Marine heat waves describe instances of extraordinarily warm waters that can linger at the surface of the ocean for months. Much like the heat waves we experience on land, marine heat waves can alter environmental chemistry and disrupt biological processes. While catastrophic losses of megafauna are hard-to-miss indicators of a system in distress, researchers are now starting to amass enough data to understand how microbial organisms at the base of the ocean’s food webs are also responding to heat waves.
A new study published in Nature Communications presents a decade of measurements documenting two successive heat waves in the northeastern Pacific Ocean. The paper’s interdisciplinary team of authors used a combination of an autonomous robotic float, a research cruise, and satellite data to understand how microbial communities in the region reorganized in response to the extreme events.
The researchers discovered that production of organic matter increased at the ocean surface during the heat waves, but the carbon-rich particles didn’t sink or swim—rather, they just stayed in place.
The Biological Carbon Pump
Phytoplankton—tiny photosynthesizing microbes—prime the biological carbon pump. By using sunlight and carbon dioxide (CO2) to grow, they draw carbon out of the atmosphere and into the ocean’s carbon cycle. Zooplankton graze on the vast fields of these plantlike organisms, transporting carbon deeper into the water column in the form of fecal pellets and chunks of half-eaten plankton. Eventually, some of these particles sink deep enough to feed ecosystems of the deep ocean.
“The capacity for the ocean to sequester carbon relies on microbes at the base of the food web.”
This carbon pump represents a globally relevant buffer against the impacts of climate change, as the ocean absorbs approximately a quarter of CO2 emitted by human activity. Some estimates suggest that our current atmospheric concentration of CO2 could increase by as much as 50% if the biological carbon pump stopped shuttling carbon to the depths of the ocean.
“The capacity for the ocean to sequester carbon relies on microbes at the base of the food web, so it’s very important that we start understanding what these impacts from marine heat waves are on the microbial communities,” explained Mariana Bif, lead author of the new study. Bif is an assistant professor at the University of Miami and was previously a researcher with the Monterey Bay Aquarium Research Institute (MBARI).
When the Food Web Gets Tangled
In both of the marine heat waves tracked in the study, researchers found that the biological carbon pump showed signs of overheating. Carbon-rich particles loitered at approximately 200 meters (660 feet) below the surface, but during the two heat waves, different mechanisms caused the pileup.
The first heat wave included in the study began in 2013, when unusually weak winds over the Pacific failed to blow the warm air of summer back to the mainland of the United States. The heat wave, dubbed “the Blob,” made headlines as warm, stagnant, oxygen-deficient waters resulted in massive die-offs of fauna from all corners of the Pacific before dissipating in 2015.
In 2019, patchy cloud cover over the ocean and a shallower mixed layer at the sea surface set the stage for another heat wave to sweep the northeastern Pacific. This second heat wave brought temperatures right back up and became known as “the Blob 2.0.”
Bif and her coauthors found that during both heat waves, the marine microbial community went through a change in its “middle managers.”
Within the initial Blob years, physical and chemical conditions favored smaller phytoplankton species, which in turn favored a new herd of zooplankton grazers. This discrete food web eventually created an ocean layer full of organic particles that were too light to sink into the denser waters of the deep.
During the Blob 2.0, concentrations of particulate organic matter were even higher, but the increase wasn’t all from primary production. This time, conditions favored thrifty species. Organisms that could opportunistically feast on detritus and lower-quality organic matter became more prevalent, showing that the system was cycling and recycling carbon to keep it at the top of the water column. Within this community, parasites thrived, and organisms (including a group of radiolarians) that had never previously been seen in the northeastern Pacific started becoming regulars.
Measuring in the Middle of Nowhere
The array of technology used in the study distinguishes it from previous efforts to catalog the effects of marine heat waves.
“We’re now moving into an era of ‘big data’ in ocean biogeochemistry, whereas before we were just restricted to what we could collect from ships.”
“We’re now moving into an era of ‘big data’ in ocean biogeochemistry, whereas before we were just restricted to what we could collect from ships,” said Stephanie Henson, a principal scientist at the National Oceanography Centre in Southampton, U.K. Henson was not involved in the study.
Henson explained that autonomous floats and other advanced monitoring systems are allowing researchers to work with datasets that span beyond the length of a research cruise.
“People have been studying marine heat wave responses in systems like coral reefs and so on,” Henson said, explaining that researchers have observed that not every biological response is the same from one marine heat wave to the next. However, she noted that this study was the first she’s seen that demonstrates that ocean carbon fluxes are also having complex responses to marine heat waves.
To check the vital signs of the Pacific before, during, and after each of the heat waves, the researchers tapped into the Global Ocean Biogeochemistry Array (GO-BGC). GO-BGC instruments are a subset of the Argo array, a global network of thousands of autonomous robotic floats. Each float drifts freely in ocean currents, keeping tabs on pH, salinity, temperature, and more.
Despite all that they can do, the floats are not able to collect microbial samples. For this, instead of Bif seeking the data, the data came to Bif.
Steven Hallam, a microbiologist at the University of British Columbia and a coauthor on the new study, reached out to Bif after reading an interview with her about her work on marine heat waves. He had a hunch that the planktonic DNA samples stored in his lab’s freezer might be helpful for Bif’s investigation into the ocean’s carbon cycle. Scientists in Hallam’s lab group had previously published research about bacterial communities in the same region, using samples collected during research cruises along the Line P transect off the coast of British Columbia.
After some back-and-forth via email, Hallam’s lab group reran the samples, expanding the analysis from bacteria to the entire community composition, resulting in a significant contribution to Bif’s study.
While the story of how the planktonic DNA came to Bif is a testament to the power of science communication and collaboration, Henson noted that the Line P transects “don’t necessarily overlap spatially with the regions of greatest impact of the marine heat waves” and combining datasets of different scales (such as shipboard data and the autonomous float datasets) should be done cautiously.
Still, Henson added, “it’s the best we can do, at the moment.”
Lingering Uncertainties
As for future research, Bif is involved in a few new projects exploring marine deoxygenated regions but said, “My focus is always the BGC-Argo floats.”
Bif noted that it will be interesting to look at BGC-Argo data from the floats that are in the middle of the marine heat wave currently affecting the North Pacific. That heat wave is already showing signs of slowing down, though scientists say it will likely hang around through the winter.
“I’m not sure if this one is going to have the legs that some of these previous marine heat waves in the region had,” said Nick Bond, who was not involved in this research but studied marine heat waves as part of his previous role as the Washington state climatologist. He is now a senior research scientist at the University of Washington.
“What we don’t measure, we can’t understand. We need more investments into monitoring the ocean.”
Bond added that while there’s “tentative evidence” that climate warming may be increasing the frequency of marine heat waves in the Pacific, there’s still much more to learn before scientists can accurately forecast how they will behave in the future.
Meanwhile, another looming unknown for this field of research is developing back onshore.
“There is a bit of a concern in the community because at the moment, for the global Argo program, the U.S. contributes about half of the floats that are deployed,” said Henson, her concern alluding to recent budget cuts to nearly all areas of federally funded research in the United States. However, she explained that other countries are stepping up with contributions to keep the Argo program afloat.
“What we don’t measure, we can’t understand. We need more investments into monitoring the ocean,” said Bif.
—Mack Baysinger (@mack-baysinger.bsky.social), Science Writer