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中生活的微生物不同，他们消耗硝酸盐离子而不是氧气进行呼吸。使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营养物质，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，但它们不能使用氮气，除了少数例外。关于海洋中发生了多少反硝化作用，是什么控制了它，以及它如何在时间和空间上变化，仍然存在一些不确定因素。传统的海洋反硝化研究受到船只可以在海上停留的时间和它们可以观测到的海洋相对较小的比例的限制。我们的项目计划通过使用被称为浮标的交通工具来解决这个问题，这种交通工具可以在海洋中随洋流漂流数百公里时自动运行三年或更长时间。我们将为10辆彩车配备传感器，以测量氧气和氮气，这些传感器将放置在墨西哥以西的整个太平洋氧气枯竭区域。这是海洋中最大的此类区域，我们从一个原型浮标获得了两年的结果，验证了我们的方法。这项研究很可能会改变我们对海洋反硝化的理解，并通过对海洋氮循环的预测以及在当前和未来条件下固定二氧化碳的能力的更大信心，最终造福于整个社会。利用商业上可用的技术应用和进一步开发浮标系统将直接有利于后续研究，并更广泛地展示利用跟水平台解决海洋难题。这项研究的进展有望推广到其他学科，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。在外展活动中，调查人员计划结合为马萨诸塞州代表性不足的人群服务的成熟的课外项目，并在华盛顿大学利用与彩车相关的展示材料建立公开展示的机会。缺氧区(ODZ)虽然只占海洋总体积的一小部分，但在调节全球海洋碳和氮循环方面发挥着重要作用，占全球固定氮损失的30%至50%。不幸的是，目前ODZ氮损失的不确定性来自于船舶测量的采样不足的速率在时间和空间上的巨大变异性。虽然氧气和有机质有效性对氮素损失的局部调节是公认的，但氮素通量的时间/空间变化可能是远程通风、季节变化和中尺度涡旋等物理强迫的影响的结果。要全面了解未来ODZ扩张的原因和后果，就需要了解物理强迫如何通过氧气和有机通量对氮损失产生影响。为了提高我们对ODZ氮损失的了解，我们将开展一项多年的、自主的浮动观测计划，以解决ODZ中生物有效氮损失的突出问题。作为我们试点部署的最大ODZ和区域，我们的作业区将是东热带北太平洋(ETNP)，我们的研究将在那里进行多年的研究，确定跨氧和表面生产力地理梯度的浮式剖面上的NM级原位氧和生物源氮。我们的研究还将首次根据拉格朗日漂流沿恒定密度表面漂移的1到2周内氮浓度的变化来确定原位氮损失率。ETNP为期两年的试点浮动部署证明了我们有能力做到这一点。最重要的是，与以前的工作相比，我们基于浮子的方法将观测的频率和分布与生物氮素生产的预期变异性更接近，并将通过获取500个氮素剖面/漂移和1000个NM氧气剖面，极大地提高该地区的数据密度。该奖项反映了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。

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.