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

their vertical structure. In particular, there has been a reduction in the depths reached
by the winter mixing over the last 40 years and, consequently, the deeper layers are
progressively less affected by seasonal variations. The deep hypolimnion of lakes
contains a “climatic memory” (represented by variations in the caloric content) from
which is revealed thermal variations on a relatively long time scale, comparable with
that of the ongoing climatic changes (Ambrosetti et al., 2003).
The climatic memories of deep Italian sub alpine lakes are represented in Figure
IV.B.2. The complete circulation in deep lakes will become more and more difficult to
achieve in the future, with major repercussion for lake hydrodynamics and turnover.
More energy is presently required than for the past mixing events and without the
possibility of re-establishing the initial thermal conditions (Ambrosetti and Barbanti,
1999).
Figure IV.B.2. Trends of the heat content in five deep Italian lakes. Values for Lake
Orta on the right axis (from Ambrosetti and Barbanti, 1999)
Direct and Indirect Effects of Temperature Changes
The direct effect of the projected increases in the winter temperatures on the
dynamics of lakes in the Atlantic Region is likely to be quite small. There will,
however, be significant indirect effects on the flux of nutrients and the growth of
plankton much later in the year.
The biological effects of the increased surface temperatures and enhanced stability
in summer depend on the physical characteristics of the individual lakes and, in
particular, their maximum depth. The very high surface temperatures recoded in
shallow lakes may well change the geographic distribution of some cold water
species but most planktonic organisms have a cosmopolitan distribution. Most of
these species already thrive at temperatures that are very much higher than those
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