Ocean heatwaves are breaking Earth’s hidden climate engine

Investigating Marine Heatwaves and Carbon Movement

To examine how heatwaves affect ocean ecosystems and carbon cycling, researchers analyzed more than ten years of biological data from the Gulf of Alaska. This region endured two major marine heatwaves over that period: the first from 2013 to 2015, known as "The Blob," and another from 2019 to 2020.

"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," said the lead author, Mariana Bif, previously a research specialist at MBARI and now an assistant professor in the Department of Ocean Sciences 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."

Tracking Changes with Robotic Floats and DNA Analysis

The team used data from the Global Ocean Biogeochemical (GO-BGC) Array, a project funded by the US National Science Foundation and led by MBARI. This initiative uses autonomous robotic floats to measure temperature, salinity, nitrate, oxygen, chlorophyll, and particulate organic carbon (POC) every five to ten days across the water column. The researchers also used ship-based surveys from Fisheries and Oceans Canada's Line P program, which collected seasonal samples to identify plankton species through pigment chemistry and environmental DNA (eDNA) sequencing.

How Heatwaves Reshaped the Ocean's Carbon Cycle

The study found that marine heatwaves disrupted the foundation of the ocean food web and altered how carbon moved through the water, though the effects differed between the two events.

Under normal circumstances, photosynthetic phytoplankton convert carbon dioxide into organic material. When these organisms are consumed by larger marine life, the resulting waste particles sink through the mesopelagic (or "twilight") zone, between 200 and 1,000 meters (about 660 to 3,300 feet), effectively locking carbon away in the deep sea for thousands of years.

During the 2013-2015 heatwave, phytoplankton productivity increased in the second year, but the resulting carbon particles accumulated around 200 meters (roughly 660 feet) below the surface instead of sinking farther down.

In contrast, during the 2019-2020 event, carbon particles built up near the surface early on. This buildup could not be explained solely by phytoplankton activity; it appeared to result from the recycling of organic matter by marine organisms and the accumulation of detritus. Eventually, this material sank into the twilight zone but stalled between 200 and 400 meters (about 660 to 1,320 feet), never reaching the deep ocean.

Cascading Effects Across the Food Web

Researchers attributed these differences to shifts in phytoplankton populations that rippled through the ecosystem. The changes favored smaller grazers that produce slow-sinking waste, keeping carbon near the surface instead of sending it to greater depths.

"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," said Bif.

This research demonstrated that not all marine heatwaves are the same. Different plankton lineages rise and fall during these warming events, underscoring the need for long-term, coordinated monitoring of the ocean's biological and chemical conditions to accurately model the diverse, and expansive, ecological impacts of marine heatwaves.

"This research marks an exciting new chapter in ocean monitoring. 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. It's a great example of how collaboration can help us answer key questions about the health of the ocean," said MBARI Senior Scientist Ken Johnson, the lead principal investigator for the GO-BGC project and a coauthor of the study.

Ocean observations and models suggest that marine heatwaves have been expanding in size and intensifying over the past few decades. The ocean absorbs a quarter of the carbon dioxide emitted each year, thanks to the steady stream of carbon particles sinking from the surface to the deep sea. A warmer ocean can mean less carbon locked away, which in turn can accelerate climate change. Beyond the changes to carbon transport, the shifts in plankton at the foundation of the ocean food web have cascading impacts on marine life and human industry too.

"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," said Bif.

This work was funded by the US National Science Foundation's GO-BGC project (NSF Award 1946578 with operational support from NSF Award 2110258), with additional support from the David and Lucile Packard Foundation, China National Science Foundation (grant number: 42406099), Fundamental Research Funds for the Central Universities (grant number: 20720240105), Danish Center for Hadal Research (Grant No. DNRF145), and Fisheries and Oceans Line P program.