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

permanent or temporary status. Shallow lakes may totally dry out or remain
ephemeral, filled with water only for short periods during sporadic rainfalls.
Temperature-related increases in evaporation from water surfaces, and in evapotranspiration
from soils and vegetation in the catchment areas will have an impact on
water demand-supply balances. In arid and semi arid regions the evapo-transpiration
potential (ETp) will exceed annual precipitation, and precipitation/ETp ratios may fall
as low as 0.25, presenting implications for how both evaporation and storage is
managed. At present, evaporation from surface water bodies alone is estimated to be
about 5% of available water supply in the dry climate areas (Dams and development,
2000). Longer-term temperature rises will influence water demand and need to be
taken into account in demand forecasts. While industrial and household water
demand is sensitive to temperature in certain circumstances, agriculture demand
(evapo-transpiration) and natural evaporation will be most critically affected as
conditions become hotter and drier.
IV.B.7. Optical Properties of Lakes
Climate change may affect the light conditions in lakes and optical properties of water
in five major ways:
1. by shortening the ice-cover duration and decreasing the thickness of snow on
the ice;
2. by increasing the leaching of dissolved organic matter from soils;
3. by increasing phytoplankton primary production and biomass;
4. by intensifying the resuspension of bottom sediments in shallow lakes through
decreasing water levels and increased storminess.
5. by increasing sediment transport from the watershed;
A reduction in the spatial and temporal extent of lake and stream ice cover as a result
of warmer winters can decrease light attenuation, which is a major limiting factor for
production in boreal aquatic systems (McCarthy et al., 2001). However, the other four
processes decrease water transparency and light availability for plants and
phytoplankton.
Ice and snow cover in high alpine lakes can be one or more meters thick effectively
blocking all light from the lakes. Without light, photosynthesis is no longer possible,
turning the entire water body into a heterotrophic system. After ice break-off, these
lakes transit to extremely bright light conditions within a very short period of time. The
higher altitude the lake, the stronger is the shortwave UV radiation (UVB, 280-320
nm). At an elevation of 3000 m, UVB is approximately 50% higher than at sea level.
In addition, ozone depletion in the stratosphere has increased UVB radiation by 10%
since 1970, which penetrates deeply into high alpine lakes because of the lack of
humic acids and other dissolved organic compounds (Psenner, 2003).
Milder winters contribute to under-ice light conditions in lakes also in Northern and
Central Europe where more frequent thaw periods melt the reflecting snow-cover and
make the ice more transparent.
In most of the lakes in the Atlantic Region the optical characteristics of water are
primarily determined by the concentration of phytoplankton in suspension (see the
remote sensing studies of George (1997) in the English Lake District). Some shallow
lakes may, however, contain high concentrations of suspended sediment whilst a few
lakes situated in upland areas are strongly coloured by dissolved humic compounds.
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