As Earth’s oceans warm, microscopic marine organisms are experiencing increasing stress due to a lack of vital nutrients. A new study combining NASA satellite observations, ocean surveys, and genetic testing on marine microorganisms suggests that warming ocean waters are limiting nutrient availability across much of the global ocean, with the potential to reshape marine ecosystems.
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 its health 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, supporting long-standing concerns 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 watch the biological effects of a warming climate unfold across the planet in near real time.
By James Riordon
NASA’s Earth Science News Team
Media contact: Elizabeth Vlock
NASA Headquarters
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James Riordon
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