Collaborative Research: Regulation of plankton and nutrient dynamics by hydrodynamics and profundal filter feeders

贝类和贻贝等底栖滤食性动物影响浅海和淡水系统中的浮游生物和营养物质动态，但它们在大型深水系统中的作用通常被认为是次要的。然而，最近的证据表明，深海长尾贻贝(Dreissena Rostriformis Bugensis)极大地改变了劳伦斯五大湖和其他大型深海水生系统的能量流动和营养循环，对食物网的许多其他组成部分产生了负面影响。此前，这些湖泊的管理策略依赖于预测湖泊对磷负荷的反应的数值模型。湖泊中底栖滤食者的激增似乎从根本上改变了磷循环和能量流动的动态，因此这些模型不再有效。该项目测量了密歇根湖的能量流动和营养循环过程，重点关注叉尾贻贝在这些过程中的作用，并利用这些测量结果来开发数字模型，以指导有关营养物质负载和鱼类放养的管理决策。除了支持三名博士生的教育和为本科生提供研究培训外，该项目还通过为学生举办跨学科研究和建模研讨会并通过一个开放的项目网站提供研讨会讲座来促进未来水生科学家的教育。这个合作的生物物理项目围绕两个主要问题构建：1)深水鱼纲贻贝在大型湖泊碳循环和营养循环中扮演什么角色？2)贻贝如何在从毫米(海底边界层)到米(整个水柱)的尺度上受到水动力学的调节，以摄食和回收营养物质的命运？研究人员推测，向湖底输送颗粒的明显增加是高过滤能力和垂直混合过程的结果，垂直混合过程将浮游植物从真光层平流到近底层。然而，水动力学的作用尚不清楚，因为这些过程在整个表层和近底层都没有很好的特征。此外，这对浮游植物和营养物质动态的影响尚不清楚，因为贻贝既是食草动物，也是营养循环者。密歇根湖正在部署最先进的仪器和分析工具，包括粒子图像测速仪、声学多普勒水流剖面仪和营养微剖面仪，以量化这些关键的动态过程，包括边界层湍流、贻贝放牧、排泄和排泄以及碳和磷的海底通量。所产生的数据被用来校准一个三维水动力-生物地球化学模型，该模型用于解决一个突出的问题，即在拥有丰富的深层滤食器的大型深水湖泊中，浮游生物和营养物质的动力学与以前的模型描述的传统模式有何不同。该项目还提供了对底边界层物理的洞察，适用于其他大型湖泊、潮间带沿海和深海。

Benthic filter feeders such as clams and mussels influence plankton and nutrient dynamics in shallow marine and freshwater systems, but their role is generally considered to be minor in large, deep systems. However, recent evidence indicates that profundal quagga mussels (Dreissena rostriformis bugensis) have dramatically altered energy flow and nutrient cycling in the Laurentian Great Lakes and other large, deep aquatic systems, with negative consequences for many other components of the food web. Previously, management strategies for these lakes relied on numerical models that predicted the lake's response to phosphorus loading. The proliferation of benthic filter feeders in the lakes appears to have fundamentally altered the dynamics of phosphorus cycling and energy flow, so that these models are no longer valid. This project measures energy flow and nutrient cycling processes in Lake Michigan, with a focus on the role of quagga mussels in these processes, and uses these measurements to develop numerical models to guide management decisions with regard to nutrient loading and fish stocking. In addition to supporting the education of three Ph.D. students and providing research training for undergraduate students, the project promotes education of future aquatic scientists by hosting a workshop on interdisciplinary research and modeling for students and making workshop lectures available through an open-access project website.This collaborative biophysical project is structured around two primary questions: 1) What role do profundal dreissenid mussels play in large lake carbon and nutrient cycles? 2) How are mussel grazing and the fate of nutrients recycled by mussels modulated by hydrodynamics at scales ranging from mm (benthic boundary layer) to meters (entire water column)? The investigators hypothesize that the apparent enhanced particle deliver rate to the lake bottom results from high filtration capacity combined with vertical mixing processes that advect phytoplankton from the euphotic zone to the near bottom layer. However, the role of hydrodynamics is unclear, because these processes are poorly characterized both within the hypolimnion as a whole and within the near-bottom layer. In addition, the implications for phytoplankton and nutrient dynamics are unclear, as mussels are both grazers and nutrient recyclers. State-of-the-art instruments and analytical tools, including particle image velocimetry, acoustic Doppler current profilers, and nutrient micro-profilers, are being deployed in Lake Michigan to quantify these critical dynamic processes, including boundary layer turbulence, mussel grazing, excretion and egestion, and benthic fluxes of carbon and phosphorus. Resulting data are used to calibrate a 3D hydrodynamic-biogeochemical model, which is used to address the overarching question of how plankton and nutrient dynamics in large, deep lakes with abundant profundal filter feeders differ from the conventional paradigm described by previous models. The project also provides insight into bottom boundary layer physics, with applicability to other large lakes, atidal coastal seas, and the deep ocean.