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

知识价值：多年卫星叶绿素图像在冬季观测到长湾外南大西洋湾（SAB）大陆架持续的浮游植物华。这部分大陆架位于“查尔斯顿隆起”的北部（在32.5-33.5°北纬之间），在那里，墨西哥湾流经常强烈地偏转到近海。由于这种离岸偏转，与墨西哥湾流锋面涡旋相关的上升流不会增加对大陆架的营养输入，而这是南大陆架其他部分营养输入的主要机制（Lee et al., 1991）。然而，先前的原位观测表明，从冬季到早春，营养物质从上斜坡反复输入到长湾外大陆架。该项目将研究外大陆架与上斜坡区域物理-生物耦合的一个基本方面。pi将检验以下假设：1)长湾外大陆架上持续的冬季藻华是养分输入和混合的反复发生的结果，这在很长一段时间内保持了营养充足的条件；2)涉及多种物理机制，包括沿上坡/陆架断裂段的内部潮汐增强了混合能；3)相对高营养、间歇性湍流的环境有利于浮游植物形成较大的水华。后者可能对更高的营养水平具有重要意义，包括在冬季到早春期间沿着长湾大陆架边缘产卵的鱼类的早期生活史策略。该项目将结合几种成熟的观测技术来解决以下问题：上坡的硝酸盐在陆架上迁移的频率和幅度是多少？2. 在冬季遇到的水文和强迫条件下，长湾附近的上坡向外大陆架区输送养分的机制是什么？3. 外大陆架水文的三维结构和相关的冬季水华特征是什么？它们是如何通过多种营养输入/混合事件演变的？4. 与冬季开花相关的硝酸盐利用率和初级生产量是多少？5. 冬季环境是否一直有利于以较大硅藻形式为主的藻华组合？将使用两台自主滑翔机进行近连续的跨大陆架和上部斜坡观测，通过一套系泊仪器（包括位于大陆架断裂处的系泊剖面系统）对外大陆架和斜坡进行时间序列测量，并使用拖曳的波动包进行重复的跨大陆架和沿大陆架的船舶测量。船上工作站的工作将包括测量初级生产和利用微流体/成像系统对浮游植物组合的关键功能特征（细胞形式、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、营养和生物领域的空间和时间分辨率，这是以前以站为基础的实地研究无法实现的，并为改进对大陆架循环养分输入的物理机制的理解以及冬季营养、混合和循环制度如何构成浮游植物群落的理解奠定了基础。更广泛的影响：该项目将加深对陆架/斜坡交换过程及其如何影响陆架生态系统的了解，产生有助于在该地区实施基于生态系统的管理的信息。海洋科学和电气与计算机工程专业的研究生将在这项研究中发挥重要作用。6-12年级的教育内容将建立在与COSEE SE项目活动之前的互动基础上，并将强调：1)海洋学固有的跨学科性质，侧重于区域背景下的物理-生物耦合；2)科学与工程的界面，通过所采用的观测技术来说明。沿海自然学家将参与实地项目的海鸟调查部分，这将增加冬季中上层海鸟的现有信息，并确定它们与南大西洋中部大陆架和斜坡的海洋学特征的联系。

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.