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

and possibly elimination of seasonal migrants to shallow, ice-covered winterkill lakes.
In temperate region, on the contrary, the risk of winter fish-kills is smaller in warmer
winters. In response to higher temperatures, northern boreal populations of cyprinid
and percid fish species are expected to increase at the expense of coldwater,
salmonid species (Lehtonen, 1996). Shallow lakes would be most susceptible to
these changes because of their lack of thermal stratification. Total freshwater fish
production is expected to increase, but with the projected changes in the composition
of fish fauna, the recreational and commercial value of catches will decrease
(Lehtonen, 1996).
IV.B.10. Ecosystem response of lakes
In conclusion, different species as well as physical and chemical properties react
differently because of their different interactions in the foodweb. However, some
changes might be relevant on a species level, but all these single effects may weigh
differently in the lake ecosystem as a whole. Petchy et al. (1999) conducted
microcosm experiments to control species composition and rates of environmental
change. They suggested that ecosystem responses are not as clear as studies of
single trophic levels indicate. Complex responses generated in entire food webs
greatly complicate inferences based on single functional groups. Here, the
consideration of more general ecological concepts is needed in order to understand
and synthesize climatic effects on lake ecosystems. The strength of food web
interactions is characterized by many weak and few strong interactions (McCann et
al., 1998). Weak links in particular act to dampen oscillations between consumers
and resources (McCann et al., 1998) and presumably also environmental stressors,
as climate extremes. This means that not all responses at a specific trophic level are
propagated to lower trophic levels or have significant impacts on ecosystem
processes (Pace et al., 1999). Additionally, a prolongation due to food web
interactions is possible as the signal of winter climate can be detected in the clear
water phase in early summer (Straile and Adrian, 2000) or in the summer
phytoplankton composition and biomass (Weyhenmeyer, 2001; Blenckner et al.,
2002). A system approach is necessary to examine the cascading effects in
response to climatic change and variability. The magnitude of a climate-driven
response of an autotrophic organism is not necessarily mediated or cascaded to the
heterotrophic species, or vice versa. The potential for misleading inferences has
been highlighted (Harrington et al., 1999). Furthermore, the non-linearity in the
response to environmental variables (including climate) of animal and plants should
be remembered (May, 1986; Mysterud et al., 2001), as smooth changes can be
interrupted by drastic switches to a contrasting state of the ecosystem function
(Scheffer et al., 2001).
80