Chapter 6. The Impact of the Changing Climate on the Thermal Characteristics of Lakes

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 meteorological forcing over longer periods of time. Other important factors that can influence the thermal characteristics of lakes include hydraulic residence time, optical 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 regulate 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 number of lakes distributed throughout Northern, Western and Central Europe. These analyses complement the ice phenology results presented in Chapter 4, the ice modelling results in Chapter 5 and the temperature modelling results in Chapter 7. Particular 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 different 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 especially valuable for evaluating the thermal responses of lakes of varying size, topography and geographical location to climate and climate change, making it possible to detect gradual as well as abrupt shifts in their thermal characteristics. Where these data sets include years with abnormal weather conditions, they are especially valuable, since this information may help us evaluate the likely response of the lakes to future extremes. Fritz (1996) postulated that lakes located in extreme habitats or near an ecotone or climatic boundary will respond most sensitively to climate change. In this respect, lakes located in the Alpine/perialpine region, on the Atlantic coast, and at high latitudes are likely to respond most sensitively to the climatic changes summarised in Chapter 2. Ecologically, even relatively small changes in the thermal characteristics of lakes ¿ e.g. in their thermal stratification ¿ can cause major shifts in phytoplankton, bacterioplankton and zooplankton populations as well as altering the rates of metabolic processes (e.g. Steinberg and Tille-Backhaus, 1990; Tulonen et al.,1994; Weyhenmeyer et al., 1999; Gerten and Adrian, 2000; Arvola et al., 2002; Jasser and Arvola, 2003). This is because organisms are often adapted to certain narrow temperature ranges and because their life-cycle strategies can be highly sensitive to variations in ambient water temperature (e.g. Chen and Folt, 1996).JRC.DDG.H.5-Rural, water and ecosystem resource