Award: OCE-2124416

The ocean plays a major role in regulating Earth’s climate by moving carbon from the surface to the deep sea, where it can remain for centuries. This process, known as the biological pump, transports organic carbon from sunlit waters into the ocean interior. Only a small fraction of this material reaches the deep ocean; most is consumed by bacteria and animals along the way. Understanding what controls the efficiency of this vertical transfer is essential for predicting how the ocean will respond to future climate change and how much atmospheric CO₂ it can absorb. This project contributed to the NASA EXPORTS program, a large, multidisciplinary effort aimed at understanding how and why the biological pump varies across ocean regions. Our work addressed EXPORTS Science Question 2: What controls the efficiency of vertical transfer of organic matter below the well‑lit surface ocean? To answer this, we used advanced chemical techniques to compare the types of organic particles sinking out of the surface ocean and the food sources of zooplankton across two contrasting regions: the highly productive North Atlantic during a spring phytoplankton bloom and the less productive North Pacific.

The project focused primarily on the North Atlantic bloom in spring 2021, when phytoplankton were at their peak and beginning to decline. We compared these observations with our earlier NSF‑funded work in the North Pacific. Using compound‑specific isotope analysis of amino acids (AA‑CSIA), which traces the chemical fingerprints of individual amino acids, we identified striking differences in the composition of sinking particles between the two regions. In the North Pacific, sinking material was dominated by zooplankton fecal pellets, which are compact, fast‑sinking particles that efficiently transport carbon to depth. In contrast, during the decline of the North Atlantic bloom, sinking particles were composed largely of dead phytoplankton and the bacteria that colonize and break them down. These differences show that the biological pump operates through distinct mechanisms depending on regional productivity and bloom stage.

Zooplankton feeding behavior also plays a critical role in carbon transfer. By analyzing amino acid isotopes in zooplankton collected at different depths, we found that North Atlantic zooplankton relied heavily on fresh phytoplankton material, even in the deep ocean. In the North Pacific, deep‑dwelling zooplankton fed more on degraded particles. We also observed that zooplankton in the North Atlantic selectively shifted their diet from diatoms to dinoflagellates as the bloom declined. These findings highlight the importance of daily vertical migrations by zooplankton, which transport surface‑derived food into the deep ocean and influence how much carbon is ultimately sequestered.

Beyond advancing ecological understanding, this project improved the tools available to study the biological pump. We developed a computational workflow to identify which components of AA‑CSIA data best distinguish zooplankton food sources and re‑analyzed published datasets from multiple ocean regions to refine global assessments of zooplankton diets. We demonstrated that AA-CSIA is capable of identifying major differences in the composition of sinking particles in different productivity regimes, and we extended this comparison to archived samples from past global sampling programs. We also compared AA‑CSIA with measurements of carbohydrates, which are carbon‑rich compounds produced in large quantities during phytoplankton blooms. During the North Atlantic bloom, we found high concentrations of specific carbohydrates associated with the formation of large, fast‑sinking aggregates. These aggregates contributed to a major pulse of sinking carbon detected by multiple EXPORTS teams, demonstrating how bloom dynamics can trigger significant carbon sequestration events.

This project supported both undergraduate and graduate students at the University of Hawai‘i and the University of Miami, providing hands‑on training in oceanographic fieldwork, laboratory analysis, and computational data interpretation. We shared our findings from the North Atlantic with the public through two episodes of Voice of the Sea, a television program broadcast across Hawai‘i and Pacific Island communities and promoted online and through social media. Across our NSF-supported projects in the North Pacific and North Atlantic, we produced a total of six episodes that were recognized with international Telly Awards for excellence in educational programming, underscoring the project’s impact on public science communication. Results have been presented at national and international scientific meetings and submitted to open‑access scientific journals, ensuring broad accessibility to the scientific community and the public.

 

Last Modified: 03/04/2026
Modified by: Brian N Popp