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，即由于全球养分循环的核心过程（如碳埋藏）在湖泊中的效率比在陆地上高出许多倍，因此湖泊在某些方面比陆地表面对这些循环更重要。它们也是独特而又往往脆弱的生境，支持着非常高的物种多样性，从而极大地促进了环境的生态健康。为了使湖泊发挥作用并为自然环境的福祉做出这些关键贡献，化学和生物材料必须有效地在水体周围移动。特别是，富含营养的物质必须从湖床（如果不受干扰的话，它会下沉）循环到靠近表面沃茨的光照处，初级生产浮游植物需要居住在那里，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流扩散实现的：溶解和颗粒物质通过湍流涡流的垂直运动。因此，为了理解，并因此能够管理湖泊，以维持其良好的健康，这是必要的理解，并能够准确地计算其垂直湍流扩散。有几种方法可以做到这一点，我们以前的研究比较了这些方法在小湖泊的特定环境中的可靠性，这一类湖泊包括世界上大多数湖泊和英国的湖泊。这项工作的结果表明，湍流涡扩散率是高度空间可变的湖泊，这种变化可能主要是由于在水面风速（这是湍流扩散的主要原因）的方式与取（风吹过水面在任何给定点的距离）变化。因此，在湖泊气象学采样的典型湖泊中位置测得的风速似乎不适合用于计算湖泊中的湍流垂直混合。该项目将更全面地研究这一理论，并将显著提高我们在小湖泊中预测这一关键变量的能力。为此，它将使用微型温度剖面仪测量扩散率，该仪器以大约1毫米的分辨率测量特定位置的垂直水温剖面，然后使用既定方法计算扩散率剖面。通过测量多个配置文件的扩散率在不同的位置在两个不同的湖泊在一系列的气象和环境水分层条件下，我们将建立一个数据集，然后我们将能够分析，以确定风取变化如何影响湍流涡流扩散率在湖泊在各种条件下。因此，我们将开发一个强大的能力，以确定正确的方式，使用风速测量来计算垂直湍流涡扩散率在小湖泊。

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.