Spatial variability of vertical eddy diffusivity in small lakes

湖泊是全球系统中至关重要的组成部分。在最近一次重要的国际湖泊科学会议上，一个主题演讲提出了令人信服的证据：由于全球营养循环的核心过程（如碳埋藏）在湖泊中的效率是在陆地上的许多倍，因此在某些方面，湖泊对这些循环比陆地表面更重要。它们也是独特且往往脆弱的栖息地，支持着非常高的物种多样性，从而对环境的生态健康作出了巨大贡献。为了使湖泊发挥作用并对自然环境的福祉作出这些关键贡献，化学和生物材料必须在其水体周围有效移动。特别是，营养丰富的物质必须从湖床（如果没有受到干扰，就会下沉到那里）循环到靠近浅水的地方，因为初级生产浮游植物需要在浅水中生存，以便能够进行光合作用，并将营养物质带入食物链。这种垂直输送主要是通过垂直湍流涡旋扩散实现的：溶解物质和颗粒物质通过湍流涡旋进行垂直运动。因此，为了理解并因此能够管理湖泊以维持其良好的健康状态，必须理解并能够准确地计算其垂直湍流扩散系数。有几种方法可以做到这一点，我们之前的研究已经比较了这些方法在小湖泊的特定环境中的可靠性，小湖泊包括世界上绝大多数的湖泊和英国的湖泊。这项工作的结果表明，湍流涡旋扩散率在湖泊之间具有高度的空间变化，这种变化可能主要是由于水面上的风速（这是湍流扩散的主要原因）随风速（风在任何给定点吹过水面的距离）而变化的方式。因此，在湖中测量的风速，通常是湖泊气象采样的地方，似乎不适合用于计算湖泊湍流垂直混合。该项目将更全面地研究这一理论，并将大大提高我们在小湖泊中预测这一关键变量的能力。它将通过使用温度微剖面仪测量扩散系数来实现这一目标，该剖面仪测量特定位置的垂直水温剖面，分辨率约为1mm，由此可以使用既定方法计算扩散系数剖面。在一系列气象和环境水分层条件下，通过测量两个不同湖泊不同位置的扩散率的多个剖面，我们将建立一个数据集，然后我们将能够分析该数据集，以确定在各种条件下风向变化如何影响湖泊中的湍流涡旋扩散率。因此，我们将发展一个强大的能力，以确定正确的方法，其中使用风速测量来计算垂直湍流涡旋扩散在小湖泊。

Lakes are vitally important elements of the global system. A keynote lecture at a recent, major international lake science conference presented compelling evidence that, since processes central to global nutrient cycles (such as carbon burial) are many times more efficient in lakes than they are on land, lakes are in some respects more important for these cycles than the land surface. They are also unique and often vulnerable habitats that support a very high species diversity and thus contribute greatly to the ecological health of the environment. In order for lakes to function and deliver these key contributions to the well-being of the natural environment, chemical and biological material must be moved efficiently around their water bodies. In particular, nutrient-rich material must be cycled up from the lake bed (to where it sinks if otherwise undisturbed) to the light, near surface waters where primary producing phytoplankton need to reside to be able to photosynthesise and bring the nutrients into the food chain. This vertical transport is achieved overridingly through vertical turbulent eddy diffusion: the vertical movement of dissolved and particulate material by turbulent eddies. Thus, in order to understand, and therefore be able to manage lakes to sustain their good health, it is imperative to understand and be able to compute accurately their vertical turbulent diffusivity. There are several ways of doing this, and our previous research has compared the reliability of these in the specific environment of small lakes, a category which includes the large majority of the world's, and the UK's, lakes. The results from this work suggest that turbulent eddy diffusivity is highly spatially variable across lakes, and that this variability may be largely due to the way in which wind speed at the water's surface (which is a primary cause of the turbulent diffusion) varies with fetch (the distance over which the wind has blown over the water surface at any given point). As a result, wind speeds measured in mid-lake locations, which are typically where lake meteorology is sampled, do not appear to be appropriate to use in calculating turbulent vertical mixing in lakes. This project will investigate this theory more fully, and will significantly enhance our ability to predict this key variable in small lakes. It will do so by measuring diffusivity using a temperature micro-profiler, which measures vertical water temperature profiles at specific locations with a resolution of approximately 1mm, from which profiles of diffusivity can be calculated using established methods. By measuring multiple profiles of diffusivity at different locations within two different lakes under a range of meteorological and ambient water stratification conditions, we will build a data set that we will then be able to anlayse to determine how wind fetch variations affect turbulent eddy diffusivity in lakes under a wide variety of conditions. Thus we will develop a robust ability to determine the correct way in which to use wind speed measurements to calculate vertical turbulent eddy diffusivity in small lakes.