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

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.