James G. (James George) Needham.

The life of inland waters; an elementary text book of fresh-water biology for students online

. (page 2 of 26)
Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 2 of 26)
Font size
QR-code for this ebook

iris which spans it, in the morning mist which rises from it, in the
deep crystalline pools which mirror its hanging shore, in the broad
lake and glancing river, finally, in that which is to all human minds
the best emblem of unwearied, unconquerable power, the wild, various,
fantastic, tameless unity of the sea; what shall we compare to this
mighty, this universal element, for glory and for beauty? or how shall
we follow its eternal cheerfulness of feeling? It is like trying to paint
a soul" RUSKIN.




ATER, the one abundant
liquid on earth, is, when
pure, tasteless, odorless
and transparent. Wa-
ter is a solvent of a
great variety of sub-
stances, both solid and
gaseous. Not only does
it dissolve more sub-
stances than any other
liquid, but, what is more
important, it dissolves
those substances which
are most needed in solution for the maintenance of
life. Water is the greatest medium of exchange in the
world. It brings down the gases from the atmosphere;
it transfers ammonia from the air into the soil for
plant food; it leaches out the soluble constituents
of the soil; and it acts of itself as a chemical agent
in nutrition, and also in those changes of putrefaction
and decay that keep the world's available food supply
in circulation.

Water is nature's great agency for the applica-
tion of mechanical energy. It is by means of water


26 Nature of Aquatic Environment

that deltas are built and hills eroded. Water is the
chief factor in all those eternal operations of flood and
floe by which the surface of the continent is shaped.

Transparency. Water has many properties that fit
it for being the abode of organic life. Second only in
importance to its power of carrying dissolved food
materials is its transparency. It admits the light of
the sun; and the primary source of energy for all
organic life is the radiant energy of the sun. Green
plants use this energy directly; animals get it in-
directly with their food. Green plants constitute
the producing class of organisms in water as on land.
Just in proportion as the sun's rays are excluded,
the process of plant assimilation (photosynthesis) is
impeded. When we wish to prevent the growth of
algae or other green plants in a reservoir or in a spring
we cover it to exclude the light. Thus we shut off
the power.

Pure water, although transparent, absorbs some of
the energy of the sun's rays passed through it, and
water containing dissolved and suspended matter
(such as are present in all natural water) impedes
their passage far more. From which it follows, that
the superficial layer of a body of water receives the
most light. Penetration into the deeper strata is
impeded according to the nature of the water content.
Dissolved matters tint the water more or less and give
it color. Every one knows that bog waters, for
example, are dark. They look like tea, even like very
strong tea, and like tea they owe their color to their
content of dissolved plant substances, steeped out of
the peaty plant remains of the bog.

Suspended matters in the water cause it to be turbid.
These may be either silt and refuse, washed in from
the land, or minute organisms that have grown up in

Transparency 27

the water and constitute its normal population. One
who has carefully watched almost any of our small
northern lakes through the year will have seen that
its waters are clearest in February and March, when
there is less organic life suspended in them than at
other seasons. But it is the suspended inorganic
matter that causes the most marked and sudden
changes in turbidity the washings of clay and silt
from the hills into a stream; the stirring up of mud
from the bottom of a shallow lake with high winds.
The difference in clearness of a creek at flood and at
low water, or of a pond before and after a storm is often
very striking.

Such sudden changes of turbidity occur only in the
lesser bodies of water; there is not enough silt in the
world to make the oceans turbic.

The clearness of the water determines the depth
at which green plants can flourish in it. Hence it is
of great importance, and a number of methods have
been devised for measuring both color and turbidity.
A simple method that was first used for comparing the
clearness of the water at different times and places
and one that is, for many purposes, adequate, and one
that is still used more widely than any other,* consists
in the lowering of a white disc into the water and record-
ing the depth at which it disappears from view. The
standard disc is 20 cm. in diameterf; it is lowered
in a horizontal position during midday light. The
depth at which it entirely disappears from view is
noted. It is then slowly raised again and the depth
at which it reappears is noted. The mean of these
two measurements is taken as the depth of its visibility

*Method of Secchi: for other methods, see Whipple's Microscopy of Drink-
ing Water, Chap. V. Steuer's Planktonkunde, Chapter III.

fWhipple varied it with black quadrants, like a surveyor's level-rod target
and viewed it through a water telescope.

