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

辽阔的海洋覆盖了近60%的表面，是由于营养稀缺而生命挣扎求生的沙漠。这些区域位于亚热带海洋的中心环流，太偏远，无法通过垂直混合从陆地或更深的水域获得显著的营养输入。营养供应不足应该会导致光合作用水平降低，食物网也不那么复杂。然而，观察表明，生命的生长速度与获得更高营养供应的地区相似。我们的项目试图确定哪些缺失的营养来源有助于维持这些生态系统。这项工作将专注于在营养丰富的地区(包括加利福尼亚州沿海)生产的溶解有机分子中结合的氮，然后横向运输到亚热带北太平洋的“海洋沙漠”。为期一个月的研究探险队将从加利福尼亚州圣地亚哥出发，在美国HI火奴鲁鲁结束，研究溶解有机氮(DON)的生产、积累、化学成分和利用，并将其与北太平洋发生的光合作用联系起来。关于DON在满足亚热带北太平洋营养需求方面的作用的新知识将被用于构建计算机模型，以衡量这种营养源对亚热带海洋生态系统的全球重要性。为了解释亚热带海洋生态系统中观测到的净群落生产量(NCP)，需要在次表层硝酸盐(NO3-)和氮气固定通量之外增加额外的氮源(N)。数值模式表明，在低营养的副热带环流中，横向供应的异地DON可以支持10-60%的NCP，但具有很大的不确定性。这项拟议的工作将实地测试关于海洋DON的生物生产和消费的假设，以及异地DON是否是维持NCP的重要有机营养源。北太平洋邮轮将使用DON浓度、叶绿素a浓度和N同位素[DON的15N/14N比率和NO3-‘S 15N/14N比率]的观测来确定DON净生产和消费的区域，分别使用O2/Ar和三重氧同位素测量来估计净生物生产力和总生物生产力。将使用新的有机地球化学工具来确定异地DON的分子组成。深度对DON消耗的影响将通过船载培养来调查，这些培养将通过表面NO3-中的梯度收集的表面DON暴露于来自几个深度区间的微生物群落。最后，一种新的半不稳定DON示踪剂将被编码到一个全球海洋生物地球化学模型中，该模型的行为方式类似于野外观察中描述的异地DON。DON和同位素数据集将用于约束半不稳定DON循环的模型参数。最先进的半不稳定DON循环模型将被用来定量评估由异地DON输送到北太平洋副热带环流所维持的NCP部分，并将这一评估扩展到全球海洋。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.