Ocean’s Carbon Conveyor Belt Jams During Heat Waves

The ocean’s biological carbon pump, normally a reliable conveyor belt carrying carbon from sunlit waters to the deep sea, keeps jamming during marine heatwaves, and scientists have finally figured out why by watching microscopic organisms for more than a decade.

A new study published today in Nature Communications reveals that marine heatwaves reshape ocean food webs in unexpected ways, causing carbon particles to pile up at intermediate depths instead of sinking to the abyss where they would be locked away for millennia. The findings matter because the ocean absorbs roughly a quarter of human carbon emissions each year, and a malfunctioning biological pump means more carbon stays in the atmosphere.

Researchers tracked two successive marine heatwaves in the Gulf of Alaska: the infamous “Blob” from 2013 to 2015, and another from 2019 to 2020. What surprised them was how differently the ocean responded each time, even though both events warmed the water substantially.

“Our research found that these two major marine heatwaves altered plankton communities and disrupted the ocean’s biological carbon pump. 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.”

That is how Mariana Bif, the study’s lead author and now an assistant professor at the University of Miami, describes what happened. The research combined data from robotic floats that measured ocean conditions every five to ten days with ship-based surveys tracking plankton populations through pigment chemistry and DNA sequencing.

When Tiny Shifts Cascade Through Food Webs

During the 2013 to 2015 heatwave, the second year saw high carbon production by photosynthetic plankton at the surface. But instead of sinking rapidly, small carbon particles accumulated around 200 meters underwater (roughly 660 feet down) in what oceanographers call the twilight zone.

The 2019 to 2020 heatwave played out differently. In the first year, carbon particles piled up at the surface in record amounts that could not be explained by plankton photosynthesis alone. Instead, the accumulation likely resulted from carbon recycling by marine life and the buildup of detritus waste. This pulse eventually sank to the twilight zone but lingered between 200 and 400 meters rather than descending to truly deep water.

The culprits behind these disruptions were shifts in phytoplankton populations that cascaded through the food web. The changes led to a rise in small grazers who produce waste particles that sink slowly or not at all. Carbon got retained and recycled near the surface and in the upper twilight zone instead of being exported to the deep ocean.

The genetic data told a revealing story. During the 2015 heatwave, two groups of radiolarians (Acantharea and especially a genus called Cladococcus) became abundant. These protozooplankton can be important carbon exporters because of their mineral skeletons, and they likely contributed to the accumulation of small, slow-sinking particles called minipellets. Small copepods like Oithona also increased, producing fecal pellets that sink slowly.

Not All Heatwaves Break Things the Same Way

By 2019, the cast of characters had changed. Parasitic organisms called Syndiniales became more abundant in deeper waters, along with heterotrophic dinoflagellates. Meanwhile, copepod sequences declined overall, which may have relaxed grazing pressure on microzooplankton and allowed parasites to flourish. With fewer copepods around, overall fecal pellet production likely dropped, reducing carbon export from the mesopelagic zone.

Even the bacteria shifted. The cosmopolitan SAR11 bacterial clade, normally abundant, declined in 2019 compared to 2015. Experimental work elsewhere has shown SAR11 does better when dissolved organic matter is less degraded, so the heavy recycling and accumulation of organic matter during the 2019 event may have limited its success.

“The ocean has a biological carbon pump, which normally acts like a conveyor belt carrying carbon from the surface to the deep ocean. This process is powered by the microscopic organisms that form the base of the ocean food web, including bacteria and plankton.”

Ken Johnson, a senior scientist at the Monterey Bay Aquarium Research Institute and lead investigator for the Global Ocean Biogeochemistry Array project, emphasized how the research represents a new chapter in ocean monitoring. To really understand how a heatwave impacts marine ecosystems, he noted, scientists need observation data from before, during, and after the event.

The study deployed robotic biogeochemical Argo floats that measured temperature, salinity, nitrate, oxygen, chlorophyll, and particulate organic carbon throughout the water column. These autonomous instruments provided continuous monitoring that would be impossible with ships alone. The research team also incorporated pigment chemistry and genetic sequencing from water samples collected during Canada’s Line P program, which has occupied a transect in the Northeast Pacific for decades.

Ocean observations and models suggest marine heatwaves have been expanding and intensifying over recent decades. Even substantial reductions in greenhouse gas emissions are unlikely to slow the spatial expansion and duration of these events for many years. The shifts in plankton at the foundation of ocean food webs have cascading impacts not just on carbon transport, but on marine life and fisheries too.

The research demonstrates that sustained, long-term monitoring using multiple platforms is essential for understanding how future marine heatwaves will reshape ocean ecosystems. The Gulf of Alaska serves as a sentinel. What happens there offers clues about how warming will disrupt the ocean’s capacity to absorb and sequester carbon worldwide.

Nature Communications: 10.1038/s41467-025-63605-w