Studying how phytoplanktons are affected when two types of water meet, and what may happen to the marine food chain
, | August 24, 2026 | Energy & Environment
A thousand miles east of Barbados, in the tropical North Atlantic, two Rutgers University oceanographers are leading a 25-day expedition to study a process called salt finger mixing — and whether it can keep nutrients flowing as the planet, and the seas, warms. What they find may matter for the marine food chain.
The ocean covers about 70% of Earth’s surface and plays a major role in regulating the climate. Phytoplankton — microscopic organisms that absorb carbon dioxide through photosynthesis — help drive a “biological pump” that transfers some of the carbon from surface waters into the deep ocean.
Salt finger mixing helps replenish the nutrients that phytoplankton need to grow.
“Our expedition is looking at how phytoplankton grow, and how the physical mixing processes in this particular environment — which are also being impacted by climate change — affect that growth,” oceanographer Corday Selden told NJ Spotlight News from the R/V Falkor (too), a research vessel operated by the Schmidt Ocean Institute. The ship was given a whimsical name in honor of its predecessor, R/V Falkor.
Oceanographer Joe Gradone explained the process with two kitchen analogies: stirring milk into coffee is turbulent mixing, while a teabag’s coloring hot water is diffusive, slower and lower-energy. Salt fingers sit closer to the teabag, which is part of why they’re so hard to detect.
“That’s actually really, really hard to measure,” said Gradone, who with Selden is co-science chief on the research. “But when you add it all up in the oceans across the world, it can have a really big impact on a variety of things.”
Salt finger mixing happens where warm, saline water sits atop colder, fresher water. An area of the upper water sinks, losing heat faster than salt. That makes it colder, and salty and dense enough to keep dropping. The colder, fresher water rises to replace it, creating what Gradone calls “interleaving layers in the ocean that really, really enhance the mixing.”
The movement carries nutrients to the sunlighted upper ocean from the deep.
“Phytoplankton — the plants of the ocean, just like plants in your garden — they need different things to grow,” Selden said. “They need water, they need light and they need nutrients like nitrogen and phosphorus that you might add to your garden to enhance growth. So in the ocean, there’s plenty of light, there’s plenty of water, but there’s not much nutrients.”
When mixing carries those nutrients up, she said, phytoplankton can grow and be eaten. “If the base of the food web is doing well, then there’s more food for everybody else to eat going on up,” Selden said. More phytoplankton also means more carbon drawn from the atmosphere.
To measure all this, the team drops instruments fitted with energy-measuring probes — Gradone described one as fragile as a potato chip — mounted on a torpedo-shaped rig. It operates as deep as 3,000 feet.
Selden said scientists expect the warming surface layer to create a barrier preventing nutrients from rising through turbulent diffusion.
“We’re curious to find out whether or not this diffusive mixing mechanism could offset that,” she said.
Joanna Gagis joined the NJ Spotlight News team in 2018. As a Senior Correspondent, her coverage includes education, health care, the environment, New Jersey politics and more.
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