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质清澈影响生态系统动态，影响许多物种关键生命阶段的基本栖息地，也具有美学和娱乐价值。在切萨皮克湾（Chesapeake Bay），水的清晰度似乎对管理措施特别没有反应，近几十年来一直在下降。矛盾的是，在无机沉积物浓度相对较低的地区，清晰度比河流沉积物负荷增加的地区下降得更多。本文提出，在切萨皮克湾和许多其他河口中，水的清晰度和沉积物的输入之间的明显脱节与常见的河口流动模式与过量有机物的存在的相互作用有关。通过对河口物理与悬浮物和无机沉积物相互作用的改进观测、建模和分析，本研究旨在转变对河口水体清晰度控制的认识。切萨皮克湾流域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流失。将来，全国的河口流域可能需要类似的投资。通过评估河口物理、有机物和沉积物如何相互作用来影响水的清晰度，本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与广义河口湍流和环流模式的关系。该研究的主要目的是更好地了解小尺度和大尺度河口物理与无机沉积物和有机物混合的相互作用，这是部分混合系统的典型特征。将开发新的实地观测和数值模型来测试这些想法，并将重点放在约克河河口，这是切萨皮克湾的一个具有物流吸引力、广泛代表性、部分混合的分支。实地观察将描绘物理海洋学（环流、分层和湍流）以及粒子群（包括大小分布、浓度、密度、沉降速度和有机成分）的特征。将实施开源、社区支持的数值模型，进一步研究河口流体动力学、有机含量和沉积物动力学之间的反馈，包括多种粒径和密度，以及成分依赖絮凝的新配方，以及最先进的粘性床行为配方。本工作将检验以下新假设：（H1）：有机物质的添加进一步增强了水平和垂直颗粒分选，这是部分混合河口物理特性的特征。（H2）：将有机物添加到低浓度的无机固体中，形成相对较小、密度低、沉降速度慢的絮凝体（类型1絮凝体）。相反，在高浓度无机絮凝体中添加有机物有利于絮凝体体积更大、密度更高，沉降速度加快（2型絮凝体）。（H3）：富有机絮凝体（类型1）在衰减阳光方面特别有效，因为它们悬浮在水柱的高度，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，有机物加入河口降低了河口下游的水质清晰度，提高了河口上游的水质清晰度，更接近河口浊度最大值。

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.