Award: OCE-1829425

Organic matter production in the upper-ocean and its vertical removal to the deep ocean sequesters atmospheric CO2 into long-timescale reservoirs thus removing CO2 from the atmosphere and storing it in the deep ocean. The biological pump is largely responsible for this vertical transport of organic carbon from the surface to the ocean interior. However, only a small fraction of organic material produced in surface waters is sequestered in the deep ocean. The rest is consumed, or respired, by bacteria and larger organisms. The efficiency of organic carbon transfer to the deep ocean is a highly sensitive parameter needed to predict future changes in the global carbon cycle and the ocean’s ability to take up CO2 from the atmosphere. We used measurements of stable isotope ratios of carbon and nitrogen in different amino acids (compound-specific isotopic analysis of amino acids, known as AA-CSIA) of organic particles and zooplankton in the subarctic Pacific, a region of relatively low carbon flux to the deep ocean, along with measurements of the abundances of different forms of amino acids, and other parameters derived from these analyses to identify different mechanisms of organic matter degradation, with important implications for nutrient availability to deep-water food webs. Our results enabled us to developed two nascent models: (1) a mixing model that uses AA-CSIA to estimate the phytodetritus, fecal pellet, and microbially degraded composition of particles, such that the vertical alteration mechanisms and size distribution of these materials can be detected; and (2) an inverse relationship between carbon flux into the deep ocean and the reliance of deep ocean food webs on small, degraded particles. This work capitalized on existing, comprehensive field programs (NASA EXPORTS) specifically focused on building a predictive framework relating surface ocean properties to the vertical flux of organic carbon. The first EXPORTS field study in the subarctic Pacific provided us the materials from which the models were developed. The tremendous amount of data collected on all aspects of the biological pump as part of the EXPORTS program aided the development and interpretation of our amino acid isotopic tools. Our work assessed the relative importance of packaging organic matter in fecal material, particle disaggregation, microbial reworking, and zooplankton dietary usage on vertical patterns of particle flux in the subarctic Pacific, using empirical methods independent of incubation techniques or metabolic rate measurements. Application and refinement of our newly-developed isotopic indicators will enable development of a globally generalized isotopic framework for assessing the degradative history of particulate organic matter and its relationship to mesopelagic dietary resources, including small, microbially degraded particles that are often not accounted for as a metazoan dietary resource. The work directly addressed EXPORTS Science Question 2: What controls the efficiency of vertical transfer of organic matter below the well-lit surface ocean? The results of this work additionally will provide observational comparisons to global models of carbon flux composition and pelagic food web resources. Results of our work have been broadly communicated via production and distribution of several episodes of Voice of the Sea, a local television program that aired in Hawaii and across many Pacific islands. Episodes were also posted online and publicized through social media to the south Florida community. This project supported graduate and undergraduate students at the University of Hawaii, which is a designated minority-serving institution. Last Modified: 12/26/2022 Submitted by: Brian N Popp