Fecal pellets play a crucial role in regulating the planet’s temperature. Phytoplankton, tiny marine organisms, utilize sunlight to absorb carbon dioxide and produce oxygen. When these phytoplankton are consumed by zooplankton, the latter excrete pellets that sink to the ocean floor. This process acts like a massive toilet, transferring carbon from the surface waters to the depths, effectively keeping atmospheric CO2 levels in check.
However, as human activities inject increasing amounts of carbon into the atmosphere and elevate ocean temperatures, significant changes in this natural process may occur. In the northeastern Pacific, off Alaska, two notable heat waves struck between 2013 and 2015 and again from 2019 to 2020. A recent study highlights how these heat events altered the composition of phytoplankton and zooplankton, obstructing the carbon transport mechanism.
“Long-term studies provide critical context and serve as important warnings,” said Anya Štajner, a biological oceanography PhD candidate at the Scripps Institution of Oceanography, who was not part of the study. “The ocean is evolving. Not only will this impact marine ecosystems, but it will eventually affect humans, as we depend on oceans for our air, food, and climate regulation.”
Although each oceanic region possesses its own distinct chemistry and ecology, the changes observed in this area may not occur universally. Nevertheless, the recent heat waves significantly impaired the ocean’s ability to sequester carbon, a particularly troubling development considering oceans absorb approximately 25% of humanity’s CO2 emissions. “While we can generalize that similar patterns may emerge in other marine heat waves, regional assessments remain critical,” remarked Colleen Kellogg, a microbial oceanographer at Canada’s Hakai Institute and a co-author of the research published today in Nature Communications.
Researchers utilized a decade’s worth of data from Biogeochemical Argo floats—autonomous devices that traverse the water column to gather ocean chemistry readings. This technology allowed scientists to obtain a continuous stream of data without constant boat presence in the often inhospitable northeastern subarctic Pacific Ocean.
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The heat waves began similarly to land-based heat waves, driven by atmospheric warming. Remarkably, oceans have absorbed nearly 90% of the excess heat generated by human activities. While only 2% of the ocean surface experienced extreme temperature spikes in the 19th century, that figure has now soared above 50%. Barring drastic reductions in greenhouse gas emissions, such heat waves are poised to grow in frequency and intensity. Recently, the northern Pacific has been breaking temperature records, potentially exacerbated by 2020 regulations that reduced aerosol emissions from ships, which typically help cool the planet by reflecting sunlight.
The lack of wind during the two heat waves significantly aggravated the situation. Normally, winter winds mix surface waters with cooler depths, maintaining a more uniform temperature. However, this didn’t occur during the heat waves, resulting in stagnant waters reminiscent of late summer conditions.
Warmer water is less dense and tends to remain at the surface, creating a thermal cap. “In the following spring and summer, that water becomes even warmer, as it didn’t cool during the winter,” explained Mariana Bif, a marine biogeochemist at the University of Miami and the lead author of the study. “The impact of marine heat waves starts in the atmosphere and is then transferred into the ocean.”
The two heat events unfolded differently. The first coincided with an El Niño phenomenon, which elevated temperatures in the northeast Pacific, while the second was marked by decreased salinity due to altered ocean circulation patterns. The lower salinity, resulting in decreased density, allowed the lighter water to remain on the surface, further reinforcing the warm cap.
Moreover, the absence of winter mixing disrupted the nutrient supply typically drawn from deeper waters, depriving surface phytoplankton of essential elements necessary for growth. The combination of elevated temperatures and low nutrient availability sparked a drastic change in the marine ecosystem.
This alteration transformed the marine ecosystem’s dynamics. Different phytoplankton species require varying nutrient amounts and ratios, akin to plants on land. “Typically, in well-mixed, nutrient-rich regions, larger phytoplankton thrive, producing substantial biomass,” noted Bif.
However, the heat waves favored smaller phytoplankton species that required less nutrition for growth, resulting in their proliferation and a decline in larger forms. Given that various zooplankton species feed on different-sized phytoplankton, this shift allowed smaller zooplankton to benefit from an abundance of food. “These smaller zooplankton produce tiny fecal pellets that are more likely to float than sink,” Kellogg explained. “This change could hinder the movement of carbon from the surface down to the deep ocean.”
Throughout the heat waves, the researchers were able to track carbon sinking patterns thanks to their extensive data from different ocean depths. They observed that during the first heat wave, carbon particles accumulated 660 feet deep, while during the second event, they stacked up between 660 and 1,320 feet. Even in these depths, zooplankton grazers continued to break down the carbon particles into smaller pieces that remained suspended and unable to sink. During the second heat wave, an uptick in smaller zooplankton further increased the manufacture of non-sinking fecal pellets.
As the ocean’s carbon toilet malfunctioned, an influx of organic waste added further complexity to the scenario, providing bacteria with ample material to decompose, which released additional CO2 back into the water. Over time, these CO2-rich waters would be mixed back to the surface, where it could escape into the atmosphere.
Going forward, scientists must study future heat waves in various ocean regions to determine if these dynamics persist and how they might impede the ocean’s carbon sequestration capabilities. Simultaneously, phytoplankton and zooplankton face additional challenges, such as ocean acidification, which may hinder certain species’ capacity to form protective shells.
A decline in phytoplankton equates to reduced oxygen production and less food availability for zooplankton, impacting a broad range of marine species, including whales. “Understanding the dynamics at the base of the food web is vital,” stressed Štajner. “It will not only provide insight into how changes will trickle up to larger marine species we care about but also extend to our understanding of climate.”
Fortunately, with thousands of Biogeochemical Argo floats deployed globally, researchers continue to gain a clearer perspective on how oceanic conditions are altering, along with the behavior of phytoplankton. “Oceans remain understudied and under-sampled,” Bif asserted. “Yet they are pivotal in our understanding of climate. We cannot grasp what we do not observe.”
