Physical Characteristics: Lake Models
39
Calculations
With the hydrometers provided, measure the specific gravity of the 0.3 % salt solution
and the tap water used to fill the aquarium. After adding the salt-produced density to
the thermally produced density following the first wind (remember to correct the
densities of the salt and tap water solutions at the time of measurement for
temperature), proceed as in Lake Modell to calculate (a) the stability ofthe lake, (b) the
thermal resistance to mixing, and (c) the theoretical periods of the internal seiches. Also,
make separate plots of density versus depth (a) as a result oftemperature variations and
(b) as a result of variations in salt concentration. Compare the results with those from
Lake Modell. See Hutchinson (1957, pp. 341-346) for additional information on the
three-layered system.
LAKE MODEL 4: TEMPERATE LAKE WITH DENSITY DIFFERENCES BY SALINITY
Purpose
The thermally dependent density stratification of moderately deep lakes of the
Temperate Zone can be simulated effectively with three strata separated by small
salinity differences. With this model the effects of wind on stratification patterns and
water movements can be observed without changing temperatures.
Procedures
1. Place the clear epilimnetic water into the model basin. Next, carefully siphon first the
red metalimnetic water (p ~ 1.015) into place below the epilimnetic water; observe
all of the precautions noted in Lake Model 3 (p. 37). Then carefully siphon the blue
hypolimnetic water (p ~ 1.050) into place below the metalimnetic water. Without
disturbance the stratified model can be held for at least 24 h without appreciable
diffusion at the interfaces. Why?
2. Distribute a few crystals of methylene blue over the surface and observe the
descending streaks of dye through the strata.
3. Apply a moderate wind to the surface and note the general size of surface waves
(amplitude and wave length). Simultaneously apply a few crystals of methylene blue
to the surface and follow the movement of water within the epilimnion at the surface
and at the epilimnetic-metalimnetic interface.
4. With strong, forced oscillations from the wind source (see p. 32), create internal
seiches in the metalimnion. Stop the wind and follow the amplitude and periodicity
of the internal seiches. With care, this exercise can be repeated without disrupting
the stratification appreciably. Why?
5. Again apply strong wind and impart a 10 to 15° slope to the epilimnionmetalimnion. Stop the wind and measure the (a) periods of the internal seiche, and
(b) the magnitude of internal waves on the metalimnion. Continue to intensify the
movements until the internal progressive waves form breakers. Compare the
wavelengths and amplitudes of the internal waves to those of the surface waves.
6. Allow the system to return to rest. With trails from dye crystals, follow the water
movements in the strata above and below each of the two density interfaces. How do
these to-and-fro movements horizontally change in magnitude with increasing
density and at the sediment-water interface? What can you infer about the relative
39
Calculations
With the hydrometers provided, measure the specific gravity of the 0.3 % salt solution
and the tap water used to fill the aquarium. After adding the salt-produced density to
the thermally produced density following the first wind (remember to correct the
densities of the salt and tap water solutions at the time of measurement for
temperature), proceed as in Lake Modell to calculate (a) the stability ofthe lake, (b) the
thermal resistance to mixing, and (c) the theoretical periods of the internal seiches. Also,
make separate plots of density versus depth (a) as a result oftemperature variations and
(b) as a result of variations in salt concentration. Compare the results with those from
Lake Modell. See Hutchinson (1957, pp. 341-346) for additional information on the
three-layered system.
LAKE MODEL 4: TEMPERATE LAKE WITH DENSITY DIFFERENCES BY SALINITY
Purpose
The thermally dependent density stratification of moderately deep lakes of the
Temperate Zone can be simulated effectively with three strata separated by small
salinity differences. With this model the effects of wind on stratification patterns and
water movements can be observed without changing temperatures.
Procedures
1. Place the clear epilimnetic water into the model basin. Next, carefully siphon first the
red metalimnetic water (p ~ 1.015) into place below the epilimnetic water; observe
all of the precautions noted in Lake Model 3 (p. 37). Then carefully siphon the blue
hypolimnetic water (p ~ 1.050) into place below the metalimnetic water. Without
disturbance the stratified model can be held for at least 24 h without appreciable
diffusion at the interfaces. Why?
2. Distribute a few crystals of methylene blue over the surface and observe the
descending streaks of dye through the strata.
3. Apply a moderate wind to the surface and note the general size of surface waves
(amplitude and wave length). Simultaneously apply a few crystals of methylene blue
to the surface and follow the movement of water within the epilimnion at the surface
and at the epilimnetic-metalimnetic interface.
4. With strong, forced oscillations from the wind source (see p. 32), create internal
seiches in the metalimnion. Stop the wind and follow the amplitude and periodicity
of the internal seiches. With care, this exercise can be repeated without disrupting
the stratification appreciably. Why?
5. Again apply strong wind and impart a 10 to 15° slope to the epilimnionmetalimnion. Stop the wind and measure the (a) periods of the internal seiche, and
(b) the magnitude of internal waves on the metalimnion. Continue to intensify the
movements until the internal progressive waves form breakers. Compare the
wavelengths and amplitudes of the internal waves to those of the surface waves.
6. Allow the system to return to rest. With trails from dye crystals, follow the water
movements in the strata above and below each of the two density interfaces. How do
these to-and-fro movements horizontally change in magnitude with increasing
density and at the sediment-water interface? What can you infer about the relative