28 Nature of Aquatic Environment

beneath the surface. Such a disc has been found to
disappear at very different depths. Witness the fol-
lowing typical examples:

Pacific Ocean 59 meters

Mediterranean Sea 42 meters

Lake Tahoe . 33 meters

"Lake Geneva . 72T meters

Cayuga Lake 5 meters

Pure Lake (Denmark), Mar 9 meters

Pure Lake (Denmark), Aug 5 meters

Pure Lake (Denmark), Dec J meters

Spoon River (111.) under ice 3.65 meters

Spoon River (111.) at flood 013 meters

It is certain that diffused light penetrates beyond the
depth at which Secchi's disc disappears. In Lake
Geneva, for example, where the limit of visibility is
2im., photographic paper sensitized with silverchloride
ceased to be affected by a 24-hour exposure at a depth
of about 100 meters or when sensitized with iodobromide
of silver, at a depth about twice as great. Below this
depth the darkness appears to be absolute. Indeed it
is deep darkness for the greater part of this depth, 90
meters being set down as the limit of "diffused light."
How far down the light is sufficient to be effective in
photosynthesis is not known, but studies of the distri-
bution in depth of fresh water algae have shown them
to be chiefly confined, even in clear lakes, to the upper-
most 20 meters of the water. Ward ('95) found 64
per cent of the plancton of Lake Michigan in the upper-
most two meters of water, and Reighard ('94) found
similar conditions in Lake St. Clair. Since the inten-
sity of the light decreases rapidly with the increase in
depth it is evident that only those plants near the sur-
face of the water receive an amount of light comparable
with that which exposed land plants receive. Less than
this seems to be needed by most free swimming algae,

Transparency 29

since they are often found in greatest number in open
waters some five to fifteen meters below the surface.
Some algae are found at all depths, even in total dark-
ness on the bottom; notably diatoms, whose heavy
silicious shells cause them to sink in times of prolonged
calm, but these are probably inactive or dying individ-
uals. There are some animals, however, normally
dwelling in the depths of the water, living there upon

150 Meters Depth
FIG. 3. Diagram illustrating the penetration of light into the water of a lake;
also, its occlusion by inflowing silt and by growths of plants on the surface.

the organic products produced in the zone of photo-
synthesis above and bestowed upon them in a consider-
able measure by gravity. To the consideration of
these we will return in a later chapter.

The accompanying diagram graphically illustrates
the light relations in a lake. The deeper it is the greater
its mass of unlighted and, therefore, unproductive
water, and the larger it be, the less likely is its upper
stratum to be invaded by obscuring silt and water

3O Nature of Aquatic Environment

Mobility Water is the most mobile of substances,
yet it is not without internal friction. Like molasses,
it stiffens with cooling to a degree that affects the
flotation of micro-organisms and of particles suspended
in it. Its viscosity is twice as great at the freezing
point as at ordinary summer temperature (77F.).

Buoyancy Water is a denser medium than air: it
is 775 times heavier. Hence the buoyancy with which
it supports a body immersed in it is correspondingly
greater. The density of water is so nearly equal to
that of protoplasm, that all living bodies will float in
it with the aid of very gentle currents or of a very little
exertion in swimming. Flying is a feat that only a
few of the most specialized groups of animals have
mastered, but swimming is common to all the groups.

Pressure This greater density, however, involves
greater pressure. The pressure is directly proportional
to the depth, and is equal to the weight of the super-
posed column of water. Hence, with increasing depth
the pressure soon becomes enormous, and wholly insup-
portable by bodies such as our own. Sponge fishers
and pearl divers, thoroughly accustomed to diving,
descending naked from a boat are able to work at depths
up to 20 meters. Professional divers, encased in a
modern diving dress are able to work at depths several
times as great; but such depths, when compared with
the depths of the great lakes and the oceans are com-
parative shoals.

