Collaborative Research: Impacts of Eddies and Mixing on Plankton Community Structure and Biogeochemical Cycling in the Sargasso Sea

0241310/OCE-0241399/OCE-0241023/OCE-0241340/OCE-0241011构成海洋中尺度的洋流、锋面和涡流，有时被称为“海洋内部天气”，是海洋环流的高能量和无处不在的特征。这些现象的动态后果包括化学和生物环境的扰动，这可能极大地影响海洋中的生物地球化学循环。调节这一反应的过程非常复杂，我们需要了解物理、生物和化学过程是如何在功能上相互联系的。最近的证据表明，中尺度涡旋是主要副热带涡旋低营养水域的一种重要的营养物质输送机制。数值模拟和基于卫星的统计估计表明，涡流驱动的营养通量的大小足以平衡对新产量的地球化学估计，这远远超过传统养分供应机制所能维持的水平。由于驱动这一过程的事件在空间和时间上具有间歇性，因此对这一过程的直接观察相对较少。现有数据表明，与某些类型的涡旋相关的等径位移可以将营养物质输送到真光层，导致上覆水域中的叶绿素积累。然而，生物反应的性质及其对耦合的生物地球化学循环和出口的影响尚未阐明。此外，涡动诱导的上升流和混合层内外的昼夜混合之间的关系仍然不清楚；这种相互作用的强度决定了涡动驱动的影响不可逆转的程度，从而影响了净生物地球化学通量。在这个项目中，一组海洋研究人员将记录浮游植物的生理反应、群落结构的变化、出口以及马尾藻海涡致上升流和混合的生物地球化学分支。在实地部署之前，将通过遥感确定目标特征。高分辨率调查将使用起伏的拖曳仪器包进行，其中包括一个快速重复频率荧光仪。这套仪器将有助于同时评估光合作用参数以及浮游植物和浮游动物的物种组合。这些测量将伴随着所选地物横截面上几组站点的生物地球化学性质的离散水样采样。出口将在中尺度结构内的选定位置进行测量。表面混合层和真光层底部水域之间的混合速率将由氦通量计推断，并用六氟化硫示踪剂释放直接测量。综上所述，这些观察结果将足以检验这样一个假设，即涡流诱导的上升流增加了光合作用速率，改变了群落结构，增加了真光带的输出，从而在亚热带海洋的生物地球化学循环中发挥了重要作用。这项研究将由来自伍兹霍尔海洋研究所、百慕大生物研究站、罗格斯大学、加州大学圣巴巴拉分校和迈阿密大学的10名主要研究人员共同努力进行。工作计划包括两年的实地观察，然后是最后一年的综合。

ABSTRACTOCE-0241310 / OCE-0241399 / OCE-0241023 / OCE-0241340 / OCE-0241011The currents, fronts and eddies that comprise the oceanic mesoscale, sometimes referred to as the "internal weather of the sea," are highly energetic and ubiquitous features of ocean circulation. Dynamical consequences of these phenomena include perturbation of the chemical and biological environment that can dramatically impact biogeochemical cycling in the ocean. The processes that regulate this response are extraordinarily complex, challenging us to understand how the physical, biological and chemical processes are functionally related.Recent evidence suggests that mesoscale eddies are an important nutrient transport mechanism in the oligotrophic waters of the main subtropical gyres. Numerical simulations and satellite-based statistical estimates indicate that the magnitude of the eddy-driven nutrient flux could be sufficient to balance geochemical estimates of new production, which far exceed that which can be sustained by traditional mechanisms of nutrient supply. Relatively few direct observations of this process are available, owing to the spatial and temporal intermittency of the events that drive it. Available data demonstrate that isopycnal displacements associated with certain types of eddies can transport nutrients into the euphotic zone, resulting in the accumulation of chlorophyll in the overlying waters. However, the nature of the biological response and its impact on coupled biogeochemical cycles and export has yet to be elucidated. Furthermore, the relationship between eddy-induced upwelling and diapycnal mixing in and below the mixed layer remains obscure; the strength of this interaction determines the degree to which the eddy-driven effects are irreversible and thereby effect a net biogeochemical flux.In this project, a team of oceanographic researchers will document phytoplankton physiological response, changes in community structure, export and the biogeochemical ramifications of eddy-induced upwelling and mixing in the Sargasso Sea. Target features will be identified prior to field deployment via remote sensing. High-resolution surveys will be undertaken with an undulating towed instrument package that includes a Fast Repetition Rate Fluorometer. This suite of instruments will facilitate simultaneous assessment of photosynthetic parameters and the species assemblage of phytoplankton and zooplankton. These measurements will be accompanied by discrete water sampling of biogeochemical properties in sets of stations along cross sections of the chosen features. Export will be measured at selected locations within the mesoscale structure. Rates of mixing between the surface mixed layer and waters at the base of the euphotic zone will be inferred from the Helium flux gauge and measured directly with a sulfur hexafluoride tracer release. Taken together, these observations will be sufficient to test the hypothesis that eddy-induced upwelling increases photosynthetic rates, changes community structure and increases export from the euphotic zone, thereby playing an important role in biogeochemical cycling of the subtropical oceans.The research will be carried out in a collaborative effort among ten principal investigators from the Woods Hole Oceanographic Institution, the Bermuda Biological Station for Research, Rutgers University, University of California, Santa Barbara, and the University of Miami. The work plan consists of two years of field observations followed by a final year of synthesis.