How marine heatwaves are stalling Earth’s carbon pump

Marine heatwaves do more than scorch coastlines and stress wildlife. New research reveals they can restructure marine food webs, slowing the descent of carbon from the sunlit surface into the deep sea.

The slowdown weakens the ocean’s ability to lock away planet-warming carbon for centuries or even millennia.

An international team stitched together more than a decade of biological and chemical observations from the Gulf of Alaska – an area hit by two major heatwaves, the 2013–2015 event dubbed “The Blob” and another in 2019–2020.

The researchers found that both events disrupted the base of the food web in ways that choked the biological “carbon pump,” though each did so differently – reminding us that no two heatwaves are alike.

The ocean’s carbon conveyor

Under normal conditions, microscopic plants called phytoplankton turn carbon dioxide into organic matter. Some of that carbon sinks when plankton die, clump together, or get eaten and repackaged into heavy, fast-sinking pellets by larger grazers. 

These particles fall through the ocean’s “twilight zone” (roughly 660–3,300 feet), sequestering carbon in the deep for thousands of years. Think of it as a conveyor belt that carries carbon out of contact with the atmosphere.

Lead author Mariana Bif of the University of Miami and colleagues wanted to see how heatwaves tug on the carbon belt’s gears, particularly the microbes and tiny animals that power it.

Tracking change from sea to sky

To capture the full picture, the team blended data from autonomous biogeochemical Argo floats and classic ship surveys.

The floats, deployed through the Global Ocean Biogeochemistry (GO-BGC) array, profile the water column every few days, measuring temperature, salinity, oxygen, nitrate, chlorophyll, and particulate organic carbon.

Ship expeditions along Canada’s Line P added seasonal snapshots of who was actually there – pigments that reveal phytoplankton groups and environmental DNA that fingerprints the community. That pairing let the scientists track what was produced at the surface, how it transformed, and where it ended up.

Two heatwaves, two carbon bottlenecks

During “The Blob,” phytoplankton were surprisingly productive in the heatwave’s second year. Yet instead of racing downward, a large share of carbon pooled around 660 feet (200 meters). The pump was running, but the belt jammed just beneath the surface layer.

The 2019–2020 event looked different from the start. The first year saw record-high buildups of carbon at the surface that couldn’t be explained by fresh photosynthesis alone. The team points to intense recycling – detritus and dissolved organic matter being chewed over and over again.

That pulse eventually sank into the upper twilight zone (about 660–1,300 feet), then stalled there rather than plunging to the deep sea.

In both cases, the choke point shifted upward in the water column. That kept more carbon within reach of currents and microbes that can return it to the air.

Tiny eaters, massive ocean impact

What changed? The community. Heatwaves reshuffled phytoplankton lineages and boosted small grazers that produce fine, slow-sinking waste. Those lighter particles linger, swirl, and get recycled instead of dropping like pebbles into the abyss.

The system becomes more “leaky,” trapping carbon near the surface and upper twilight zone instead of exporting it to depth.

“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.”

Heatwaves reshape ocean-climate ties

The ocean absorbs roughly a quarter of humanity’s annual CO₂ emissions, in part thanks to that steady rain of organic carbon into the deep.

If warming events keep pushing that rain shallower, less carbon will be locked away and more may loop back to the atmosphere, amplifying climate change.

Because plankton sit at the foundation of marine food webs, their reshuffling also ripples upward to fish, seabirds, and the industries that depend on them.

Crucially, the study shows heatwaves aren’t a single phenomenon with a single outcome. Different winners and losers in the plankton world can create different carbon pathways – even in the same region.

Tools for a changing ocean

This work is also a proof of concept for how to keep pace with rapid ocean change. Autonomous floats deliver year-round, depth-resolved chemistry; ships add who’s-who detail through pigments and genetics.

Together, they provide a “before-during-after” storyline that neither approach can offer alone.

“To really understand how a heatwave impacts marine ecosystems and ocean processes, we need observation data from before, during, and after the event,” said Ken Johnson, a co-author and lead investigator for GO-BGC. “This is what collaboration across tools and teams can deliver.”

Hotter seas, weaker carbon pump

Marine heatwaves are growing larger and more frequent. In the Gulf of Alaska, two events hit in less than a decade.

Each heatwave throttled the biological carbon pump, but in different ways. They reshaped plankton communities and tipped the balance toward recycling rather than export.

That makes sustained, coordinated monitoring essential, both to sharpen climate models and to help coastal communities and fisheries plan for a warmer, more variable ocean.

The study is published in the journal Nature Communications.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–