Collaborative Research: GLOBEC-01: Tidal Front Mixing and Exchange on Georges Bank: Controls on the Production of Phytoplankton, Zooplankton, and Larval Fishes

乔治斯海岸支持丰富的渔业，因为：(1)海岸的大部分足够浅，浮游植物的光限制通常不重要；(2)富含无机营养物质的深水可用于混合到岸上；以及(3)海岸的顺时针循环可以保留重要鱼类物种的浮游阶段。潮汐混合锋(TMF)是乔治斯浅滩生产力的中心，通过北部的营养物质注入和南侧的幼虫滞留过程。这两个区域由沿锋面的循环通道连接，在该通道中，营养物质导致浮游植物和浮游动物的生长，在顶部周围形成了一个甜甜圈状的高产区域。这表明该途径的生产力是北缘养分注入的结果，并受气候影响该地区的养分供应。该项目的总体目标是了解TMF内维持乔治斯浅滩生物生产力和目标物种鳕鱼和黑线鳕成功的过程。这需要了解TMF内的混合和循环如何提供新的营养物质、支持初级生产和留住幼虫。GLOBEC染料示踪实验首次直接测量了TMF中近底部的拉格朗日环流和混合。结果表明，锋面垂直混合与岸上流经TMF底部的流动是动态联系的。这项研究是在这些测量的基础上研究TMF的三维动力学。模型将有助于评估跨等深环流和沿等深环流的强度如何设定与鳕鱼和黑线鳕幼体发育相适应的时间和空间尺度。该项目由数据分析和建模活动组成。首先，染料分散数据和简单的剪切分散模型被用来理解跨岸流动和垂直混合之间的联系。其次，有限体积沿海海洋模式(FVCOM)将被用来计算进入TMF的营养物质通量的时空结构，对比南北两侧的输入。FVCOM-NPZ(营养物-浮游植物-浮游动物)耦合模型正被用来检验以下假设：(I)北部的营养物质注入平流到岸顶周围，导致浮游植物和浮游动物产量增加。(Ii)羽流丰富了南翼幼虫夹带的区域。如果上述说法属实，那么羽流中的产量可能会因缅因湾东北航道源水的营养物质含量而发生变化，而这些变化将影响鳕鱼和黑线鳕幼体的摄食环境。最后，结合测量的三维流场和湍流场的模型正被用于检查幼虫滞留的空间模式，并定义幼虫在这一过程中经历的环境转变的种类。

Georges Bank supports a rich fishery because: (1) large portions of the bank are shallow enough that light-limitation of phytoplankton is usually not important; (2) deep waters rich in inorganic nutrients are available for mixing onto the bank; and (3) the Bank's clockwise circulation can retain the planktonic stages of important fish species. The tidally mixed front (TMF) is central to the productivity of Georges Bank through the processes of nutrient injection in the north and retention of larvae on the south flank. These two regions are connected by a circulation pathway along the front in which nutrients lead to phytoplankton and zooplankton growth, creating a donut-shaped region of high production surrounding the crest. It is suggested that the productivity of this pathway is the result of northern edge nutrient injections and is susceptible to climatic influences on nutrient supply in that region. The overall objective of this project is to understand the processes within the TMF that sustain the biological productivity of Georges Bank and the success of the target species, cod and haddock. This requires understanding how mixing and circulation within the TMF supplies new nutrients, supports primary production, and retain larvae. GLOBEC dye tracer experiments have, for the first time, measured directly the near-bottom Lagrangian circulation and mixing in the TMF. Results show that vertical mixing in the front, and the on-bank flow through the base of the TMF, are dynamically connected. This study is examining the 3-dimensional dynamics of the TMF based on these measurements. Models will help assess how the strength of the across- and along-isobath circulation sets time and space scales compatible with the development of cod and haddock larvae. This project consists of a mix of data analysis and modeling activities. First, dye dispersion data and simple shear dispersion models are being used to understand the link between cross-bank flow and vertical mixing. Second, a finite-volume coastal ocean model (FVCOM) will be used to calculate the temporal and spatial structure of nutrient flux into the TMF, contrasting northern and southern flank inputs. A coupled FVCOM- NPZ (nutrient-phytoplankton-zooplankton) model are being used to test the following hypotheses: (i) Nutrient injections in the north are advected around the crest of the bank and lead to a plume of elevated phytoplankton and zooplankton production. (ii) The plume enriches the area of larval entrainment on the south flank. If the above statements are true, then production in the plume can be altered by the nutrient content of source waters in the Northeast Channel of the Gulf of Maine, and these changes will affect the feeding environment of larval cod and haddock. Finally, models incorporating the measured 3-D flow and turbulence fields are being used to examine spatial patterns of larval retention and define the kinds of environmental transitions that larvae experience during this process.