Award: OCE-2124415

Life in the ocean plays a major role in regulating Earth’s climate by transforming atmospheric carbon into biomass and moving it from the surface to the deep sea, where it can remain for centuries. This process, known as the biological pump, is largely responsible for the vertical transport of organic carbon from well-lit surface to the ocean interior. However, only a small portion 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 the ocean’s response to future changes in the global carbon cycle. This project contributed to the NASA EXPORTS program, a large, multidisciplinary effort to understand how and why the biological pump varies across ocean regions. Our work focused on EXPORTS Science Question 2: What controls the efficiency of vertical transfer of organic matter below the well‑lit surface ocean? 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.

 

This project focused mainly on the productivity bloom in the North Atlantic Ocean in spring 2021, when phytoplankton, the single-celled photosynthesizers that dominate ocean productivity, were at their peak of activity and starting to die off (the decline phase). We compared the results with our previous NSF-funded work in the less productive North Pacific. We applied a technique called compound‑specific isotope analysis of amino acids (AA‑CSIA), which uses the chemical fingerprints of individual amino acids to trace the origins and transformations of organic matter. Using this approach, we discovered striking differences in the composition of sinking particles between the two regions. In the North Pacific, sinking material was dominated by zooplankton fecal pellets, a form of compact, fast‑sinking carbon. In contrast, during the decline of the spring phytoplankton bloom in the North Atlantic, sinking particles were composed largely of dead phytoplankton and the bacteria that colonize and break down the particles. These differences reveal that the biological pump operates through different mechanisms depending on the productivity of the region and the stage of the phytoplankton bloom.

 

The feeding behavior of zooplankton also plays a key role in carbon transfer. By analyzing their amino acid isotopes 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. Our results also showed that zooplankton selectively fed on different phytoplankton groups as the North Atlantic bloom declined, shifting from diatoms to dinoflagellates. 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.

 

In addition to advancing ecological understanding, this project improved the tools used to study the biological pump. We developed a computational workflow to identify which components of AA‑CSIA data best distinguish zooplankton food sources, and we 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. For the first time, we also compared AA-CSIA results with findings from carbohydrates. During the North Atlantic bloom, phytoplankton tend to produce abundant carbohydrate-rich material, which is carbon-rich and nitrogen-poor. We examined the carbohydrate content of particles as a complement to our analysis of amino acids, which are nitrogen-rich. We found a high concentration of carbohydrates in a size fraction of particles corresponding to the size of diatoms, and this concentration varied over time during the bloom in parallel with the types of phytoplankton present. During the mid-bloom, we found high concentrations of specific forms of carbohydrates that can produce large, sinking particles (known as 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 Miami and the University of Hawaii, providing hands‑on training in oceanographic fieldwork, laboratory analysis, and computational data interpretation. We shared our findings with the public through a Miami-based episode of Voice of the Sea, a local television program broadcast across Hawaii and Pacific Island communities and promoted online and publicized through social media to the south Florida community. 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: 02/13/2026
Modified by: Hilary G Close