Collaborative Research: Isotopic Indicators for Mechanisms of Organic Matter Degradation under High Productivity and High Carbon Flux Conditions (EXPORTS)

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The downward settling of organic material transports carbon out of the ocean surface, as part of a process called the biological pump. However, only a small fraction of organic material produced by organisms in surface waters makes it to the deep ocean. The rest can be fragmented or consumed (respired) by bacteria or larger organisms; the role of each process remains in question. Guided by recent results from the Pacific Ocean, the investigators will use the stable isotopes of carbon and nitrogen in amino acids to identify the input of fresh algal material, zooplankton feces, and bacteria to the biological pump in the North Atlantic spring bloom. With data from contrasting locations, the investigators will test and develop their isotopic models so they can be used to help predict global patterns in carbon transport. The work will be part of a large oceanographic field program (NASA EXPORTS). The tremendous amount of data collected in this program will aid the development and interpretation of the isotopic models. To share results broadly, the investigators will produce and distribute several episodes of Voice of the Sea, a local television program that will air in Hawaii and the Pacific islands. Episodes will be posted online and publicized through social media to the south Florida community. The project will support a Ph.D. student and an undergraduate student at University of Miami, which serves a 25% Hispanic population, and a Ph.D. student and an undergraduate student at University of Hawaii, a designated minority-serving institution.The proposed work will assess the relative importance of packaging organic matter in fecal material, particle disaggregation, microbial reworking, and zooplankton dietary usage on vertical patterns of particle flux across contrasting oceanic provinces, using empirical methods independent of incubation techniques or metabolic rate measurements. From their existing work in relatively low-flux environments of the Pacific Ocean, the investigators have developed two nascent models: (1) a mixing model that uses the compound-specific isotope analysis of amino acids (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 mesopelagic food webs on small, degraded particles. In this project, the investigators will test these two models by applying the same methods to the recent NASA EXPORTS field study in a high productivity, high flux regime, the North Atlantic spring bloom. The first EXPORTS field study in the subarctic Pacific provided some of the materials from which the models were developed. Application and refinement of the investigators’ 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. This work capitalizes on existing, comprehensive field programs specifically focused on building a predictive framework relating surface ocean properties to the vertical flux of organic carbon. The proposed work directly addresses 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.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.