Editor’s note: This article was updated on June 5.
A new study combining NASA satellite observations, ocean surveys, and genetic testing on marine microorganisms found evidence that warming ocean waters may be limiting nutrient availability across much of the global ocean. The researchers report that this nutrient stress affects microscopic marine organisms and could influence marine ecosystems over time.
The research, published June 5 in Science Advances, tracked the condition of phytoplankton, which form the base of ocean food webs. Rather than measuring nutrients like nitrogen, iron, and phosphorus directly, the researchers inferred stress by tracking subtle shifts in the ratio of carbon to chlorophyll in phytoplankton observed from space. When the amount of chlorophyll decreases relative to carbon as seen in satellite data, it’s an indication that the plankton are stressed.
“As our ocean continues to change, the ability to observe and track ocean conditions through sustained, high quality remote sensing observations has never been more important,” said Laura Lorenzoni, Program Scientist for NASA’s Ocean Biology and Biogeochemistry Program at NASA Headquarters in Washington. “This is fundamental, as plankton communities are the base of the marine food web on which important economic activities rely.”
The research team combined two decades of data from NASA’s Aqua satellite’s Moderate Resolution Imaging Spectroradiometer (MODIS) sensor with plankton samples collected on research cruises around the world. The approach linked large-scale satellite observations with genetic markers in Prochlorococcus, a tiny but abundant marine microbe that shows signs of nutrient stress in its DNA. The result is a global map revealing where phytoplankton are thriving and where they’re struggling.
The strongest indications of nutrient stress on plankton appeared in the subtropical gyres, which are vast, relatively calm regions of the Atlantic, Pacific, and Indian oceans. In these areas, a layer of warm surface water stifles the flow of colder water from deeper in the ocean.
“When the surface of the ocean warms, it generates this very stable situation where a layer of low-density water sits on top of higher-density cold water,” said study coauthor Adam Martiny, an oceanographer at the University of California, Irvine. “You’ve probably experienced that if you’ve ever been to a lake in the summertime—it’s super warm right on the surface, and very cold deeper down when you stick your legs in.”
This layering blocks the upward flow of nutrient-rich water, limiting the availability of ocean surface nutrients that are crucial for plankton. In the South Pacific, one of the most nutrient-poor regions, a layer of warm surface water contributed to nitrogen and iron shortages, producing the most severe nutrient-related stress that the team discovered.
But the researchers were surprised to find that parts of the North Atlantic experience less nutrient stress than expected. Although there was evidence of a lack of phosphorus, the impact on microorganisms was comparatively mild.
That difference may reflect the biology of the organisms themselves. Phytoplankton can partially compensate for phosphorus shortages by recycling phosphorus more efficiently or replacing phosphorus-rich molecules inside their cells. Nitrogen shortages are harder to overcome because nitrogen is crucial for the proteins and cellular machinery required for photosynthesis and nutrient uptake.
The study revealed that nutrient stress is strongly correlated with seasons and major weather cycles such as El Niño and the Pacific Decadal Oscillation, which lead to warming waters in the Pacific Ocean. During La Niña events, which cool water over a large part of the Pacific, stronger upwelling brought more nutrients to surface waters and reduced stress in some regions. Superimposed on those multi-year cycles, however, was a longer-term trend.
From 2002 through 2021, average sea-surface temperatures increased across nearly 90% of the ocean area examined in the study. Over the same period, nutrient stress generally intensified, consistent with previous hypothesis that warming oceans may become increasingly stratified and less able to replenish surface nutrients.
In many nutrient-poor regions of the Southern Hemisphere, however, the researchers found evidence that nutrient stress had not increased as much as expected despite significant warming. They suspect that microbes capable of capturing nitrogen from the air may partially offset the effects of reduced nutrient mixing.
That finding hints that marine ecosystems may possess more resilience to warming climates than some models predict. It also underscores the complexity of forecasting how ocean biology will respond to continued warming.
“We have two really powerful tools,” said study coauthor Michael Behrenfeld, a biochemist with Oregon State University in Corvallis, Oregon. The tools include satellite observations and cellular studies. “Both produce big data sets, but they are kind of opposites. We have very detailed data about microscopic phytoplankton … and then we have global coverage with satellites.”
By combining satellites that monitor the entire ocean with genetic clues carried inside microscopic plankton, the researchers say they are gaining a new way to track biological responses to changing environmental conditions near real time.
By James Riordon
NASA’s Earth Science News Team
Media contact: Elizabeth Vlock
NASA Headquarters
The post NASA Satellites Help Map Ocean Nutrient Stress appeared first on NASA Science.
