What can high frequency measurements tell us about mixing events in lakes?

The dynamics of lake temperature structure, driven by the interplay of surface wind and heat fluxes, are crucial to many aspects of ecology. It has long been observed that this temperature structure is characterised by an isothermal ‘mixed’ layer at the top of a lake, varying in depth from centimetres to the full lake depth across different lakes and through different seasons. While the concept of this mixed layer is ubiquitous and the importance of rapid changes in it is acknowledged, there is no unique quantitative definition of a mixing depth or event. This hampers comparisons and also raises questions about definitiondependence of results. Widespread, multi-year, high frequency studies examining mixed depths and mixing events have not previously been possible. The recent proliferation in deployment of high resolution temperature sensors provides an exciting opportunity to compare differences in these physical features across many different lakes at both short and long time-scales to improve understanding of extrinsic and intrinsic drivers of mixing. It also highlights the need to understand the implications of the use of a wide variety of definitions of mixed depth and mixing events. Here we show data from three very different European lakes, to demonstrate the importance of considering the diel cycle in mixing, the large interannual variability in mixing and the influence that definition has on the interpretation of mixing. These results provide a starting point for a large-scale analysis of mixing across European lakes.