it contracts until it reaches 4°C, and then begins to expand again until it
reaches the freezing point, O°C. As a result, water is at its maximum density
at 4°C, and ice or water at any other temperature will oat on top of it. This
too, is a result of hydrogen bonding: Hydrogen bonds hold water molecules
apart; unbonded molecules can pack more closely. In liquid water there are'
Î
an average of two to three bonds per molecule, while in ice there are four per
molecule (that is, all the water molecules are held in the three—dimensional,
hydrogen-bonded
structure and are therefore less densely packed than in wa—
ter).
The density properties of water become particularly important when con—
sidering the aquatic ecosystem found in a lake. Because cold water is denser
than warm water, many lakes (if they are relatively deep) become stratified.
l
The cooler, denser water forms the bottom layer, called the hypolimnion (literally, in the Greek from which both “hypo” and “limnion” are borrowed,
this means “under the pond,” an accurate description). The warmer, lighter,
uppermost layer of water is called the epilimnion (“over the pond”). The
%
transitional water zone is the themwcline, a layer of steeply decreasing tem—
perature. Cases and nutrients do not readily pass from one layer to another.
The contents of such a lake are completely mixed during only two periods of
the year—spring and fall turnover times. In the spring, as ice melts in temperate regions, the epilimnion becomes even cooler and denser than the hypo—
li;imion; the epilimnion therefore ows to the bottom and vertical mixing oc—
ours. In the fall, the upper layers of the water are again cooled, this time by
%
the drop in atmospheric temperature; the epilimnion becomes denser than
3
the water below it and again mixing occurs.
Heat as a Pollutant
Water, then, is unique in its ability to moderate the effects of heat on the environment. Situations may occur, however, in which so much heat is applied
to a body of water that its temperature rises above the level to which the
_
plants and animals in it can adapt. Among the sources of thermal pollution
»
are industrial cooling systems which withdraw water from a stream or lake to
cool certain types of industry and electric generating plants and then return
Ël
the water to its original source at much higher temperatures. Removal of ve—
getation from river banks removes shade, and the addition of sediment
from erosion heats the water because sediment particles absorb light energy.
:Ël
The magnitude of temperature changes that may occur from shade removal
and increased sediment erosion was shown in a study carried out in an Oregon
logging area. Logging activities along stream banks increased the mean
monthly maximum temperatures for ]uly from approximately 57°F in 1965,
before logging had occurred, to 71°F in 1967 after logs had been cut and
cleared away from the area [14].
However, heat discharged from electric generating plants that can raise
stream temperatures 10—30°F is potentiay the most dangerous source Of
42
An Uncommon Friend
reaches the freezing point, O°C. As a result, water is at its maximum density
at 4°C, and ice or water at any other temperature will oat on top of it. This
too, is a result of hydrogen bonding: Hydrogen bonds hold water molecules
apart; unbonded molecules can pack more closely. In liquid water there are'
Î
an average of two to three bonds per molecule, while in ice there are four per
molecule (that is, all the water molecules are held in the three—dimensional,
hydrogen-bonded
structure and are therefore less densely packed than in wa—
ter).
The density properties of water become particularly important when con—
sidering the aquatic ecosystem found in a lake. Because cold water is denser
than warm water, many lakes (if they are relatively deep) become stratified.
l
The cooler, denser water forms the bottom layer, called the hypolimnion (literally, in the Greek from which both “hypo” and “limnion” are borrowed,
this means “under the pond,” an accurate description). The warmer, lighter,
uppermost layer of water is called the epilimnion (“over the pond”). The
%
transitional water zone is the themwcline, a layer of steeply decreasing tem—
perature. Cases and nutrients do not readily pass from one layer to another.
The contents of such a lake are completely mixed during only two periods of
the year—spring and fall turnover times. In the spring, as ice melts in temperate regions, the epilimnion becomes even cooler and denser than the hypo—
li;imion; the epilimnion therefore ows to the bottom and vertical mixing oc—
ours. In the fall, the upper layers of the water are again cooled, this time by
%
the drop in atmospheric temperature; the epilimnion becomes denser than
3
the water below it and again mixing occurs.
Heat as a Pollutant
Water, then, is unique in its ability to moderate the effects of heat on the environment. Situations may occur, however, in which so much heat is applied
to a body of water that its temperature rises above the level to which the
_
plants and animals in it can adapt. Among the sources of thermal pollution
»
are industrial cooling systems which withdraw water from a stream or lake to
cool certain types of industry and electric generating plants and then return
Ël
the water to its original source at much higher temperatures. Removal of ve—
getation from river banks removes shade, and the addition of sediment
from erosion heats the water because sediment particles absorb light energy.
:Ël
The magnitude of temperature changes that may occur from shade removal
and increased sediment erosion was shown in a study carried out in an Oregon
logging area. Logging activities along stream banks increased the mean
monthly maximum temperatures for ]uly from approximately 57°F in 1965,
before logging had occurred, to 71°F in 1967 after logs had been cut and
cleared away from the area [14].
However, heat discharged from electric generating plants that can raise
stream temperatures 10—30°F is potentiay the most dangerous source Of
42
An Uncommon Friend