Beyond these depths, however, even in the bottom
of the seas, animals live, adjusted to the great pressure,
which may be that of several hundred of atmospheres.
But these cannot endure the lower pressure of the
surface, and when brought suddenly to the surface they
burst. Fishes brought up from the bottom of the
deeper freshwater lakes, reach the surface greatly

Maximum Density 31

swollen, their scales standing out from the body, their
eyes bulging.

Maximum density Water contracts on cooling, as do
other substances, but not to the freezing point only
to 4 centigrade (39.2 Fahrenheit). On this pecu-
liarity hang many important biological consequences.
Below 4 C. it begins to expand again, becoming lighter,
as shown in the accompanying table:

Temperature Weight in Ibs.

C F percu. ft. Density

35 95 62.060 .99418

21 70 62.303 .99802

10 50 62.408 -99975

4 39 62.425 i.ooooo

o 32 62.417 .99987

Hence, on the approach of freezing, the colder lighter
water accumulates at the surface, and the water at the
point of maximum density settles to the bottom, and
the congealing process, so fatal to living tissues generally
is resticted to a thin top layer. Here at o C. (32 F.)
the water freezes, expanding about one-twelfth in bulk
in the resulting ice and reducing its weight per cubic
foot to 57.5 pounds.

Stratification of the water Water is a poor conductor
of heat. We recognize this when we apply heat to the
bottom of a vessel, and set up currents for its distribution
through the vessel. We depend on convection and not
on conduction. But natural bodies of water are heated
and cooled from the top, when they are in contact with
the atmosphere and where the sun's rays strike.
Hence, it is only those changes of temperature which
increase the density of the surface waters that can pro-
duce convection currents, causing them to descend, and
deeper waters to rise in their place. Minor changes
of this character, very noticeable in shallow water, occur

Nature of Aquatic Environment

every clear day with the going down of the sun, but
great changes, important enough to affect the tempera-
ture of all the waters of a deep lake, occur but twice a
year, and they follow the precession of the equinoxes.
There is a brief, often interrupted, period (in March
in the latitude of Ithaca) after the ice has gone out, while
the surface waters are being warmed to oC.; and
there is a longer period in autumn, while they are being
cooled to oC. Between times, the deeper waters of



FIG. 4. Diagram illustrating summer and winter temperature conditions in

Cayuga Lake. The spacing of the horizontal lines represents

equal temperature intervals.

a lake are at rest, and they are regularly stratified
according to their density.

In deep freshwater lakes the bottom temperature
remains through the year constantly near the point of
maximum density, 4 C. This is due to gravity. The
heavier water settles, the lighter, rises to the top.
Were gravity alone involved the gradations of tempera-
ture from bottom to top would doubtless be perfectly
regular and uniform at like depths from shore to shore.
But springs of ground water and currents come in to

Lake Temperatures


disturb the horizontal uniformity, and winds may do
much to disturb the regularity of gradations toward the
surface. Water temperatures are primarily dependent
on those of the superincumbent air. The accompany-
ing diagram of comparative yearly air and water
temperatures in Hallstatter Lake (Austria) shows
graphically the diminishing influence of the former on
the latter with increasing depth.


FIG. 5. Diagram illustrating the relation of air and water temperatures at
varying depths of water in Hallstatter Lake (after Lorenz).

34 Nature of Aquatic Environment

FIG. 6. Diagram illustrating the distribution of temperature in Cayuga Lake
throughout the year. (Extremes: not normal).

The yearly cycle The general relation between sur-
face and bottom temperatures for the year are graphi-
cally shown in the accompanying diagram, wherein the
two periods of thermal stratification, "direct" in summer
when the warmer waters are uppermost, and "inverse"
in winter when the colder waters are uppermost, are
separated by two periods of complete circulation, when
all the waters of the lake are mixed at 4 C. The range
of temperatures from top to bottom is much greater in
the summer "stagnation period"; nevertheless there

The Yearly Cycle 35

is more real stagnation during the winter period; for,
after the formation of a protecting layer of ice, this
shuts out the disturbing influence of wind and sun and
all the waters are at rest. The surface temperature
bears no further relation to air temperature but remains
constantly at o C.

