RAPID: Evolution of critical shear stress in the seabed of an urbanized estuary and natural estuary after the passage of Hurricane Ian

沿海沃茨中的许多污染物附着在细小的沉积物上。为了预测与沉积物有关的污染物是如何迁移的，有必要知道沉积物是如何移动的。使表层沉积物重新悬浮所需的波浪和水流摩擦力的大小是沉积物运移的一个关键因素。所需摩擦力的大小取决于沉积物颗粒的大小和床面的生物过程。藻类垫和细菌膜会使沉积物更难再悬浮。然而，一个强大的物理干扰，如飓风可以打破垫子，使再悬浮更容易。该项目将研究受伊恩飓风影响的佛罗里达沿海沉积物。时间序列样本将显示沉积物床对再悬浮的敏感性在一年内如何演变。这将有助于更好地预测强风暴后沉积物和与沉积物有关的污染物的迁移地点。这项研究亦会探讨被红树林边缘环绕的未发展地区与以海堤为界的已发展地区之间海床演变的差异。先前的研究表明，红树林能捕获细小的沉积物。被红树林包围的海湾比没有红树林的发达地区的海湾有更大的沉积物颗粒。发达海湾的细颗粒沉积物更容易再悬浮。这可以加强与沉积物有关的污染物的移动，飓风已被证明可以调动与沉积物有关的重金属。这项研究将产生更广泛的影响，有助于预测污染物是否有可能被困或运输远离受大风暴袭击的地区。该研究还将为沿海发展选择提供信息，以便在水边实施红树林。这是及时的，因为许多房屋在西南佛罗里达海岸附近被毁，很快就会恢复。了解海底的临界剪切应力对于预测沉积物和与沉积物有关的污染物的迁移是必要的。这一点尤其重要，但在风暴对河床造成强烈扰动后，还没有得到很好的研究。拟议的研究评估了飓风伊恩通过后，在佛罗里达西南部的两个沿海海湾的海床的临界剪切应力的演变。临界剪切应力不仅取决于沉积物的大小，但藻类垫和细菌生物膜生产与生物扰动的相反效果。 迅速作出反应是必不可少的，因为生物的发展也可能很快，如果不尽快测量，飓风的影响就会消失。该研究将比较一个沿海海湾的河床演变如何不同，该海湾已进行了广泛的开发，包括用硬稳定结构取代边缘红树林，附近有一个未开发的海湾，没有人为开发。该研究将使用阵风侵蚀室对两个地点的三个重复岩心进行测量，每季度进行一次。生物扰动量也将每季度评估一次，使用每个地点三个岩心的X射线照片和铍-7剖面。据推测，临界剪切应力最初将随着细菌和藻类薄膜的发展而增加，然后随着大型底栖动物在底栖生物中的重新建立而减少。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

Many contaminants in coastal waters adhere to fine sediments. To predict how sediment-associated contaminants are transported it is necessary to know how the sediment moves. The amount of friction by waves and currents that is necessary to resuspend surface sediments is a key factor for sediment transport. The amount of friction needed depends on the size of the sediment grains and on biological processes at the surface of the bed. Algal mats and bacterial films can make it harder to resuspend sediment. However, a strong physical disturbance such as a hurricane can break down the mats and make resuspension easier. This project will study Florida coastal sediments impacted by Hurricane Ian. Time series samples will show how the sediment bed susceptibility to resuspension evolves over the course of a year. This will enable improved predictions about where sediment and sediment-associated contaminants are transported after strong storms. The study also will examine the difference in the evolution of the bed between an undeveloped area surrounded by a mangrove fringe and a developed area that is bordered by seawalls. Prior studies suggest that mangroves trap fine sediments. Bays surrounded by mangroves have larger sediment grains than bays in developed areas without mangroves. The fine sediments in developed bays are more easily available for resuspension. This can enhance the movement of sediment-associated contaminants, and hurricanes have been shown to mobilize sediment-associated heavy metals. This study will have broader impacts by helping predict whether contaminants are likely to be trapped or transported away from regions that are struck by large storms. The study also will inform coastal development choices regarding implementation of mangrove fringes at the water’s edge. This is timely since many homes were destroyed near the southwest Florida coast and will soon be restored. Understanding the critical shear stress of the seabed is necessary to predict the transport of sediment and sediment-associated contaminants. This is especially important, but not well studied, after intense disturbance to the bed caused by storms. The proposed research assesses the evolution of the critical shear stress of the seabed in two coastal bays in southwest Florida after the passage of Hurricane Ian. The critical shear stress depends not only upon the size of the sediment, but the opposing effects of algal mat and bacterial biofilm production versus bioturbation. A rapid response is essential because the biological development may also be rapid, and the effect of the hurricane will be lost if not measured soon. The study will compare how the evolution of the bed differs in a coastal bay that has had extensive development, including the replacement of fringe mangroves with hard stabilization structures, with a nearby but undeveloped bay with no anthropogenic development. The study will measure the critical stress using a Gust erosion chamber on three replicate cores from each of the two sites on a quarterly basis. The amount of bioturbation will be assessed quarterly as well, using X-radiographs of triplicate cores at each site and beryllium-7 profiles. It is hypothesized that the critical shear stress will initially increase as bacterial and algal films develop, then decrease as macrofauna reestablish within the benthos.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.