Interactions of Estuarine Physics, Sediment, and Organic Matter in Determining Suspended Particle Properties, Their Spatial and Temporal Distribution, and Resulting Water Clarity

海岸和河口系统的水透明影响生态系统动态，影响许多物种关键生命阶段的基本栖息地，也对美学和娱乐有价值。在切萨皮克湾，水的清晰度似乎尤其对管理做法反应迟钝，而且在最近几十年里一直在下降。矛盾的是，与河流沉积物负荷增加的地区相比，无机沉积物浓度相对较低的地区透明度下降得更多。本文认为，切萨皮克湾和其他许多河口的水体透明度与沉积物输入之间的明显脱节，与河口常见流态与过量有机质的存在相互作用有关。通过改进河口物理与悬浮有机物和无机沉积物的相互作用的观测、模拟和分析，这项研究旨在改变人们对河口水域透明度控制的理解。切萨皮克湾分水岭的各州将在未来十年投资数十亿美元，以进一步减少营养物质和沉积物的径流。未来，全国范围内的河口流域可能需要类似的投资。通过评估河口物理、有机物和沉积物相互作用如何影响水的清晰度，这项研究将为有效投资这些资金提供有价值的指导。该项目将资助两名博士生进行与社会相关的论文项目，并为本科生研究人员提供引人入胜的项目。在许多沿海环境中，决定水清晰度的相互作用对颗粒浓度、大小、密度、组成、有机含量和沉降速度很敏感，但很少有研究明确考虑这些因素与一般河口湍流和环流模式的关系。这项研究的主要目的是更好地了解河口小规模和大尺度物理与部分混合系统中典型的无机沉积物和有机质混合物之间的相互作用。将开发新的现场观测和数值模型来检验这些想法，并将重点放在约克河河口，这是切萨皮克湾的一个物流上有吸引力、具有广泛代表性的部分混合分支。现场观测将描述物理海洋学(环流、层化和湍流)以及颗粒种群(包括粒度分布、浓度、密度、沉降速度和有机成分)的特征。将实施开源的、社区支持的数值模型，以进一步研究河口水动力、有机含量和沉积物动力学之间的反馈，包括多种颗粒大小和密度，与成分有关的絮凝新配方，以及粘性床行为的最新配方。这项工作将检验以下新的假设：(H1)：有机材料的添加进一步增强了水平和垂直颗粒分类，这是部分混合河口物理学的特征。(H2)：将有机物添加到低浓度的无机固体中会导致相对较小的低密度絮体，具有较慢的沉降速度(类型1絮体)。相反，将有机物添加到高浓度的无机絮体中有利于更大、更高密度的絮体，并增加沉降速度(类型2絮体)。(H3)：富含有机物质的絮体(类型1)在衰减阳光方面特别有效，因为它们悬浮在水柱中很高，单位质量的横截面积很大。与河口物理相互作用的最终结果是，河口有机物的添加降低了河口下部的水透明度，提高了上部河口的水透明度，更接近河口的最大浑浊度。

Water clarity in coastal and estuarine systems impacts ecosystem dynamics, influences essential habitat for key life stages of many species, and is also valuable for aesthetics and recreation. In Chesapeake Bay, water clarity has seemed particularly unresponsive to management practices, and has continued to decline over recent decades. Paradoxically, clarity has decreased more in areas with relatively low concentrations of inorganic sediment than in locations where riverine sediment loads have increased. Here it is proposed that the apparent disconnect between water clarity and input of sediment in the Chesapeake Bay and many other estuaries is related to the interaction of common estuarine flow patterns with the presence of excess organic matter. Through improved observation, modeling, and analysis of the interaction of estuarine physics with suspended organic matter and inorganic sediment, this study aims to transform understanding of the controls on water clarity in estuaries. States in the Chesapeake Bay watershed will invest billions of dollars over the next decade to further reduce runoff of nutrients and sediment. In the future, analogous investments may be required for estuarine watersheds nationwide. By evaluating how estuarine physics, organics, and sediment interact to influence water clarity, this study will provide valuable guidance on effectively investing these funds. This project will fund two PhD students on societally-relevant dissertation projects and provide engaging projects for undergraduate researchers. In many coastal settings, the interactions that determine water clarity are sensitive to particle concentration, size, density, composition, organic content, and settling velocity, yet few studies have explicitly considered how these connect to generalized estuarine turbulence and circulation patterns. The main objective of the study is to better understand the interaction of small and large scale estuarine physics with mixtures of inorganic sediment and organic matter typical of partially-mixed systems. Novel field observations and numerical models will be developed to test these ideas and focus on the York River estuary, a logistically attractive, broadly representative, partially-mixed branch of the Chesapeake Bay. Field observations will characterize physical oceanography (circulation, stratification, and turbulence), as well as particle populations (including size distribution, concentration, density, settling velocity and organic constituents). Open-source, community-supported numerical models will be implemented to further investigate feedbacks between estuarine hydrodynamics, organic content, and sediment dynamics, incorporating multiple particle sizes and densities, and new formulations for composition-dependent flocculation, as well as state-of-the-art formulations for cohesive bed behavior. This work will test the following novel hypotheses: (H1): Addition of organic material further enhances the horizontal and vertical particle sorting that characteristic to the physics of partially-mixed estuaries. (H2): Organic matter added to low concentrations of inorganic solids results in relatively small, low density flocs having slow settling velocities (Type 1 flocs). Conversely, adding organic matter to high concentrations of inorganic flocs favors larger, higher density flocs, with increased settling velocity (Type 2 flocs). (H3): Organic rich flocs (Type 1) are especially effective at attenuating sunlight because they are suspended high in the water column and have a large cross-sectional area per unit mass. The end result of interactions with estuarine physics is that the addition of organic matter to the estuary degrades water clarity in the lower estuary and improves water clarity in the upper estuary, closer to the estuarine turbidity maximum.