Larval Response to Turbulence During Dispersal and Settlement

底栖海洋无脊椎动物的浮游幼虫阶段为偏远种群之间的个体交换提供了一种机制。扩散受游泳行为的影响，特别是那些改变幼虫在水中的垂直位置的行为。有些种类的幼虫改变其垂直位置，以响应湍流停止游泳和下沉（潜水）。通过这样做，它们可以改变其在海流中的水平运输，并增加其对海底的供应。本研究的主要目的是调查牡蛎（Crassostrea virginica）幼虫的行为反应，在水柱和海底的湍流，并确定这些行为如何影响沉降。研究人员假设，潜水行为可以促进定居到合适的栖息地，即使平均床剪切力很高。他们预计，一旦幼虫接近底部，它们可以利用时间和空间的避难所（如粗糙元素背风处的湍流平静）在其他恶劣条件下定居。研究幼虫对湍流的反应是一个挑战，因为它需要同时测量随时间变化的流动和幼虫的行为。研究人员将修改传统的粒子图像测速（PIV）方法，使其可用于同时跟踪幼虫运动和流体速度。PIV提供了有关流动运动学的信息（例如，旋转和应变率），以及散装耗散率和泰勒和积分长度尺度的措施，可能会影响幼虫的加速。当这些测量与幼虫轨迹相结合时，它们提供了幼虫经历的流体环境的历史，并且可以用于确定湍流的什么特征触发潜水行为。它们还可以计算单个幼虫在遇到底部并试图定居时所经历的底部剪切应力。研究人员将使用网格搅拌罐来检查湍流对水柱中幼虫行为的影响。他们将使用一个跑道水槽来测试这一假设，即幼虫定居的成功取决于幼虫附着的足够持续时间的平静频率。智力上的优点。实验室实验将提供幼虫行为的机械理解，可用于一般的理论模型，探索行为如何影响扩散和种群连接。牡蛎的量化游泳反应是耦合的生物物理模型在现场的扩散的关键输入。了解幼虫的行为有助于我们预测自然和人为扰动（其中一些与全球气候变化有关）对沿海生态系统中底栖生物群落的影响，这些生态系统中的湍流和栖息地适宜性在空间上存在差异。这些信息对于贝类管理和海洋保护区设计的知情决策至关重要。同时PIV和幼虫跟踪开发的技术将在幼虫生态学中开辟新的问题，并广泛适用于浮游生物与湍流相互作用的研究。将开展各种培训和外联活动，纳入这项研究的各个方面。研究人员将根据生物体对流动的反应的概念，为麻省理工学院/WHOI生物海洋学课程开发一个主动学习模块，并将开发一个低技术版本，为卡尔顿学院的SERC做出贡献（地球科学教师的网络资源）。他们将为研究生和一系列暑期本科研究员提供指导和跨学科研究的经验。Co-PI Helfrich将使用支持来帮助维护WHOI的地球物理流体动力学实验室，该实验室被认为是同类最好的设施之一，并具有协助学生和科学家进行各种物理海洋学，地质学和生物物理相互作用流体动力学实验的传统。

The planktonic larval stage of benthic marine invertebrates provides a mechanism for exchange of individuals between remote populations. Dispersal is affected by swimming behaviors, particularly those that alter the larva's vertical position in the water. Larvae of some species change their vertical positions in response to turbulence by ceasing to swim and sinking downward (diving). By doing so, they can alter their horizontal transport in currents and increase their supply to the seafloor. The main objectives of this study are to investigate behavioral responses of oyster (Crassostrea virginica) larvae to turbulence in the water column and at the seafloor, and to determine how these behaviors affect settlement. The investigators hypothesize that diving behavior enhances settlement into suitable habitat, even where mean bed shear stress is high. They expect that once larvae approach the bottom, they can take advantage of temporal and spatial refuges (such as turbulent lulls in the lee of roughness elements) to settle in otherwise harsh conditions. Investigating larval responses to turbulence is a challenge because it requires simultaneous measurement of time-variant flows and larval behaviors. The investigators will modify a conventional particle image velocimetry (PIV) approach so it can be used to track larval motions and fluid velocities simultaneously. PIV provides information on flow kinematics (e.g., rotation and strain rate) in the immediate vicinity of a larva, as well as bulk dissipation rates and measures of Taylor and integral length scales that likely influence larval acceleration. When these measurements are coupled with a larval trajectory, they provide a history of the fluid environment a larva experiences, and can be used to determine what characteristic of turbulence triggers the diving behavior. They also make it possible to calculate the bottom shear stress an individual larva experiences when it encounters the bottom and attempts to settle. The investigators will examine turbulence effects on larval behaviors in the water column using a grid-stirred tank. They will use a racetrack flume to test the hypothesis that larval settlement success depends on the frequency of lulls of sufficient duration for larval attachment. Intellectual merit. Laboratory experiments will provide a mechanistic understanding of larval behavior that can be used in general theoretical models exploring how behavior influences dispersal and population connectivity. The quantified swimming responses of oysters are critical input for coupled bio-physical models of dispersal in the field. An understanding of larval behavior contributes to our ability to predict the effects of natural and anthropogenic perturbations (some of which are linked to global climate change) on benthic communities in coastal ecosystems where turbulence and habitat suitability vary spatially. This information is critical for informed decision making on shellfish management and design of marine reserves. The technique developed for simultaneous PIV and larval tracking will open new questions in larval ecology and be broadlyapplicable to studies of plankton interactions with turbulence.Broader impacts. Various training and outreach activities that incorporate aspects of this research will be conducted. The investigators will develop an active learning module for the MIT/WHOI Biological Oceanography course based on concepts of organismal responses to flow, and will develop a low-tech version to contribute to the SERC at Carlton College (a web resource for Geoscience faculty). They will provide the graduate student and a series of summer undergraduate research fellows with mentoring and an experience with interdisciplinary research. Co-PI Helfrich will use support to help maintain the Geophysical Fluid Dynamics Laboratory at WHOI, which is recognized as one of the finest facilities of its type and has a tradition of assisting students and scientists in a variety of experiments in fluid dynamics in physical oceanography, geology, and bio-physical interactions.