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

The projected increase in the atmospheric concentration of CO2 will not have a major
effect on the dynamics of lakes since they do not rely on the atmosphere as their
primary carbon source. However, it has been recurrently shown that doubling of
atmospheric CO2 concentration expected during a time-span of about one hundred
years (McCarthy et al., 2001) may strongly impact the structure and functioning of
terrestrial ecosystems (Lincoln, 1993, Agrell et al., 2000). Recent work has
demonstrated that changes in stoichiometric elemental ratios in terrestrial and
aquatic primary production can substantially impact the structure and functioning of
aquatic food webs. Tuchman et al. (2002) showed that nutritional quality of foliage
and leaf litter from plants grown under elevated pCO2 is lower for aquatic
decomposers and insects because of higher levels of structural compounds and
lower N:C ratio. Larval craneflies (Tipula abdominalis) fed elevated CO2-grown
leaves grew 12 times slower than their ambient fed counterparts. Changes in leaf
litter composition may affect ecosystem functioning in streams but also and small
lakes located in woodland.
In addition, plankton experiments (Urabe et al., 2003) showed that increased partial
pressure of carbon dioxide (pCO2) stimulated algal growth but reduced algal P:C
ratio. When feeding on algae grown under high pCO2, growth of Daphnia, an
important planktonic herbivore, decreased regardless of algal abundance. Thus, high
pCO2-raised algae were poor food for Daphnia. Both results suggest that, in
freshwater ecosystems with low nutrient supplies, increased CO2 may reduce energy
and mass transfer efficiency to higher trophic levels by decreasing nutritional quality
of the plant biomass.
Elevated temperatures reduce the solubility of CO2 in water leading to chemical
calcite precipitation in hard waters. Of much greater importance, however is the
biogenic de-calcification due to rising pH-values as a consequence of enhanced
photosynthesis (Dokulil et al., 1993; Schröder et al., 1983).
In summer, lower dissolved oxygen levels can be expected in lakes due to higher
water temperatures. Incomplete mixing in spring can result in near-bottom oxygen
depletion (Livingstone, 1997; Livingstone and Imboden, 1996). Increased water
column stability and a longer period of thermal stratification in summer will further
exacerbate oxygen availability in the hypolimnia of stratified lakes. In winter, on the
contrary, a reduction in the spatial and temporal extent of lake and stream ice cover
in boreal region can reduce winter anoxia that typically occurs in shallow lakes
(McCarthy et al., 2001).
IV.B.9. Coloured Dissolved Organic Carbon (CDOM)
In recent years, the colour of water has increased in streams and lakes throughout
the UK (Evans and Monteich, 2001). An increase in concentrations of coloured
dissolved organic carbon (CDOM) has been observed in the UK lakes and also in
Central Europe. Model predictions for a South Bohemian river (Hejzlar et al., 2003)
suggested a 7% increase in CDOM concentration under the scenarios of possible
future climate change related to doubled CO2 concentration in the atmosphere.
There is clear evidence that this is a consequence of the changing climate. Mitchel
and McDonald (1992) have shown experimentally that the drying and re-wetting of
peat has a major effect on the amount of coloured dissolved organic carbon (CDOM)
released from upland soils. The timing of the release is strongly influenced by the
hydrological characteristics of the catchment. In the Atlantic Region, the most serious
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