Collaborative Research: Contribution of allochthonous DON to biological nitrogen demand in the subtropical North Pacific

Vast stretches of the ocean, covering almost 60% of its surface, are ‘deserts' where life struggles to survive due to nutrient scarcity. Located in the central gyres of the subtropical ocean, these regions are too remote to receive significant nutrient inputs from land or from deeper waters by vertical mixing. The low supply of nutrients should result in reduced levels of photosynthesis and less complex food webs. However, observations instead indicate that life grows at rates similar to regions which receive higher nutrient supplies. Our project seeks to determine what the missing sources of nutrients are that help sustain these ecosystems. This work will focus on nitrogen bound within dissolved organic molecules that are produced in nutrient-rich regions, including coastal California, and then transported laterally to the ‘ocean desert’ of the subtropical North Pacific Ocean. A one-month research expedition will leave from San Diego, CA and end in Honolulu, HI USA to study the production, accumulation, chemical composition, and utilization of dissolved organic nitrogen (DON) and link it to photosynthesis occurring across the North Pacific. The new knowledge gained about the role of DON in satisfying the nutrient requirements of the subtropical North Pacific will be used to construct computer models that gauge the global importance of this nutrient source to subtropical ocean ecosystems. Additional nitrogen (N) sources beyond subsurface nitrate (NO3-) and N2 fixation fluxes are required to explain observed net community production (NCP) within subtropical ocean ecosystems. Numerical models indicate that laterally supplied allochthonous DON may support 10-60% of NCP in oligotrophic subtropical gyres, but with large uncertainties. The proposed work will field test hypotheses concerning the biological production and consumption of marine DON, and whether allochthonous DON is a significant organic nutrient source sustaining NCP. A North Pacific cruise will use observations of DON concentration, chlorophyll a concentration, and N isotopes [DON's 15N/14N ratio, and NO3-'s 15N/14N ratio] to identify regions of net DON production and consumption in the context of net and gross biological productivity estimated using O2/Ar and triple Oxygen isotopic measurements, respectively. Novel organic geochemistry tools will be used to identify the molecular composition of the allochthonous DON. The influence of depth on DON consumption will be investigated using ship-board incubations that expose surface DON collected across gradients in surface NO3- to microbial communities from several depth intervals. Finally, a new semi-labile DON tracer will be encoded in a global ocean biogeochemistry model, fashioned to behave like the allochthonous DON characterized from the field observations. The DON and isotopic data sets will serve to constrain model parameterizations of semi-labile DON cycling. The state-of-the-art model of semi-labile DON cycling will be used to quantitatively assess the portion of NCP sustained by allochthonous DON delivery to the North Pacific subtropical gyre and extend this assessment across the global ocean.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.