Take a plunge into Australia’s Great Barrier Reef, and you’ll come face to face with a what looks like a gorgeous rocky garden. This dazzling array of corals is the largest living structure on Earth. Some, like staghorn coral, look like leafless bushes sprouting from the ocean floor. Others, such as honeycomb coral, resemble rocks dotted with tiny holes. Then there’s brain coral, which, you guessed it, look like brains. And that’s just a small sample of the brilliant diversity to be found in a reef.
Despite their appearance, corals are animals. These invertebrates are close relatives of jellyfish and sea anemones. Most corals live in colonies of thousands of tiny individuals called polyps growing together. Each polyp sports a cylindrical body topped with a ring of tentacles surrounding its mouth.
“You can think of them as upside-down jellyfish,” says Peter Cowman. At Queensland Museum Tropics in Townsville, Australia, this marine biologist studies the evolutionary history of corals.
Like their jellyfish cousins, coral polyps use tentacles to feed and defend themselves. These appendages are packed with specialized stinging cells called nematocysts. Each contains a coiled, barbed thread. When touched, the cell “fires” this thread to inject venom into its target. Most corals hunt at night, snagging tiny zooplankton, floating eggs and small fish.
Many corals have a more laid-back way of feeding, too. Inside their tissues, they house algae called zooxanthellae (Zoh-uh-ZAN-thuh-lay). (This is an example of symbiosis, where two creatures benefit by associating together.) These single-celled organisms are photosynthetic. They use light and carbon dioxide to make sugars and proteins. Most of these nutrients are offered to the coral. “In return, the coral gives them a safe apartment to live in,” says Cowman. “They’re the solar-powered roommates.”
Corals that rely on light-powered algae live in shallow, warm waters. It’s here where you find coral reefs. These structures are made by a type of coral called hard or stony corals. “They are the engineers of the reef system,” says Cowman.
Stony corals produce hard skeletons of aragonite, a mineral made of calcium carbonate. Over hundreds to thousands of years, generations of polyps can build up massive reef systems.
Soft corals, like their name implies, don’t make these stony skeletons. Instead, they have bendy structures that move with ocean currents. They can often be found living among reefs, though. “The soft corals are like the decorators,” says Cowman. “They look like underwater swaying trees and fans.”
Some corals — both soft and hard — can live in deep water, up to 6,000 meters (20,000 feet) below the water surface. Stony corals found at these depths are often smaller than their shallow-water counterparts. Most tend to live as single polyps. Some species do create reefs, but they’re less of a team effort. Called “mounds,” these deep reefs grow over the skeletons of old or dead corals on the ocean floor.
The first true corals appeared about 500 million years ago. But many of today’s reef-building corals can be traced back to the end of the Cretaceous Period some 66 million years ago. That’s when an asteroid impact wiped out about 80 percent of all animals (including the nonbird dinosaurs).
“The mass extinction had an effect in the oceans as well,” says Cowman. Existing reef-builders died out. Their extinction allowed stony corals to take their place. Today, corals are found in all of the world’s oceans.
But many are at risk from human activities. These dangers include pollution and destructive or heavy fishing. One of the biggest threats is climate change. High water temperatures and too much sunlight can stress corals, leading to a phenomenon called coral bleaching.
At higher temperatures, the algae produce toxic compounds. “When the water gets too hot, the coral gets stressed and they evict their zooxanthellae,” says Cowman. These microbes are also responsible for a coral’s bright colors. In their absence, the coral’s tissues turn transparent. All that remains visible is the coral’s white skeleton.
Bleaching doesn’t kill the coral immediately. If conditions improve, the coral may recover and even take the algae back in. But if coral go too long without their algal partners, they may get sick or die. “If they can’t get the zooxanthellae back in again, they [will] actually starve to death,” says Cowman.
This bleaching affects coral worldwide. Since 2023, the world’s corals have been experiencing the largest bleaching event on record.
Many scientists and conservationists are concerned about corals’ future. Coral reefs are among Earth’s most diverse ecosystems. Though they cover less than one percent of the seafloor, coral reefs support at least one-fourth of all marine species. “On land, you think of rainforests as having all that diversity,” says Cowman. “The rainforests of the sea are the coral reefs.”
Corals are important for people, too. Humans rely on many reef-dwelling species for food. Reefs can also buffer coasts against damage from strong waves or storms, says Cowman. Chemical compounds from reef animals have been found to help fight pain and treat cancer. And reefs support tourism around the globe. The food, products, jobs and coastal protection provided by corals are estimated to be worth $2.7 trillion per year.
Says Cowman, “as humans, we rely on reefs a lot in our daily lives, even if we don’t think of it.”
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