Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 47

(a) (b)

7.0 17.0

Winter Summer

10 m (0.15°C / decade)

6.0 16.0

10 m (0.42°C / decade)

5.0 15.0

14.0

4.0

100 m (0.06°C/decade)

1950 1960 1970 1980 1990 2000

13.0

(c)

Summer

6.0

12.0

5.0

100 m (0.03°C / decade)

11.0

4.0

10.0

1950 1960 1970 1980 1990 2000

1950 1960 1970 1980 1990 2000

Figure 3.7 Warming trends in Lake Zurich (Switzerland) showing linear regression lines
and their gradients: (a) at 10 and 100 m depth in winter (December, January, February),
(b) at 10 m depth in summer (June, July, August) and (c) at 100 m depth in summer.

provides a good example of the response of deep temperate lakes to long-term
changes in air temperature. The lake has undergone long-term warming at all
depths. However, the temperature increase has been more rapid in the surface
layers than in the deep-water layers, resulting in increased thermal stability in all
seasons and in an extended period of summer stratification by about 2-3 weeks
from the 1950s to the 1990s (Livingstone 2003).
In winter (December-February), highly significant increasing trends in water
temperature have occurred at all depths (Mann-Kendall test (MK), p < 0.001),
but the highest long-term winter warming rates (

∼

0.15°C per decade) are found
in the uppermost part of the water column (0-10 m) (Fig. 3.7a). From 10 to
80 m, long-term warming rates decrease with increasing depth, but from 80 m to
the lake bottom, no further change occurs; warming rates remain approximately
constant at about 0.06°C per decade. In summer, the highest rate of long-term
warming (0.42°C per decade; MK, p < 0.01) has occurred at 10-m depth
(Fig. 3.7b), while at the lake surface, the temperature increase since the 1950s
has been comparatively low (

∼

0.07°C per decade; MK, p < 0.01). Surface water
temperatures approximately reflect the behaviour of the regional summer air
temperature, and reveal a period of rapid cooling from 1945 to 1970 (−1.0°C
per decade) followed by strong warming from 1970 to 2006 (0.5°C per decade).