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

increased predation from some species like perch and roach that currently produce
strong year classes in very warm years (Sarvala and Heliminen, 1996). One ‘food
supply’ effect that has a major effect on the dynamics of planktonic herbivores is the
episodic growth of edible algae. In most lakes, such species are only abundant for a
few weeks in the year and there is typically a clear correlation between the
appearance of strong cohorts of grazers and the seasonal dynamics of these edible
forms (George and Reynolds, 1997). Warmer surface water can reduce the
nutritional value of edible phytoplankton, but it also may shift primary production
toward green algae and cyanobacteria, which are less favoured by secondary
consumers (McCarthy et al., 2001).
Zooplankton that graze on phytoplankton as a food resource, appear shortly after the
phytoplankton spring bloom. Since the phytoplankton spring bloom shifted as a result
of warmer winters, in some Swedish lakes the zooplankton biomass peak shifted as
well, occurring earlier after warm winters. This is reflected by a higher zooplankton
biomass in May after warm winters in the large Swedish lakes (Weyhenmeyer, 2001).
Increased water temperatures can have negative impacts on zooplankton because of
enhanced metabolic requirements and adverse effects on development and growth
(Dokulil et al., 1993). In Lake Constance for example, the interannual variability of
Daphnia growth rate is closely related to water temperature and hence to climate
signals. This correlation is due to physiological response of Daphnia egg
development and growth rates to increased water temperatures and is not mediated
by better food supply (Gaedke et al., 1998; Straile, 2000).
In temperate lakes, asynchronous cycles in surface water temperatures and incident
ultraviolet (UV) radiation expose aquatic organisms to damaging UV radiation at
different temperatures. The enzyme systems that repair UV-induced DNA damage
are temperature dependent, and thus potentially less effective at repairing DNA
damage at lower temperatures (MacFadyen et al., 2004). The important implication is
that aquatic organisms that depend heavily on DNA repair processes may be less
able to survive high UV exposure in low temperature environments. Photoprotection
may be more effective under the low temperature, high UV conditions such as are
found in early spring or at high elevations. In high alpine lakes zooplankton is
pigmented and deep living to avoid exposure to high doses of UVB radiation
(Praptokardiyo, 1979).
Zoobenthos
The response of freshwater benthos to climatic changes is not well documented in
the perialpine region. Fossil remains have mainly been used by Lotter et al. (1997) to
reconstruct the recent past environmental conditions in alpine lakes. Similar
circumstantial evidence comes from a high mountain lake in Spain where fossil
chironomids indicate recent warming (Granados and Toro, 2000).
Fish
Thermal stress is defined as any temperature change that produces a significant
disturbance in the normal functions of a freshwater fish and thus decreases the
probability of survival. According to Alabaster & Lloyd (1980), an increase in water
temperature from about 0°C to 2°C in winter at the time of reproduction would
severely affect spawning of Coregonus sp. and burbot (Lota lota). Summer
temperature of 20 to 21°C is the upper permissible temperature for salmonids of the
genus Salmo. Coregonids can withstand a rise of temperature of 5 to 6°C but the
maximum for the summer months should not exceed 22 to 23°C. For many cyprinids,
the permissible increase of temperature is about 6°C above the natural ambient
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