After the melting of the ice in late winter the surface
waters begin to grow warmer; so, they grow heavier,
and tend to mingle with the underlying waters. When
all the water in the lake is approaching maximum
density strong winds heaping the waters upon a lee
shore, may put the entire body of the lake into complete
circulation. How long this circulation lasts will depend
on the weather. It will continue (with fluctuating
vigor) until the waters are warm enough so that their
thermal stratification and consequent resistance to
mixture are great enough to overcome the disturbing
influence of the wind. Thereafter, the surface may be
stirred by storms at any time, but the deeper waters of
the lake will have passed into their summer rest.

On the approach of autumn the cooling of surface
waters starts convection currents, which mix at first the
upper waters only, but which stir ever more deeply as
the temperature descends. When nearly 4C., with
the aid of winds, the entire mass of water is again put
in circulation. The temperature is made uniform
throughout, and what is more important biologically,
the contents of the lake, in both dissolved and suspended
matters, are thoroughly mixed. Nothing is thereafter
needed other than a little further cooling of the surface
waters to bring about the inverse stratification of the
winter period.

Vernal and autumnal circulation periods differ in
this, that convection currents have a smaller share, and
winds may have a larger share in the former. For the
surface waters are quickly warmed from o C. to 4 C.,

36 Nature of Aquatic Environment

and further warming induces no descending currents,
but instead tends toward greater stability. It some-
times happens that in shallow lakes there is little vernal
circulation. If the water be warmed at 4 C. at the
bottom before the ice is entirely gone, and if a period
of calm immediately follow, so that no mixing is done
by the wind, there may be no general spring circulation

The shallower the lake, other things being equal, the
greater will be the departure of temperature conditions
from those just sketched, for the greater will be the
disturbing influence of the wind. In south temperate
lakes, temperature conditions are, of course, reversed
with the seasons. In tropical lakes whose surface
temperature remains always above 4 C., there can be
no complete circulation from thermal causes, and in-
verse stratification is impossible. In polar lakes, never
freed from ice, no direct stratification is possible.

It follows from the foregoing that gravity alone may
do something toward the warming of the waters in the
spring, and much toward the cooling of them in the fall.
By gravity they will be made to circulate until they
reach the point of maximum density, when going either
up or down the scale. Beyond this point, however,
gravity tends to stabilize them. The wind is responsi-
ble for the further warming of the waters in early sum-
mer, and the heat in excess of 4 C. has been called by
Birge and Juday "wind-distributed" heat. They esti-
mate that it may amount to 30,000 gram-calories per
square centimeter of surface in such lakes as those
of Central New York, and the following figures for
Cayuga Lake show its distribution by depth in August,
1911, in percentage remaining at successive ten-meter
intervals below the surface :

Below o 10 20 30 40 50 60 70 80 100 133 meter?
% 100 50.2 16.7 7.1 3.7 2.4 1.8 1.2 .7 .3 remaining

Thermocline 37

These figures indicate the resistance to mixing that
gravity imposes, and show that the wind is not able to
overcome it below rather slight depths.

Vernal and autumnal periods of circulation have a
very great influence upon the distribution of both
organisms and their food materials in a lake; to the
consideration of this we will have occasion to return

The thermocline In the study of lake temperatures
at all depths, a curious and interesting peculiarity of
temperature interval has been commonly found per-
taining to the period of direct stratification (mid-
summer). The descent in temperature is not regular
from surface to bottom, but undergoes a sudden acceler-
ation during a space of a very few meters some distance
below the surface. The stratum of water in which
this sudden drop of temperature occurs is known as the
thermocline (German, Sprungschichf) . It appears to
represent the lower limit of the intermittent summer
circulation due to winds. Above it the waters are more
or less constantly stirred, below it they lie still. This
interval is indicated by the shading on the right side of
figure 4. Birge has designated the area above the
thermocline as the epilimnion; the one below it as

Further study of the thermocline has shown that it is
not constant in position. It rises nearer to the surface
at the height of the midsummer season and descends a
few meters with the progress of the cooling of the
autumnal atmosphere. This may be seen in figure 7,
which is Birge and Juday's chart of temperatures of
Lake Mendota as followed by them through the season
of direct stratification and into the autumnal circula-
tion period in 1906. This chart shows most graphically
the growing divergence of surface and bottom tempera-
tures up to August, and their later approximation and

Nature of Aquatic Environment

final coalescence in October. Leaving aside the not
unusual erratic features of surface temperature (repre-
sented by the topmost contour line) it will be noticed
that there is a wider interval somewhere between 8 and
1 6 meters than any other interval either above or below
it. Sometimes it falls across two spaces and is rendered
1 ess apparent in the charting by the selection of inter-
vals. It first appears clearly in June at the 10-12 meter
interval. It rises in July above the 10 meter level.

