. "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."
Research Report:Southern Ocean net primary production influenced by seismically modulated hydrothermal iron
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