Physical Characteristics: Lake Models
37
Apparatus and Supplies
1. Aquarium filled with tap water adjusted uniformly to about 20°C from top to
bottom.
2. Heat lamp, thermometers, etc., positioned as in Lake ModelL For more rapid
heating, the lamp may be positioned nearer the water surface, but use caution.
Calculations
As in Lake Modell, calculate (a) the stability of the lake, (b) the thermal resistance to
mixing, (c) the heat budget, and (d) the theoretical periods of the internal seiches. Also
make plots of temperature versus depth, and the thermally produced density versus
depth for the values determined after the two winds. Compare with the results from
Lake Model 1.
LAKE MODEL 3: MEROMICTIC LAKE
Purpose
The phenomenon of meromixis will be illustrated. The exercise will also show saltproduced density currents sinking and moving along the upper part ofthe chemocline
and will simulate the phenomenon of "I a bataillere." (La bataillere is the name given to
a subsurface density current in the lake of Geneva where cold, silt-laden, glacial
meltwaters move as a discrete stream along the thermocline region far out into the
lake.) Being cold and containing suspended clay, the waters should be dense and
descend as density currents along the bottom of the lake. However, the content of
dissolved solids is low, producing an intermediate density, and the water flows into the
lake as a subsurface stream along the bottom of the epilimnion. This phenomenon has
no particular connection with meromixis-it merely is convenient to illustrate it at this
same time when we are using a highly stratified lake.
Procedures
1. Fill the aquarium two-thirds full with tap water. Add ice liberally and uniformly
cool the water to 4°C. Allow the water mass to become quiet.
2. Prepare a siphon arrangement from a 0.3% salt (NaCl) solution (at room
temperature) containing methylene blue. An L-shaped glass tube with a short and
unpolished end on the horizontal part is attached to the lower end of the siphon.
The vertical part of the glass tube must be long enough so that it can rest diagonally
on one side of the aquarium with the bottom of the tube resting in a corner at the
bottom of the other side. The hose connection is provided with a screw clamp
attached near the glass tube for regulation of the flow. Start the siphon, allowing the
blue liquid to come close to the end of the glass tube. Close the clamp and then
lower the tube in such a way that the dense, salt-laden liquid does not flow into the
clear aquarium water. Position the tube so that the opening is pointed in the
direction of the long axis of the aquarium. Slowly open the screw clamp, allowing
the liquid to flow out in a nonturbulent stream. Approximately 20 min will be
required to fill the aquarium one-third full with the salt layer. (The flow may be
increased somewhat as the level rises in the tank.) When the surface water in the
tank has reached the desired level Gust above the base of the metal rim), close the
37
Apparatus and Supplies
1. Aquarium filled with tap water adjusted uniformly to about 20°C from top to
bottom.
2. Heat lamp, thermometers, etc., positioned as in Lake ModelL For more rapid
heating, the lamp may be positioned nearer the water surface, but use caution.
Calculations
As in Lake Modell, calculate (a) the stability of the lake, (b) the thermal resistance to
mixing, (c) the heat budget, and (d) the theoretical periods of the internal seiches. Also
make plots of temperature versus depth, and the thermally produced density versus
depth for the values determined after the two winds. Compare with the results from
Lake Model 1.
LAKE MODEL 3: MEROMICTIC LAKE
Purpose
The phenomenon of meromixis will be illustrated. The exercise will also show saltproduced density currents sinking and moving along the upper part ofthe chemocline
and will simulate the phenomenon of "I a bataillere." (La bataillere is the name given to
a subsurface density current in the lake of Geneva where cold, silt-laden, glacial
meltwaters move as a discrete stream along the thermocline region far out into the
lake.) Being cold and containing suspended clay, the waters should be dense and
descend as density currents along the bottom of the lake. However, the content of
dissolved solids is low, producing an intermediate density, and the water flows into the
lake as a subsurface stream along the bottom of the epilimnion. This phenomenon has
no particular connection with meromixis-it merely is convenient to illustrate it at this
same time when we are using a highly stratified lake.
Procedures
1. Fill the aquarium two-thirds full with tap water. Add ice liberally and uniformly
cool the water to 4°C. Allow the water mass to become quiet.
2. Prepare a siphon arrangement from a 0.3% salt (NaCl) solution (at room
temperature) containing methylene blue. An L-shaped glass tube with a short and
unpolished end on the horizontal part is attached to the lower end of the siphon.
The vertical part of the glass tube must be long enough so that it can rest diagonally
on one side of the aquarium with the bottom of the tube resting in a corner at the
bottom of the other side. The hose connection is provided with a screw clamp
attached near the glass tube for regulation of the flow. Start the siphon, allowing the
blue liquid to come close to the end of the glass tube. Close the clamp and then
lower the tube in such a way that the dense, salt-laden liquid does not flow into the
clear aquarium water. Position the tube so that the opening is pointed in the
direction of the long axis of the aquarium. Slowly open the screw clamp, allowing
the liquid to flow out in a nonturbulent stream. Approximately 20 min will be
required to fill the aquarium one-third full with the salt layer. (The flow may be
increased somewhat as the level rises in the tank.) When the surface water in the
tank has reached the desired level Gust above the base of the metal rim), close the