FIG. 7. Temperature of the water at different depths in Lake Mendota in
1906. The vertical spaces^ represent degrees Centigrade and the figures
attached to the curves indicate the depths in meters. (Birge and Juday) .

In the middle of August it lies above the 8 meter level,
though it begins to descend later in the month. It
continues to descend through September, and is found
in early October between 16 and 18 meters. It dis-
appears with the beginning of the autumnal circulation.
The cause of this phenomenon is not known. Richter
has suggested that convection currents caused by the
nocturnal cooling of the surface water after hot summer
days may be the cause of it. If the surface waters were

Circulation 39

cooled some degrees they would descend, displacing the
layers underneath and setting up shallow currents
which would tend to equalize the temperature of all the
strata involved therein. And if the gradation of tem-
peratures downward were regular before this mixing,
the result of it would be a sudden descent at its lower
limit, after the mixing was done. This would account
for the upper boundary of the thermocline, but not for
its lower one. Perhaps an occasional deeper mixing,
extending to its lower boundary, and due possibly to
high winds, might bring together successional lower
level s of temperature of considerable intervals . Perhaps
the thermocline is but an accumulation of such sort of
thermal disturbance-records, ranged across the vertical
section of the lake, somewhat as wave-drift is ranged in
a shifting zone along the middle of a sloping beach.
At any rate, it appears certain that the thermocline
marks the lower limit of the chief disturbing influences
that act upon the surface of the lake. That it should
rise with the progress of summer is probably due to the
increasing" stability of the lower waters, as differences
in temperature (and therefore in density) between upper
and lower strata are increased. Resistance to mixing
increases until the maximum temperature is reached,
and thereafter declines, as the influence of cooling and
of winds' penetrates deeper and deeper.

In running water the mixing is more largely mechani-
cal, and vertical circulation due to varying densities is
less apparent. Yet the deeper parts of quiet streams
approximate closely to conditions found in shallow
lakes. Such thermal stratification as the current
permits is direct in summer and inverse in winter, and
there are the same intervening periods of thermal over-
turn when the common temperature approaches 4 C.
In summer and, in winter there is less "stagnation" of
bottom waters owing to the current of the stream.

40 Nature of Aquatic Environment

The thermal conservatism of water Water is slower
to respond to changes of temperature than is any other
known substance. Its specific heat is greater. The
heat it consumes in thawing (and liberates in freezing)
is greater. The amount of heat necessary to melt one
part of ice at o C. without raising its temperature at all
would be sufficient to raise the temperature of the same
when melted more than 75 degrees. Furthermore, the
heat consumed in vaporization is still greater. The
amount required to vaporize one part of water at 100 C.
without raising its temperature would suffice to raise
534 parts of water from o C. to i C. ; and the amount
is still greater when vaporization occurs at a lower
temperature. Hence, the cooling effect of evaporation
on the surrounding atmosphere, which gives up its
heat to effect this change of state in the water; hence,
the equalizing effect upon climate of the presence of
large bodies of water; hence the extreme variance
between day and night temperatures in desert lands;
hence the delaying of winter so long after the autumnal,
and of summer so long after the vernal equinox.
Water is the great stabilizer of temperature.

The content of natural waters Water is the common
solvent of all foodstuffs. These stuffs are, as every-
body knows, such simple mineral salts as are readily
leached out of the soil, and such gases as may be washed
down out of the atmosphere. And since green plants
are the producing class among 1 organisms, all others
being dependent on their constructive activities, water
is fitted to be the home of life in proportion as it con-
tains the* essentials of green plant foods, with fit condi-
tions of warmth, air and light.

Natural waters all contain more or less of the elemen-

Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 2 of 26)