Collaborative Research: Multiyear autonomous measurement of N-loss in the ETNP ODZ

深海的几个区域自然几乎不含氧气。由于缺乏氧气，生活在这些地区的微生物的生活方式与那些在含氧水域的微生物不同，它们消耗硝酸盐离子，而不是呼吸氧气。利用硝酸盐进行微生物呼吸会产生氮气，这被称为反硝化作用。由于氮是控制植物生长的重要养分，因此硝酸盐的去除对整个海洋都有影响；然而，尽管植物可以利用硝酸盐形式的氮，但除了少数例外，它们不能利用氮气。关于海洋中发生了多少反硝化，是什么控制了它，以及它在时间和空间上的变化，仍然存在许多不确定性。传统的海洋反硝化研究受到船舶在海上的时间和它们所能观察到的海洋相对较小比例的限制。我们的项目计划通过使用一种叫做漂浮物的交通工具来解决这个问题，这种漂浮物可以在海洋中自主运行三年或更长时间，因为它们会随着数百公里的水流漂移。我们将在十个浮标上安装传感器来测量氧气和氮气，这些浮标将被放置在墨西哥西部的太平洋缺氧地区。这是海洋中最大的这样一个区域，我们用两年的时间从一个原型浮子那里得到了结果，验证了我们的方法。这项研究可能会很好地改变我们对海洋反硝化的理解，并最终使整个社会受益，因为它可以更有信心地预测海洋的氮循环和在当前和未来条件下固定二氧化碳的能力。使用商业技术的浮子系统的应用和进一步发展将直接有利于后续研究，并更广泛地展示使用水跟踪平台来解决困难的海洋学问题。这项研究的进展有望延续到其他学科，包括海洋生物地球化学模拟。该项目包括外展活动、对早期职业科学家的支持和学生培训。在拓展活动方面，调查人员计划与马萨诸塞州为弱势群体服务的成熟的课后项目联系起来，并在华盛顿大学利用花车相关的展示材料建立公开演讲的机会。缺氧区（odz）虽然只占海洋总量的一小部分，但在调节全球海洋碳和氮循环方面发挥着重要作用，包括承载了全球固定氮损失的30%至50%。不幸的是，目前ODZ氮损失率的不确定性来自于船上测量的大量时间和空间变异性。虽然氧气和有机质有效性对氮损失的局部调控已被广泛接受，但氮通量的时空变异可能是远程通风、季节变异和中尺度涡旋等物理强迫影响的结果。了解物理强迫如何通过氧和有机通量介导氮损失，将需要充分了解任何未来ODZ扩张的原因和后果。为了提高我们对ODZ氮损失的理解，我们将开展一项为期多年的自主浮子观测计划，以解决ODZ中生物有效氮损失的突出问题。作为试验部署中最大的ODZ和区域，我们的作业区域将是热带北太平洋东部（ETNP），我们的研究将在多年的时间内确定漂浮剖面上的纳米级氧气和生物氮，这些剖面跨越了氧气和地表生产力的地理梯度。我们的研究还将首次确定拉格朗日浮子沿恒定密度表面漂移1至2周期间氮浓度变化的原位氮损失率。在ETNP中进行的为期两年的浮动部署试点证明了我们这样做的能力。至关重要的是，与之前的工作相比，我们基于浮子的方法更接近于观测频率和分布与生物源氮产量的预期变异性，并且通过获得500个氮气剖面/漂移和1000个nM氧气剖面，将大大增加该地区的数据密度。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。

Several regions of the deep ocean naturally contain almost no oxygen. Because of this lack of oxygen, microbes living in these regions live in ways that differ from those in oxygenated waters consuming nitrate ions instead of oxygen for respiration. Use of nitrate for microbial respiration results in the production of nitrogen gas which is called denitrification. The resulting removal of nitrate has consequences for the whole ocean as nitrogen is an important nutrient controlling plant growth; however, whereas plants can use nitrogen in the form of nitrate, they cannot, with a few exceptions, use nitrogen gas. There remains a number of uncertainties regarding how much denitrification occurs in the ocean, what controls it, and how it varies in time and space. Traditional studies of ocean denitrification have been limited by the time ships can be at sea and the relatively small proportion of the ocean they can observe. Our project plans to remedy this problem by using vehicles called floats that can operate autonomously in the ocean for three years or more as they drift with currents over hundreds of kilometers. We will outfit ten floats with sensors to measure oxygen and nitrogen gas which will be placed throughout the oxygen-depleted region of the Pacific Ocean to the west of Mexico. This is the largest such region in the ocean from which we have two years of results from a prototype float which validated our approach. This study may well transform our understanding of ocean denitrification and ultimately benefit society as a whole through greater confidence in predictions of the ocean's nitrogen cycle and capacity to fix carbon dioxide under current and future conditions. Application and further development of float systems using commercially available technology will directly benefit successor studies, and more broadly showcase the use of water-following platforms to tackle difficult oceanographic problems. Advances from this study are expected to carry over to other disciplines including ocean biogeochemical modeling. Outreach activities, support for an early career scientist, and student training are included in the project. For the outreach activities, the investigators plan to tie into well-established after-school programs serving underrepresented populations in Massachusetts and established opportunities for public presentations using float related display materials at the University of Washington. Oxygen deficient zones (ODZs), despite constituting a small fraction of total oceanic volume, play important roles in regulating global ocean carbon and nitrogen cycles including hosting 30 to 50% of the global loss of fixed nitrogen. Unfortunately, current uncertainty in ODZ nitrogen loss derives from substantial temporal and spatial variability in rates that remain under-sampled by ship-based measurements. While local regulation of nitrogen loss by oxygen and organic matter availability are well accepted, temporal/spatial variability in the nitrogen flux is likely a result of the influence of physical forcings such as remote ventilation, seasonal variability, and mesoscale eddies. Understanding how the impact of physical forcings on nitrogen loss as mediated through oxygen and organic flux will be required to fully understand the causes and consequences of any future ODZ expansion. To improve our understanding of ODZ nitrogen loss, we will carry out a multiyear, autonomous float-based observational program to address outstanding questions regarding bioavailable nitrogen loss in ODZs. As the largest ODZ and region of our pilot deployments, our operation area will be the Eastern Tropical N. Pacific (ETNP) where our study will determine over a multi-year period, in-situ nM-level oxygen and biogenic nitrogen on float profiles spanning geographic gradients in oxygen and surface productivity. For the first time, our study will also determine in situ nitrogen loss rates from changes in nitrogen concentration during 1 to 2 week Lagrangian float drifts along a constant density surface. A pilot 2 yr float deployment in the ETNP documents our ability to do so. Critically, our float-based approach more closely matches the frequency and distribution of observations to the expected variability in biogenic nitrogen production as compared to prior work and will dramatically increase the data density for this region by acquiring 500 profiles/drifts for nitrogen and 1000 profiles for nM oxygen.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.