Impacts of hydrological variability on material transfers through the River-Estuary Transition Zone

河口具有生物生产力，容易受到气候变化的影响，也容易受到营养物质丰富的影响。这些特性因其捕获、过滤和回收颗粒物的能力而得到加强。因此，提供给河口的颗粒物的处理很重要，而且取决于河口的主要材料来源。在这方面，悬浮颗粒物(SPM)及其固有的化学特性有助于控制从河流集水区到沿海地区的物质的反应性、迁移和生物影响。在向海岸运输的过程中，SPM的物理性质(颗粒大小、密度、沉降速度)在短时间和短长度尺度上发生变化，特别是当陆地颗粒遇到以河流-河口过渡带(RETZ)为特征的陡峭的水动力、化学和生物梯度并进入河口时。这是因为SPM主要以絮体的形式存在：无机物、有机物和水的聚集体。絮体长得比它们各自的无机成分大得多。当淡水在REZ遇到盐水时，就会发生聚集，并进一步受到生物效应的影响。当单个粒子由于弱湍流而碰撞时会产生絮状物，而强湍流则会使其破裂。大块的絮体迅速下沉，从而将物质转移到沉积物床上。因此，陆地SPM的命运(即，它是否被河流保留、转移到河口或出口到沿海海洋)强烈地受到絮体特性的影响，而这些特性反过来又取决于它们的来源和成因。由于絮体在取样时是脆弱和破裂的，所以很难研究它们。由于絮体在短时间和短长度尺度上改变其性质，因此需要高分辨率测量。这很难实现，但现在已经有了光学技术，可以对絮体特性(尺寸、体积浓度)进行高分辨率的现场测量，再加上质量浓度和沉降速度的测量，就可以评估絮体强度。该项目调查从河流集水区到河口的SPM及其颗粒物成分(碳、氮、重金属)的转移。这些陆地成分对沿海地区的水质、生态系统功能和健康很重要，并可能因气候变化引起的河流状况变化--主要洪水事件的发生频率增加--而发生严重变化。其目的是确定(I)河流SPM的不同来源，这种来源在季节性和周期性洪水事件期间发生变化，当时材料被动员起来，以前沉积在暴露在空中的泛滥平原和高岸上。我们将使用以C：N比、稳定碳同位素和有机质木质素衍生单体产率为形式的多种地球化学指标，以及以重金属形式的无机地球化学示踪剂。(2)絮体性质和地球化学成分在穿过雷兹河和河口时如何演变。(3)絮团和物质转移如何应对海洋强迫(潮汐、风暴)和河流强迫，特别是短期(间歇性)事件。来自集水区、RETZ和河口的仪器采样点的高分辨率观测数据集将用于验证一维垂直交换模型，该模型将嵌入三维水动力学模型，用于情景测试河流洪水和风暴对通过河流-河口系统的物质传输的影响。实地研究区域是大潮汐Dyfi系统(西威尔士)，那里有一个由集水和沿海研究中心建立的示范项目，该项目将为该项目提供后勤基础设施。该项目将在环境署和CEFAS的参与下进行，这两个机构将是产出的重要最终用户。

Estuaries are biologically productive, susceptible to climatic variability, and vulnerable to nutrient enrichment. These characteristics are reinforced by their ability to trap, filter, and recycle particulate matter. The processing of particulate matter supplied to estuaries is therefore important and dependent on the major sources of material to estuaries. In this respect, suspended particulate matter (SPM) and its inherent chemical characteristics are instrumental in controlling the reactivity, transport, and biological impact of substances from river catchments to the coastal zone. During transport to the coast, the physical properties of SPM (particle size, density, settling velocity) change on short time and length scales, especially when terrestrial particles encounter the steep hydrodynamic, chemical, and biological gradients that characterise the River-Estuarine Transition Zone (RETZ), and enter the estuary. This is because SPM is largely in the form of flocs: aggregates of inorganic matter, organic matter, and water. Flocs grow to a much larger size than their individual inorganic components. Aggregation occurs when fresh water meets salt water in the RETZ and is further mediated by biological effects. Flocs are created when individual particles collide due to weak turbulence, and are ruptured by strong turbulence. Large flocs sink rapidly, thus transferring material to the sediment bed. So, the fate of terrestrial SPM (i.e., whether it is retained by rivers, transferred to estuaries, or exported to the coastal ocean) is strongly influenced by floc properties, which, in turn, are dependent on their provenance and genesis. Since flocs are fragile and ruptured by sampling, they are difficult to study. High resolution measurements are needed since flocs change their properties on short time and length scales. This has been difficult to achieve, but optical techniques are now available which allow high resolution in situ measurement of floc properties (size, volume concentration), which, coupled with measurements of mass concentration and settling velocity, provide assessment of floc strength. This project investigates the transfer from river catchments to estuaries of SPM and its particulate components (carbon, nitrogen, heavy metals). These terrestrial components are important for water quality and ecosystem functioning and health in the coastal zone, and are likely to be severely altered by changes to the fluvial regime - increased frequency of major flood events - driven by climate change. The aims are to determine (i) The varying provenance of riverine SPM, which changes seasonally and during episodic flood events, when material is mobilised, previously deposited on aerially exposed floodplains and high banks. We will use multiple geochemical proxies in the form of the C:N ratio, stable carbon isotopes, and the lignin-derived monomer yields of organic matter, as well as inorganic geochemical tracers in the form of heavy metals. (ii) How floc properties and geochemical composition evolve as they pass through the RETZ and the estuary. (iii) How flocs and material transfers respond to marine forcings (tidal, storm) and fluvial forcings, especially short duration (episodic) events. High resolution observational datasets from instrumented sampling sites in the catchment, RETZ, and estuary will be used to validate a 1-D vertical exchange model, which will be embedded in a 3-D hydrodynamic model for scenario testing of impacts of episodic river floods and storms on material transfers through the river-estuary system. The field study area is the macrotidal Dyfi system (West Wales), where there is a demonstration project set up by the Centre for Catchment and Coastal Research, which will provide a logistical infrastructure in support of this project. The project will be carried out with the participation of the Environment Agency and CEFAS, who will be important end users of the outputs.