Spatial variability of vertical eddy diffusivity in small lakes

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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.