Migrating Zooplankton May Help Drive Carbon Transport

Every day, as the sun sets, billions of small animals make their way from the depths of the ocean to the surface to feed. As the next day begins, these zooplankton swim back down. It’s the largest synchronous migration on the planet, responsible for carrying vast amounts of carbon from the ocean surface to the deep.

Despite growing interest in tools to capture atmospheric carbon dioxide, this “active transport” of carbon by zooplankton is poorly understood and inadequately considered in conversations around leveraging natural processes in the ocean for carbon sequestration.

An interdisciplinary team of ecologists, modelers, and carbon accounting experts is working to change that.

Led by Karen Stamieszkin, a senior research scientist at Bigelow Laboratory, the team was funded last year by the Advanced Research Projects Agency-Energy to develop models to improve estimates of carbon transport by zooplankton.

That information can, in turn, inform tools for monitoring, reporting, and verification of marine carbon dioxide removal (mCDR) activities to maximize their efficacy — and minimize their impact. These prediction tools could also serve other users, such as helping the shipping industry with route planning to avoid marine mammals that feed on zooplankton or providing information on fish migration and food availability for the seafood and aquaculture industries.

The project, called Zooplankton-Mediated Export Pathways for Carbon Sequestration, or Z-Trace, features collaborators from UCLA and Arizona State University’s Bermuda Institute of Ocean Sciences and Center for Negative Carbon Emissions.

“A lot of people just think that zooplankton migration isn’t important to these strategies, but we need to consider it to fully understand how much carbon is actually being transported to the deep sea,” Stamieszkin said. “Having more accurate models will highlight where the uncertainties are and give us the information to confidently know where — and even if — we should do mCDR.”

The team is focused on ocean iron fertilization, an mCDR strategy that involves adding iron to seawater to stimulate phytoplankton growth. Those microscopic plants absorb carbon dioxide and then sink with it when they die. Given the focus on phytoplankton, models of how much carbon can be removed through iron fertilization have emphasized the passive sinking of phytoplankton.

“When you look at the magnitude of carbon from active transport by zooplankton compared to the amount that sinks passively, it’s not that impressive,” said Daniel Clements, a postdoctoral scientist working on the project with Stamieszkin. “But we’re learning that we should be less concerned about magnitude than long-term storage. And when you look at storage capability of zooplankton, and how efficient they are at putting carbon deeper into the water, it’s a different story.”

But quantifying that long-term sequestration potential is a challenge.

In an upcoming paper in the Annual Review of Marine Science, Clements and the team identified four pathways through which zooplankton influence the cycling of carbon: they’re breathing, exchanging oxygen for carbon dioxide; they’re excreting dissolved carbon through pee and poop; they’re absorbing carbon into their body to grow; and they’re dying and being broken down by the vast pool of deep ocean microbes. To model carbon transport, the authors argue, one needs to accurately estimate how much — and at what depth — carbon is released through each pathway.

The paper also serves as the blueprint for the steps that the project team plans to take.

So far, researchers from Bigelow Laboratory and the Bermuda Institute of Ocean Sciences have been examining the relationship between zooplankton physiology and their behavior in order to develop a novel, fine-tuned model that quantifies with unprecedented detail how much carbon moves during the daily migration. That information will then be incorporated by researchers at UCLA to improve the accuracy of a regional biogeochemical model of the subarctic northeast Pacific, a priority area for iron fertilization.

“Most existing models assume there’s just one type of generalist zooplankton, but that’s not representative of what’s happening,” Clements said. “We’re trying to create something new that’s really rooted in the physiology of these animals with as few generalizations as possible.”

Meanwhile, the team from the Center for Negative Carbon Emissions is looking at the feasibility of using these tools to verify and monetarily quantify carbon storage.

“The question is whether you can account for the carbon sequestered through iron fertilization well enough to make it valuable in a carbon credit market,” Stamieszkin said. “Bringing CNCE into the conversation at this point, and getting the ecology and social science to interface in a quantitative way, increases the chances that what we do will be useful for guiding mCDR.”

Parallel to research efforts, the team is participating in outreach events, including the annual ARPA-E Energy Innovation Summit and the Ocean Visions Biennial Summit, both held this past March. These events give the researchers an opportunity to connect with potential investors and funders. More importantly, they’re a chance to bring together everyone in the mCDR space to get on the same page about what unknowns remain before any of these strategies move forward.

“Our goal is to bring good scientific knowledge to bear in the mCDR sphere,” Stamieszkin said. “We have to put science first to ensure these strategies are proven effective without causing harm to the environment.”

The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy (Award Number DE-AR0001834). The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Photo Captions:

Photo 1: Microscopy of a zooplankton from the copepod family Eucalanidae from the Gulf of California (Credit: Amy Maas, BIOS/ASU).

Photo 2: Microscopy of the zooplankton Thecosome pteropod, also called a sea butterfly, from the Gulf of California (Credit: Amy Maas, BIOS/ASU).

Photo 3: An assemblage showing the diversity of different zooplankton species from around the world (Credit: Hannah Gossner, BIOS/UNH).