Development of low Nitrogen:Phosphorus ratios in the euphotic zone - the Phosphorus side of the story

这项研究将研究导致上层海洋中氮(N)和磷(P)的相对生物可利用性在墨西哥湾北部从海岸向近海移动时发生变化的过程。氮和磷都是必需的养分。因此，上层海洋的氮磷比对植物生长和海洋食物网结构具有重要影响。通常情况下，随着海水向近海移动，生物有效氮的下降速度快于生物有效磷。虽然改变任何一种元素的过程都会影响氮磷比，但以前的研究主要集中在氮方面的关系，研究海洋微生物对水中不同形式氮的循环和选择性去除。相反，该项目将侧重于研究较少的上层海洋氮磷比的P面。它将使用船上的实验来量化微生物过程，这些微生物过程在氮的可用性下降的情况下保持了上层海洋中磷的可用性。考虑到相对于磷的低氮有效性限制了大多数海洋阳光照射的地表水中的植物生长，了解这种化学比例如何随着水向近海移动而发展，对于研究全球海洋生态系统具有重要意义。教育影响将包括至少七名学生直接参与研究，提供动手和跨学科培训，以及海上实践经验。两名中学教师也将参加海洋巡游。他们将把实地调查结果和个人经验纳入课程计划和教师研讨会。该项目还将开展公众宣传活动，重点关注墨西哥湾海洋生态系统的独特价值。该项目将测试这样一个假设，即地表水中生物可利用氮磷比例的下降是由于优先保留磷而不是去除生物可利用形式的氮。作为与这一中心假设相关的研究的一部分，该项目将在墨西哥湾北部的两次海洋巡航中定量比较不同的磷保留机制的相对重要性。以往对氮磷有效性的空间变化的观测是常见的。研究人员将用拉格朗日漂移仪跟踪离散的水团进行时间过程采样，并使用物理海洋学测量来量化垂直和水平混合造成的潜在氮磷比变化。船上的孵化实验将量化和比较被认为影响上层海洋中磷滞留的关键微生物过程的速率。这种关注潜在的磷保留过程，而不是氮损失，作为普遍观察到的海洋表层氮磷比下降的解释，代表了对营养循环和海洋生态系统功能之间复杂反馈机制的完整理解的独特贡献。

This research will study the processes that cause the relative biological availability of nitrogen (N) and phosphorus (P) in the upper ocean to change as water moves from the coast to offshore in the northern Gulf of Mexico. Both N and P are required nutrients. Consequently, the ratio of N to P in the upper ocean has important consequences for plant growth and the marine food web structure. Typically, as water moves offshore bioavailable N declines faster than bioavailable P. While processes that alter either element will affect the N to P ratio, previous research has focused mainly on the N side of the relationship, examining cycling and the selective removal of different forms of N from the water by marine microorganisms. This project will focus instead on the less-studied P side of the N to P ratio in the upper ocean. It will use shipboard experiments to quantify microbiological processes that maintain P availability in the upper ocean, even as N availability declines. Given that low N availability relative to P limits plant growth in most of the ocean's sunlit surface waters, understanding how this chemical ratio develops as water moves offshore is of fundamental importance for the study of marine ecosystems worldwide. Educational impact will include at least seven students' direct participation in the research, providing hands-on and cross-disciplinary training, as well as practical experience at sea. Two middle school teachers will also participate in the oceanographic cruises. They will incorporate field results and personal experiences into lesson plans and teachers' workshops. The project will also develop public outreach activities that focus on the unique value of marine ecosystems of the Gulf of Mexico.This project will test the hypothesis that the decline in the ratio of bioavailable N to P in surface water as it moves offshore develops from preferential phosphorus retention as opposed to removal of biologically-available forms of nitrogen. As part of the research associated with this central hypothesis, the project will quantitatively compare the relative importance of different phosphorus-retention mechanisms during two oceanographic cruises in the northern Gulf of Mexico. Previous observations of spatial changes in N and P availability are common. The researchers will track discrete water masses with Lagrangian drifters for time course sampling, and use physical oceanographic measurements to quantify potential N to P ratio changes contributed by vertical and horizontal mixing. Shipboard incubation experiments will quantify and compare rates for the key microbiological processes thought to affect phosphorus retention in the upper ocean. This focus on potential P-retention processes rather than N loss as an explanation of commonly observed declines in surface ocean N to P ratio represents a unique contribution to the complete understanding of the complex feedback mechanisms between nutrient cycles and marine ecosystem function.