Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 105

Temperature ( o C)

6 8 10 12 14 16 18 2 0

0

3

5

8

After hurricane
Before hurricane

10

13

15

FIGURE 6.12 Stability of the thermocline in Linsley Pond, Connecticut, before and after a
hurricane on September 21, 1938, with wind speeds up to 100 km h 1 (from G. E. Hutchin-
son, A Treatise on Limnology, Vol. 1, Geography, Physics and Chemistry . Copyright © 1957
John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.).

surface. The cooled surface water is denser than the water immediately be-
low, so it sinks. The wind can mix the lake once the entire lake is isother-
mal, and the fall mixing period begins. The lake will continue to cool and
mix until formation of an ice cover on the surface of the lake.
The surface of the lake can freeze when the temperature of the entire
lake is below 3.9°C. If the lake is warmer, cool water from the surface will
continue to sink and mix with the less dense water below it. If the lake
temperature is 3.9°C, the water is at its densest, so cooler water will sit on
the surface of the lake as long as the wind does not mix it. The surface of
the lake can freeze if there is a cold, calm night. The low-density ice and
cold, low-density water will sit on the surface of the lake once the surface
has frozen. No more wind reaches the surface of the water so no more mix-
ing occurs. Winter stratification is the second period during the year when
a cold-temperate lake does not mix (Fig. 6.10), and it lasts as long as cold
weather maintains the ice cover.
Duration of ice cover provides one of the best long-term records of the
effects of global warming on freshwater systems. There are good data on
dates of formation and breakup of ice cover for many lakes and rivers for
the past 100 years, and records exist for Lake Suwa in Japan almost con-
tinuously since 1450. These data suggest that freeze dates have become
later by 5.8 days over the past 100 years and breakup dates 6.5 days ear-
lier (Magnuson et al., 2000).
Much terminology is used to describe the mixing regimes of lakes
(Table 6.1). Lakes that mix twice a year are called dimictic . Lakes that only
mix once during the year are called monomictic and are common in tem-
perate and subtropical regions where winters are not cold enough to freeze
lake surfaces but are cool enough to allow the lakes to become isothermal.
Lakes that never mix are called amictic. Polymictic lakes stratify or mix
several times a year and are found mainly in tropical regions. Meromictic