ESTIMATING VERTICAL EDDY DIFFUSIVITY IN A SMALL LAKE: A COMPARISON OF METHODS

湍流是水中一种极为重要的现象，主要是因为它是将粒子、热量和溶解物质从一个地方传播到另一个地方的最有效方式。湍流在大气和海洋中有着明显的重要性，它通过在全球范围内传递热量、污染物和许多其他东西，对世界的健康和气候起着至关重要的作用。此外，湍流也是了解湖泊的核心。湖泊不仅是人类的水资源，而且是生物多样性非常丰富的地方，特别是在构成自然生态系统基石的微生物水平上。在湖泊中，湍流扮演着多种角色：它提供从湖床到湖泊上层的营养物质，那里是大多数浮游生物存在的地方；它将湖中的氧气混合到较低的水平，使生物能够在那里生存，否则它们就会窒息；它通过湖水传播热量，使生物活动得以进行；它是许多关键化学物质循环和污染物扩散的核心。虽然它也在水平方向混合，但它的关键作用是垂直方向混合，将湖床上的营养供应——食物来源——与大多数生物栖息的靠近水面的明亮、温暖、富氧的水域连接起来。因此，很明显，测量由湍流引起的垂直混合速率（我们称之为“垂直扩散率”，用Kz表示）对于理解湖泊如何工作，从而使我们能够有效地管理湖泊至关重要。然而，这是非常困难的，而且已经使用的技术主要是在比英国大得多的湖泊和海洋中发展起来的。在较小的湖泊中，水通常更接近最近的湖床，流体中的湍流通常在固体边界附近最高，因为它是由流体刮擦边界产生的，这意味着小湖泊中的Kz模式可能与大湖泊或海洋中的Kz模式大不相同。因此，在大湖泊中有效的技术可能不适用于较小的湖泊，反之亦然。因此，我们在这个项目中的目标是在一个典型的英国湖泊（以及在世界各地非常常见的大小和类型）中测试这些技术中的几种，以找到一种合适的方法，可转移到其他湖泊，并且便宜且易于使用。我们特别感兴趣的是研究一种利用湖床自然排放的放射性气体氡的技术，因为据我们所知，这之前还没有在英国的湖泊中进行过试验。

Turbulence is a pre-eminently important phenomenon in water, largely because it is the most effective way of spreading particles, heat and dissolved substances from one place to another. As well as its obvious importance in the atmosphere and oceans, where it plays a crucial role in the world's health and climate by transferring heat, pollutants and many other things around the globe, turbulence is also central to understanding lakes. Lakes are very important both as water resources for humans and as locations of remarkably high levels of biodiversity, especially at the microbiological level which forms the foundation stone of natural ecosystems. In lakes, turbulence performs a multitude of roles: it provides nutrients from the lakebed to the upper levels of the lake where most plankton exist; it mixes oxygen down to lower levels in the lake allowing organisms to exist there that otherwise would suffocate; it spreads heat through the lake water enabling biological activity; and it is central to the cycling of many key chemicals and the diffusion of pollutants. Although it also mixes things horizontally, its key role is to mix vertically, connecting the nutrient supply - the food source - at the lake bed with the light, warm, oxygen rich waters near the surface which most organisms inhabit. Clearly, therefore, measuring the rate of vertical mixing caused by turbulence (which we call the 'vertical diffusivity' and denote Kz) is of crucial importance for understanding how lakes work and thus enabling us to manage them effectively. Nevertheless, this is very difficult to do, and moreover the techniques that have been used have largely been developed in lakes much larger than those found in the UK, as well as in the oceans. In smaller lakes, the fact that the water is in general much closer to the nearest bit of lake bed, and turbulence in a fluid is in general highest near a solid boundary, because of its creation by the fluid scraping against the boundary, means that the patterns of Kz in small lakes are likely to be quite different from those in larger lakes or oceans. So techniques that work well in large lakes may not work so well in smaller ones, and vice versa. Our aim in this project, therefore, is to test several of these techniques in a lake typical of those found in the UK (and of a size and type very common around the world) to find a method that is appropriate, transferrable to other lakes and cheap and easy to use. We are particularly interested in looking at a technique that uses the natural emission of the radioactive gas radon from the lake bed, as this has not previously been tried in UK lakes to our knowledge.