A new study published in Nature Communications reveals that marine heatwaves can significantly alter ocean food webs, reducing the ocean's capacity to capture and store carbon. The findings highlight how these extreme events can slow down carbon transport to the deep sea -- weakening one of Earth's natural climate buffers.
The research was conducted by an international team from the Monterey Bay Aquarium Research Institute (MBARI), University of Miami Rosenstiel School, Hakai Institute, Xiamen University, University of British Columbia, University of Southern Denmark, and Fisheries and Oceans Canada.
To assess how marine heatwaves affect marine ecosystems, the researchers analyzed over a decade of biological and chemical data from the Gulf of Alaska -- an area that experienced two major heatwaves: "The Blob" (2013-2015) and another from 2019-2020.
"The ocean has a biological carbon pump, which normally acts like a conveyor belt carrying carbon from the surface to the deep ocean," explained lead author Mariana Bif, former MBARI research specialist and now an assistant professor at the Rosenstiel School. "For this study, we wanted to track how marine heatwaves affected those microscopic organisms to see if those impacts were connected to the amount of carbon being produced and exported to the deep ocean."
The study used data from the Global Ocean Biogeochemical (GO-BGC) Array, a large-scale project funded by the US National Science Foundation and led by MBARI. This project deploys autonomous biogeochemical Argo (BGC-Argo) floats that collect data on ocean temperature, salinity, oxygen, nitrate, chlorophyll, and particulate organic carbon every few days.
The researchers also incorporated seasonal data from Fisheries and Oceans Canada's Line P program, which included pigment chemistry analysis and environmental DNA (eDNA) sequencing to track changes in plankton communities.
The analysis revealed that marine heatwaves disrupted the base of the food web -- especially plankton communities -- and consequently altered carbon cycling processes.
Under normal conditions, phytoplankton convert carbon dioxide into organic material, forming the foundation of the marine food chain. Their waste and remains sink through the ocean's "twilight zone" (200-1,000 meters), sequestering carbon for millennia.
However, the two heatwaves had contrasting effects:
These variations were attributed to changes in plankton populations. A rise in smaller grazers meant slower-sinking waste particles, resulting in carbon being recycled near the surface instead of transported downward.
"Our research found that these two major marine heatwaves altered plankton communities and disrupted the ocean's biological carbon pump," said Bif. "The conveyor belt carrying carbon from the surface to the deep sea jammed, increasing the risk that carbon can return to the atmosphere instead of being locked away deep in the ocean."
The study underscores that not all marine heatwaves have the same ecological consequences, as different plankton groups dominate during each event. This makes long-term, coordinated monitoring essential for predicting the diverse impacts of warming oceans.
"This research marks an exciting new chapter in ocean monitoring," said Ken Johnson, MBARI Senior Scientist and lead principal investigator for the GO-BGC project. "To really understand how a heatwave impacts marine ecosystems and ocean processes, we need observation data from before, during, and after the event. This research included robotic floats, pigment chemistry, and genetic sequencing, all working together to tell the entire story."
As marine heatwaves grow more intense and frequent, their effects on the ocean's carbon storage capacity could amplify climate change. The ocean currently absorbs about a quarter of global carbon emissions each year. A reduction in carbon export to the deep sea means more carbon could remain in the atmosphere.
"Climate change is contributing to more frequent and intense marine heatwaves, which underscores the need for sustained, long-term ocean monitoring to understand and predict how future marine heatwaves will impact ecosystems, fisheries, and climate," added Bif.
This work was supported by the US National Science Foundation's GO-BGC project (NSF Award 1946578 and 2110258), with additional funding from the David and Lucile Packard Foundation, China National Science Foundation, Fundamental Research Funds for the Central Universities, Danish Center for Hadal Research, and Fisheries and Oceans Line P program.
Founded in 1987 by philanthropist David Packard, the Monterey Bay Aquarium Research Institute (MBARI) is a nonprofit research organization dedicated to advancing ocean science and technology to better understand and protect our changing ocean. Learn more at mbari.org.