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= 16.0°C
Epi/metalimnetic
temperature (
T
em
)
μ
Hypolimnetic
temperature (
T
h
)
μ
= 4.8°C
Temperature
difference (
T
em
-
T
h
)
μ
= 11.2°C
Schmidt
stability (
S
)
= 8780 J m
-2
μ
Hypolimnetic
oxygen depletion (HOD)
μ
= 4.3 g m
-3
-4
2
Number of standard deviations
-2
0
4
6
8
Figure 3.8
Standardized values of water temperature, Schmidt stability (the quantity
of work required to mix a lake to a uniform vertical density with no addition or
substraction of heat) and hypolimnetic oxygen depletion in Lake Zurich (Switzerland).
Data are expressed as standard deviations from the long-term mean (
) of 1956-2002.
The summer 2003 values are shown in red. (Modified from Jankowski
et al
. 2006.
Reproduced with permission of the American Society of Limnology and Oceanography.)
μ
In the hypolimnion (i.e. below 20 m), summer temperatures show a low long-
term warming trend of 0.03°C-0.06°C per decade, which is, however, not
statistically significant even at the
p
< 0.1 level (Fig. 3.7c).
As in most deep, temperate-zone lakes, temperatures in the uppermost 10-20 m
of Lake Zurich are influenced directly by weather in all seasons, whilst
temperatures in the deeper water respond most strongly to weather in winter and
spring. Physically, Lake Zurich appears to be quite sensitive to climate variability
since it can behave either as a dimictic, monomictic or oligomictic lake, mixing
twice or once per year or not mixing at all, respectively. Over the past few
decades, the frequency of dimixis (after ice melt and in autumn) has decreased,
while the frequency of years with incomplete mixing has increased as winters
have become warmer (Livingstone 1997; Peeters
et al
. 2002).
In Central Europe, the summer of 2003 was exceptionally hot, with air
temperatures similar to those predicted for an average summer during the late
21st century (Schär
et al
. 2004). During that summer, Lake Zurich experienced
the highest epilimnetic temperatures ever recorded, exceeding the long-term
mean (1856-2002) by almost three standard deviations (Fig. 3.8). By contrast,
hypolimnetic temperatures were slightly lower than average. The resulting high
thermal stability of the water column resulted in hypolimnetic oxygen depletion
exceeding its long-term mean by more than seven standard deviations. The
potential ecological consequences of this, such as the release of phosphorus from
the sediments, possibly ultimately resulting in an increase in the intensity of algal
blooms, may thus counteract management and restoration efforts undertaken in
the past to mitigate anthropogenic eutrophication.
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