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

predicted by the climate models (UKCIP, 2002). Deep, thermally stratified lakes also
provide spatial refuges for organisms that cannot tolerate high temperatures so the
direct effects of the projected increases in temperature may be quite small. The
annual abundance and long-term survival of these organisms may, however, be
compromised if there is a ‘mismatch’ between their seasonal dynamics and a key
resource (George and Harris, 1985).
IV.B.6. Hydrology
All four scenarios used in the simulations within the PRUDENCE project (Räisänen et
al., 2004) agree on a general increase in winter precipitation in Northern and Central
Europe. They also agree on a general and in some areas large (up to 70%) decrease
in summer precipitation in Central and Southern Europe, and of a smaller decrease
in summer precipitation north to Central Scandinavia. The climate change scenarios
suggest that that there will be a significant increase in winter rainfall throughout the
Atlantic Region and a corresponding decrease in the summer rainfall. In winter, some
areas in eastern UK that are currently rather dry will thus become much wetter. The
most pronounced summer reductions are projected to occur in the south of Ireland
and the UK where there may be a 30% reduction in the rainfall within the next twenty
years.
Changes in water richness affect lakes in two principal ways: by changing the water
residence time and/or changing the water level. The rate of water renewal affects
nutrient dynamics and has a critical effect on eutrophication (Dillon, 1975;
Vollenweider, 1975) while changes in the water level, especially in shallow lakes,
affect the strength of sediment resuspension, light conditions, and the areas of
macrophyte colonisation (Nõges and Nõges, 1999).
Residence time
The residence time of a lake is expressed as an annual average, which takes no
account of the short-term fluctuations in the rainfall. The effects of the projected
changes in the weather will, however, be critically dependent on the seasonal
distribution of the rainfall and the frequency of extremes. George and Hurley (2003)
Retention time (days)
160
120
80
40
0
20
1960 1970 1980 1990 2000
Year
Figure IV.B.3. Winter residence time of Esthwaite Water (UK)
66
0
5
10
15
Rainfall (mm day -1 )