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

water colour problems are thus likely to occur in the ‘dry’ eastern areas where they
will be closely correlated with the frequency of extreme weather events.
Transport of DOM to freshwaters represents a significant carbon flux from the soils
global carbon pool to the hydrosphere (Freeman et al., 2001). Increased CDOM input
would be beneficial for heterotrophic organisms, but at the same time it also affects
the quality of water resources (Vik and Eikebrokk, 1989), reduces light penetration,
including that of damaging UV-B radiation (Schindler and Curtis, 1997), changes the
vertical distribution of solar heating (Schindler et al., 1996), and in this way
accelerates the effects of global climate warming on the thermal structures of lakes.
IV.B.10. BIOLOGY
Small variations in climate can have dramatic effects on environmentally sensitive
high latitude and high altitude lakes. In these extreme habitats, whether shallow or
deep, many species are living at the limit of their capabilities and will respond
immediately to changes in ice regime. Big changes can be predicted also in other
extreme environments like, e.g., temporary lakes in the Mediterranean area or
shallow soda lakes in Austria (Kirschner et al., 2002). Seasonal systems that
presently can cope with occasional or periodic drought will experience additional
stress that some species might not be able to survive (Brock and van Vierssen,
1992). Changes in lake permanence cause temporal habitat fragmentation (Thomas,
2003) that has drastic consequences to the biota. Most of the species die out and will
be replaced by species, which have special adaptations to tolerate intermediate dry
periods.
Microcosm experiments (Petchey et al., 1999) showed that extinction risk in warming
environments depends on trophic position. Warming (+2ºC per week during 7 weeks)
significantly increased primary production, directly through increased temperaturedependent
physiological rates and indirectly through changes in trophic structure.
Warming greatly increased herbivore and predator extinction frequencies but had
little effect on the composition of producers and bacterivores leaving the latters overrepresented
in warmed communities.
Phytoplankton
Climate impacts on phytoplankton via catchment and lake processes are
conceptually summarised in Figure IV.B.5. In general, effects of global warming on
phytoplankton dynamics seem to be not fundamentally different in different regions of
the world (Gerten and Adrian, 2002). In winter, the most important weather-related
effects are those connected with light conditions: for ice-covered lakes biggest
changes are projected in ice duration while in ice-free lakes light conditions depend
mostly on the interannual variations in the intensity of wind-induced mixing. In
summer, the most important weather-related effects are those associated with the
projected increases in the water temperature and the enhanced physical stability of
the water column in stratified lakes.
Timing of spring bloom
In winter ice-covered lakes the timing of the phytoplankton spring bloom is mainly
triggered by light availability, controlled by the ice characteristics and the snow on the
ice. Therefore, strong relationships between the timing and the winter climate were
found in North European lakes (Weyhenmeyer et al. 1999). The timing of ice breakup
and duration of the overturn control light climate and the transport of the nutrients
from hypolimnion to epilimnion (Kilman et al. 1996). In spring, the water column is
commonly dominated by diatoms, that typically favour high nutrient conditions, low
light and low temperature. Generally, large, heavily silicified species, such as
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