CAREER: Multiple Scales of Nitrogen Cycle in Oxygen Minimum Zones

海洋中的氮循环是海洋生产力、碳储存和向大气排放一氧化二氮（一种强效温室气体）的关键。连接海洋中氮物种的化学过程对海水中溶解的氧气量很敏感。这些反应在氧最小区变得更激烈？海洋中溶解氧很少或没有溶解氧的区域。氧气最小带受到电流的影响，范围从几百公里到不到几公里。这些洋流创造了微栖息地，在那里氮循环和一氧化二氮排放更高。该项目研究了小尺度海洋环流、氧气有效性和氮循环之间的相互作用。它使用了一系列越来越精细的太平洋数值模拟，在那里发现了两个最大的氧气最小带。这些模拟提供了从不到一年到几十年的时间尺度和从几公里到盆地尺度的空间尺度上的氮转化和氧化亚氮排放的信息。这项研究将提高我们模拟和预测海洋对自然和人类干扰的反应的能力，并对社会产生影响。该项目的教育部分每年为加州大学洛杉矶分校约100名本科生举办一系列海洋化学海洋学活动。实地考察包括在圣莫尼卡湾进行半天的巡游，在那里学生们可以对各种生物地球化学特性进行取样。在实地考察中收集的观察结果将被用作课堂活动和学生研究项目的资源。实地考察和教育材料提供了探索海洋生物地球化学前沿问题的机会，增加了学生对海洋科学和研究的兴趣，增强了学生的学习和自我效能感，最终促进了海洋学和STEM的保留。最低氧区承载着主要的氮转化，包括反硝化、厌氧氨氧化和氧化亚氮的产生，这对生物地球化学和气候至关重要。这些反应在缺氧范围内沿氧气梯度强烈地分散，使它们对变化的洋流驱动的通风和化学非均质性敏感。然而，人们对这种敏感性的本质知之甚少。该项目的目的是验证这样一个假设，即从几十公里（中尺度）到不到一公里（亚中尺度）的尺度上的物理循环对形成这些氮循环转化至关重要。为了验证这一假设并研究其影响，我们将根据最近的一系列观测结果优化一个新的氮循环模型，并在一个现实的三维水动力-生物地球化学模型中实现它。我们将采用嵌套策略，将太平洋范围内的历史模拟缩小到分辨率低至几公里或更低的一系列区域域，解决氧气最小带边界及其精细尺度变率。通过分析这些模型解，我们将：(1)约束微生物氮循环对氧气、通风和化学异质性的敏感性；(2)考虑到这种敏感性，量化中尺度和亚中尺度过程在形成氮转化和跨氧最小带边界运输中的作用；(3)研究氮循环对气候变率的响应，特别是固定氮损失和氧化亚氮排放到大气中。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Multiple Scales of Nitrogen Cycle in Oxygen Minimum Zones The nitrogen cycle in the ocean is key to ocean productivity, carbon storage, and emissions of nitrous oxide, a potent greenhouse gas, to the atmosphere. The chemical processes that connect nitrogen species in the ocean are sensitive to the amount of oxygen dissolved in seawater. These reactions become more intense within oxygen minimum zones ?areas of the ocean with little or no dissolved oxygen. Oxygen minimum zones are affected by currents that range in scale from hundreds to less than few kilometers. These currents create microhabitats where nitrogen cycling and nitrous oxide emissions are higher. This project investigates the interaction between small-scale ocean circulation, oxygen availability, and the nitrogen cycle. It uses a series of increasingly finer-scale numerical simulations of the Pacific Ocean, where two of the largest oxygen minimum zones are found. These simulations provide information about nitrogen transformations and nitrous oxide emissions on timescales from less than one year to several decades, and spatial scales from a few kilometers to the basin scale. This research will increase our ability to simulate and predict ocean responses to natural and human disturbances, with implications for society. The educational component of the project establishes a series of ocean-going chemical oceanography activities for approximately 100 undergraduate students at the University of California at Los Angeles each year. The field trips involve half-day cruises in the Santa Monica Bay, where students sample a variety of biogeochemical properties. Observations collected during the field trips will be used as a resource in classroom activities and student research projects. The field trips and educational materials offer opportunities to explore cutting-edge questions in ocean biogeochemistry, increase student interest in ocean sciences and access to research, and enhance student learning and self-efficacy, ultimately promoting retention in oceanography and STEM.Oxygen minimum zones host major nitrogen transformations, including denitrification, anammox, and nitrous oxide production, which are essential for biogeochemistry and climate. These reactions are strongly partitioned along oxygen gradients in the suboxic range, making them sensitive to ventilation and chemical heterogeneity driven by variable ocean currents. However, the nature of this sensitivity is poorly understood. The objective of this project is to test the hypothesis that physical circulation at scales from tens of kilometers (mesoscale) to less than one kilometer (submesoscale) is critical in shaping these nitrogen cycle transformations. To test the hypothesis and investigate its implications, we will optimize a new model of the nitrogen cycle against a range of recent observations, and implement it in a realistic three-dimensional hydrodynamic-biogeochemical model. We will adopt a nesting strategy to downscale a Pacific-wide historical simulation to a series of regional domains at resolutions down to few kilometers or less, resolving the oxygen minimum zone boundaries and their fine-scale variability. By analyzing these model solutions, we will: (1) constrain the sensitivity of the microbial nitrogen cycle to oxygen, ventilation, and chemical heterogeneity; (2) in light of this sensitivity, quantify the role of mesoscale and submesoscale processes in shaping nitrogen transformations and transport across oxygen minimum zone boundaries; and (3) investigate the response of the nitrogen cycle to climate variability, in particular fixed-nitrogen losses and nitrous oxide emissions to the atmosphere.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.