Climate Change and the European Water Dimension - Agri ...

Windermere in the English Lake District (George, 2000a). In the future, only a few
lakes in the more mountainous areas of the Atlantic Region will freeze every year
and even here there will be a marked reduction in the duration of ice-cover.
Temperature
One of the most important physical parameters in any lacustrine system is
lake temperature, because it reflects meteorological forcing immediately and
sensitively (Dokulil, 2000). In temperate region the highest surface water
temperatures in winter will be recorded in deep lakes that retain heat and the lowest
temperatures in shallower lakes that loose heat to the atmosphere. In ice-covered
lakes the signals of climatic forcing are mostly reflected in ice-cover duration and less
pronounced in water temperature. However, winter temperature controls whether it
predominantly snows or rains in winter, and this is crucially important for winter light
conditions and gas regime of ice-covered lakes.
Long-term measurements in the English Lake District demonstrate that the winter
temperature of the larger lakes has increased by at least 0.6 o C over the last forty
years. Much of this warming can be related to year-to-year variations in the NAO
(George et al, 2000), an effect described in some detail by Livingstone (2000) and
Straile et al (2003).
Comparing a relatively cold period in Sweden, 1982 to 1988, with the warm period
1989 to 1995, surface water temperatures in May in Sweden's largest lakes
increased by 0.6 to 4.7 °C (Weyhenmeyer 2001). Significant increasing trends in the
water temperature in April and May since the 1950s have been reported also for
shallow lakes in the Baltic countries (Nõges & Järvet, in press).
Despite the often-pronounced geomorphological differences of lakes in alpine
regions, their physical parameters show still a high degree of synchrony (Dokulil and
Teubner, 2002). For lakes in the Northern Perialpine Area a high degree of spatial
coherence in lake surface temperature among deep lakes has been established.
Correlations of the lake surface temperature with air temperature, and with seasonal
indices of the NAO suggest that lake surface temperature in winter and spring is
related to large-scale atmospheric processes occurring over the North Atlantic
(Livingstone and Dokulil, 2001).
Deep alpine lakes orographically situated in more remote, narrow valleys show
generally clear trends of deep water warming, like several others in the Perialpine
Region north and south of the Alps (Blanc et al., 1990; Livingstone, 1993,1997;
Ambrosetti and Brabanti, 1999). Hypolimnetic temperatures are correlated to climate
signals with a time lag of about one year. Reduced winter cooling can result in the
persistence of small temperature gradients that may resist complete mixing (Straile et
al., 2003). Mixing in turn can determine the trophic status of lakes (Salmaso et al.,
2003), and thus might have essential implications for the assessment of ecological
status over time relative to the Water Framework Directive.
Stratification
Thermal stratification, resulting from heating by the sun, is the most important
physical process in the lake’s annual cycle and dominates most aspects of lake
structure in summer. Thus, the most important weather-related effects in summer will
be those associated with the increase in the water temperature and the enhanced
physical stability of thermally stratified lakes. The effects of the increased
temperature will be most pronounced in shallow, isothermal lakes and the effects of
the change in stability in moderately deep lakes that are also relatively productive.
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