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

The available evidence suggests that the projected changes in climate will reduce the
transparency of most lakes in the region but the factors responsible for this change
will vary from lake to lake. In deep lakes that are moderately productive the key factor
will be the quantitative and qualitative changes in the seasonal composition of the
phytoplankton. Seasonal and interannual changes in the composition of the
phytoplankton will also influence the transparency of shallow lakes but here the
overall effect will be complicated by the periodic resuspension of the bottom
sediments. The wind speed scenarios currently available for the Atlantic region
suggest that wind speeds will tend to increase in winter and decrease during the
summer. These changes could well lead to a general reduction in the transparency of
the lakes with more sediment being brought into suspension during the winter and
more algal blooms appearing during the summer.
Intensified rainfall events result in higher erosion and, consequently, in higher
sediment content in runoff water. In clay-rich areas the increase in water turbidity in
lakes and reservoirs may last for weeks after a heavy rainfall event affecting the light
conditions, gas regime, and primary productivity of these water bodies (Hargreaves,
1999).
Sedimentation exacerbated by climate change and desertification is becoming a
major issue, in particular, in reservoirs. In its 2002 State of the Environment Report,
the United Nations Environment Programme (UNEP) singled it out as a significant,
emerging issue for many regions of the world, where sediment reduces storage
capacity, reducing the reliability of water supply and power generation, and reducing
flood control effectiveness (Haas, 2002).
The present annual loss of storage in existing reservoirs is estimated at 0.5 to 1.0
percent globally (Dams and Reservoirs, 2003) though the “best guess” is 2%
annually in some Mediterranean countries. At a 1.0 percent loss rate there will be a
25 percent reduction in the existing freshwater storage capacity (6,000 km 3 of surface
storage provided by large dams at present) globally in the next 25-50 years (Dams
and development, 2000). And it will occur mainly in countries that are water stressed
and most vulnerable to climate change effects. In particular, rates of sediment yield
from large catchments with high induced erosion rates, such as prevalent in arid and
semi-arid southern and eastern parts of the Mediterranean, as well as parts of semiarid
or drought prone Northern Mediterranean countries (e.g. parts of Southern Spain
and Italy) are projected to increase from the combine effects of elevated
temperatures that would affect soil retention characteristics, cycles of drought, land
use change – and more concentrated, intense rainfall and flood events.
IV.B.7. Lake Chemistry
Signals of climatic and weather impacts cascade and weaken coherence among
lakes from physical and hydrological variables, via chemical to biological parameters.
Nutrients
Climate-induced changes of nutrient dynamics in lakes may be divided into two
groups:
1. alterations in catchment processes (leaching, runoff, retention) that
affect the external loading of nutrients, and water residence time;
2. in-lake changes because of changed water column stability in
stratified lakes, changed sediment resuspension rate in shallow lakes
and in littoral areas of deeper lakes leading to alterations in nutrient
cycles.
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