Collaborative Research: Mechanisms of nutrient input at the shelf margin supporting persistent winter phytoplankton blooms downstream of the Charleston Bump

知识价值：在多年的卫星叶绿素图像中，在冬季观察到沿南大西洋外湾(SAB)外大陆架的浮游植物持续繁殖。这一段大陆架位于“查尔斯顿凸起”(32.5-33.5°N)以北，墨西哥湾流经常在那里强烈转向近海。由于这种离岸偏转，这一区域的营养物质输入不会因与墨西哥湾流锋面涡旋相关的上升流而得到加强，这是SAB大陆架其他部分营养物质输入的主要机制(Lee等人，1991年)。然而，先前的现场观测表明，从冬季到初春，营养物质从上坡反复输入到龙湾外的外陆架。本项目将研究外陆架至上坡地区的物理-生物耦合的一个基本方面。PIS将检验以下假设：1)龙湾外陆架冬季水华的持续是由于营养物质输入和混合的反复事件，使营养物质在较长时间内保持充足的条件；2)涉及几个物理机制，包括沿这段上坡/陆架断裂的内潮增强的混合能量；3)相对高营养物质，间歇性湍流环境将有利于更大的水华形成浮游植物。后者可能会对更高的营养水平产生重要影响，包括冬季到早春在长湾外陆架边缘产卵的鱼类的早期生活史策略。这个项目将结合几种成熟的观测技术来解决以下问题：1.从上坡向外大陆架输送硝酸盐的频率和幅度是什么？2.在冬季遇到的水文和强迫条件下，从上坡向外大陆架带作业的营养物质输送机制是什么？3.外架水文的三维结构和相关的冬季水华特征是如何通过多个养分输入而演变的。混合事件？4.与冬季水华有关的硝酸盐利用率和初级生产量是多少？5.冬季状况是否一贯有利于以较大的硅藻类型为主的水华组合？将使用两架自动滑翔机获得近乎连续的跨大陆架和上陆架观测，通过一套系泊仪器(包括搁板断裂处的系泊剖面系统)对外部大陆架和斜坡进行时间序列测量，并使用拖曳、起伏的组件重复进行跨陆架和沿陆架的船舶勘测。船站工作将包括使用微流体/成像系统测量初级生产量和对浮游植物组合的关键功能特征(细胞形态、丰度、大小和生物体积分布)进行船上分析。结合起来，这些系统将提供物理、营养和生物场的空间和时间分辨率，这在早期的基于站点的实地研究中是无法实现的，并为更好地理解循环输入到陆架的营养物质的物理机制，以及冬季的营养、混合和循环状况如何构建浮游植物群落奠定了基础。海洋科学和电气与计算工程的研究生将在这项研究中发挥重要作用。6-12年级的教育部分将建立在以前与COSEE SE计划活动互动的基础上，并将强调：1)海洋学固有的跨学科性质，侧重于区域范围内的物理-生物耦合；2)科学与工程的界面，通过所采用的观测技术加以说明。沿海博物学家将通过实地考察方案中的海鸟调查部分参与工作，该部分将补充现有关于冬季远洋海鸟的信息，并确定它们与南大西洋海湾中部大陆架和斜坡上的海洋学特征的关系。

Intellectual Merit: Sustained phytoplankton blooms along the outer South Atlantic Bight (SAB) continental shelf off Long Bay are observed in winter in multi-year satellite chlorophyll imagery. This section of the shelf lies north of the "Charleston Bump" (between 32.5-33.5°N), where the Gulf Stream is often strongly deflected offshore. Due to this offshore deflection, this is not an area where nutrient input to the shelf would be enhanced by upwelling associated with Gulf Stream frontal eddies, a major mechanism of nutrient input in other parts of the SAB shelf (Lee et al., 1991). Yet prior in situ observations suggest that there is recurring input of nutrients from the upper slope to the outer shelf off Long Bay from winter to early spring. This project will investigate a fundamental aspect of physical-biological coupling in the outer shelf to upper slope region. The PIs will test the hypotheses that: 1) the persistence of winter blooms on the outer shelf off Long Bay results from repeated episodes of nutrient input and mixing which maintains nutrient-sufficient conditions for extended periods; 2) several physical mechanisms are involved, including enhanced mixing energy from the internal tide along this section of the upper slope/shelf break; 3) the relatively high nutrient, intermittently turbulent environment will favor larger bloom-forming phytoplankton. The latter could have important implications for higher trophic levels, including early life history strategies of fish that spawn along the shelf margin off Long Bay in winter to early spring.This project will combine several maturing observational technologies to address the following:1. What is the frequency and magnitude on on-shelf transport of nitrate from the upper slope? 2. What are the mechanisms of nutrient delivery from the upper slope to the outer continental shelf zone that are operating off Long Bay under the range of hydrographic and forcing conditions encountered in winter? 3. What is the 3-D structure of outer shelf hydrography and associated winter bloom features and how do these evolve through multiple nutrient input/mixing events? 4. What are the rates of nitrate utilization and primary production associated with the winter blooms? 5. Does the winter regime consistently favor a bloom assemblage dominated by larger diatom forms? Near-continuous cross-shelf and upper slope observations will be obtained with two autonomous gliders, time-series measurements on the outer shelf and slope from a set of moored instruments (including a moored profiling system at the shelf break), and repeated cross- and along-shelf ship surveys using a towed, undulating package. Ship station work will include measurements of primary production and on-board analyses of key functional characteristics of the phytoplankton assemblage (cell forms, abundance, size and bio-volume distributions) using a microfluidics/imaging system. In combination, these systems will provide a level of spatial and temporal resolution of physical, nutrient and biological fields that could not be achieved in earlier, station-based field studies and the basis for improved understanding of physical mechanisms of recurring nutrient input to the shelf, and how the nutrient, mixing, and circulation regime in winter structures the phytoplankton community.Broader Impacts: This project will provide a deeper understanding of shelf/slope exchange processes and how these influence shelf ecosystems, generating information that will contribute to implementation of ecosystem-based management in the region. Graduate students in Marine Sciences and in Electrical and Computing Engineering will have important roles in the study. A grade 6-12 education component will build on prior interactions with the COSEE SE program activities and will emphasize: 1) the inherently interdisciplinary nature of oceanography, focusing on physical-biological coupling in a regional context, and 2) the interface of science and engineering, illustrated by observational technologies employed. Coastal naturalists will be engaged through a seabird survey component of the field program that will augment existing information on pelagic seabirds in winter and define their association with oceanographic features on the central South Atlantic Bight shelf and slope.