Chapter 6 The Impact of the Changing Climate on the Thermal Characteristics of Lakes Lauri Arvola, Glen George, David M. Livingstone, Marko Järvinen, Thorsten Blenckner, Martin T. Dokulil, Eleanor Jennings, Caitriona Nic Aonghusa, Peeter Nõges, Tiina Nõges, and Gesa A. Weyhenmeyer 6.1 Introduction Meteorological forcing at the air-water interface is the main determinant of the heat balance of most lakes (Edinger et al., 1968; Sweers, 1976). Year-to-year changes in the weather therefore have a major effect on the thermal characteristics of lakes. However, lakes that differ with respect to their morphometry respond differently to these changes (Gorham, 1964), with deeper lakes integrating the effects of mete- orological forcing over longer periods of time. Other important factors that can influence the thermal characteristics of lakes include hydraulic residence time, opti- cal properties and landscape setting (e.g. Salonen et al., 1984; Fee et al., 1996; Livingstone et al., 1999). These factors modify the thermal responses of the lake to meteorological forcing (cf. Magnuson et al., 2004; Blenckner, 2005) and regu- late the patterns of spatial coherence (Chapter 17) observed in the different regions (Livingstone, 1993; George et al., 2000; Livingstone and Dokulil, 2001; Järvinen et al., 2002; Blenckner et al., 2004). In this chapter, we summarise the long-term thermal changes observed in a num- ber of lakes distributed throughout Northern, Western and Central Europe. These analyses complement the ice phenology results presented in Chapter 4, the ice mod- elling results in Chapter 5 and the temperature modelling results in Chapter 7. Par- ticular attention is paid to the interannual and seasonal variations in the surface and bottom temperatures of the lakes. In Europe, lake surface waters are typically at their warmest in July or early August. Surface water temperatures in low-altitude lakes in central Europe then often exceed 25 ◦ C (Livingstone and Lotter, 1998; Livingstone and Padisák, 2007), but are usually lower in lakes at high altitudes or high latitudes (Livingstone et al., 1999; Korhonen, 2002; George et al., 2007b). Long-term water temperature records are available from lakes in several dif- ferent regions (e.g. Livingstone, 1993; Bengtsson et al., 1996; George et al., 2000; Livingstone and Dokulil, 2001; Nõges, 2004). These long-term data sets are L. Arvola (B ) University of Helsinki, Lammi Biological Station, Pääjärventie 320, FI-16900 Lammi, Finland e-mail: Lauri.Arvola@helsinki.fi 85 D.G. George (ed.), The Impact of Climate Change on European Lakes, Aquatic Ecology Series 4, DOI 10.1007/978-90-481-2945-4_6, C  Springer Science+Business Media B.V. 2010