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James G. (James George) Needham.

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

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results that the bulk of the plancton in a lake lives in its



Distribution in Depth



309



uppermost part, the thickness of this productive
stratum varying directly with the transparency of the
water.

It is not at the surface, however, but usually a little
below it a depth of a meter, more or less at which
the greatest mass of the planet on is found. Full sun-
light is perhaps too strong; for average planctonts a
dilution of it is preferred. Free-swimming planctonts
such as rotifers and entomostraca move
freely upward or downward with changes
of intensity of light. Anyone who has seen
Daphnes in a sunlit pool congregating in
the shadow of a water-lily pad will under-
stand this. These animals rise nearer to
the surface when the sun goes under a
cloud, and sink again when the cloud
passes. The extent of their regular diur-
nal migrations appears to be directly relat-
ed to the transparency of the water.

Temperature also is an important factor
determining vertical distribution. Forms
requiring the higher temperatures are
summer planctonts that live at or near
the surface. Others that are attuned to lower tem-
peratures may find a congenial summer home at a
greater depth. Thus the flagellate Mallomonas (fig.
185) in Cayuga Lake is rarely encountered in summer
in the uppermost twenty feet of water, though it is com-
mon enough at depths between 30 and 40 feet, where
the temperature remains low and constant. The
average range of Daphne pulicaria is said to be deeper
than that of other Daphnias.

The gases of the water have much to do with the
distribution of animal planctonts, especially below the
thermocline, where the absence of oxygen from some
lakes during the summer stagnation period excludes




FIG. 185. Mal-
lomonas
ploessi.
(After Kent.)



3io Aquatic Societies



practically all entomostraca. Certain hardy species of
Cyclops and Chydorus appear to be least sensitive to
stagnation conditions. The insect Corethra, (fig. 183)
is remarkable for its ability to live in the depths, where
practically no free oxygen remains.

Age appears to be another factor in vertical distribu-
tion. On the basis of his studies of the Entomostraca
of Lake Mendota, Birge ('96) has formulated for them
a general law of distribution, to the effect that (i)
broods of young appear first in the upper waters of the
lake (quite near the surface) ; (2) increase of population
results in extension downward, and the mass becomes
most uniformly distributed at its maximum develop-
ment; (3) with decline of production there is relative
increase of numbers in the lower waters.

Perhaps this shifting downward merely corresponds
to the wane of vigor and progressive cessation of swim-
ming activities with advancing age.

In the case of many plants spore development or
encystment may follow upon a seasonal wave of produc-
tion, with a resulting change in vertical distribution.
Filamentous blue-green algae develop spores. The
ordinary vegetative filaments are buoyed up in part by
vacuoles within the cells, that lessen their specific
gravity ; but spores lack these. Hence the spore-bear-
ing filaments settle slowly to the bottom, and may be
found in numbers in the lower waters ere they have
reached their winter resting place. Dinobryon main-
tains itself at the surface in part by means of the lash-
ings of its flagella, but when its cells encyst, the flagella
stop, and the fragmenting colonies slowly settle. Thus,
both internal and external conditions have much to do
with vertical distribution. In general it may be said
that during their period of highest vegetative activity
all plants are necessarily confined to surface waters;
that most animals are closely associated with them,



Distribution in Depth 311

but that the constant fall of organic material toward
the bottom makes it possible for some animals to dwell
in the depths, if they can endure the low temperature
and the other conditions found there. There are some
animal planctonts, such as species of Cyclops and
Diaptomus, that range the water (oxygen being pre-
sent) from top to bottom. There are many that are
confined during periods of activity to the warmer
region above the thermocline. There are a few like
Leptodora that seem to prefer intermediate depths,
and there are a few
(Heterocope, Limno-
calanus, Mysis, etc.)
that dwell in the cold
water below the ther-
mocline.

Collectively, this
extraordinary assem-
blage of organisms
that we know as
plancton recalls in

, ,1 i- r r FIG. 1 86. Leptodora.

miniature the life of

the fields. It has, in its teeming ranks of minute
chlorophyl-bearing flagellates, diatoms and other algae,
a quick-growing, ever-present food supply that, like
the grasses and low herbage on the hills, is the mainstay
and dependence of its animal population. It has in
some of its larger algae the counterparts of the trees
that support more special foragers, are less completely
devoured, and that, through death and decomposition,
return directly to the water a much larger proportion
of their substance. It has in its smaller herbivorous
rotifers and entomostraca, the counterpart of the hordes
of rodents that infest the fields. It has in its large,
plant-eating Cladocerans, such as Daphne, the equiva-
lent of the herds of hoofed animals of the plains; and




312 Aquatic Societies

it has at least one great carnivore, that, like the tiger,
ranges the fields, selecting only the larger beasts for
slaughter. This is Leptodora (fig. 186). It is of phan-
tom-like transparency, and though large enough to be
conspicuous, only the pigment in its eye and the color
of the food it has devoured are readily seen. It ranges
the water with slow flappings of its great, wing -like
antennae. It can overtake and overpower such forms
as Cyclops and Daphne and it eats them by squeezing
out and sucking out the soft parts of the body, rejecting
the hard shell. Leptodora, in a small way, functions
in this society as do the fishes of the necton.

The total population of plancton in any lake is very
considerable. Kofoid ('03) reported the maximum
plancton production found by himself in Flag Lake near
Havana, 111., as 667 cubic centimeters per square meter
of surface: found by Ward ('95) in Lake Michigan,
176 do.; found by Juday ('97) in the shoal water of
Turkey Lake in Indiana, 1439 do. Kofoid estimated
the total run-off of plancton from the Illinois River as
above 67,000 cubic meters per year this the produc-
tion of the river, over and above what is consumed
by the organisms dwelling in it.

If we imagine the organisms of a lake to be pro-
jected downward in a layer on the bottom, this thick
layer would probably represent a quantity of life equal
to that produced by an average equal area of dry land.

There is hardly another ecological group of organisms
that lends itself so readily to quantitative studies, since
the entire fauna and flora of the plancton may be
gathered by merely straining or filtering the water.
All over the world, therefore, quantitative studies have
been made in every sort of lake and in many sorts of
streams. Extensive data have been gathered concern-
ing the distribution and numerical abundance of the



The Necton 313



planctonts ; but we still are sadly lacking in knowledge
of the conditions that make for their abundance.

II
THE NECTON

The large free swimming animals of the fresh waters
are all fishes. Indeed, as we have already noted (p.
233), but a few of the fishes range through the open
waters. Such are the white-fish, the ale-wife and the
ciscos, all plancton feeders, and a few more piratical
species, like the lake trout and the muskellonges that
feed mainly on smaller fishes.

Necton, it will thus be seen, is not a natural society.
It contains no producing class. It is sustained by the
plancton and by the products of the shores.

These fishes all have a splendid development of
stream-line form. They all swim superbly. And
according as they feed on plancton or on other fishes
they are equipped with plancton strainers or with
raptorial teeth. Excellent plancton strainers are those
of the lake fishes. They are composed of the close-set
gill-rakers on the front of the gill arches, and they
strain the water passing through. This mesh is adapt-
ed for straining the larger animal planctonts while let-
ting the lesser chlorophyl-bearing forms slip thru.
Thus the fishes reap the crop of animals that is ma-
tured, without destroying the sources for a crop to come.




LITTORAL
SOCIETIES



INDER the sheltering
influence of shores the
vascular plants may
grow. Animals elude
the eyes of their ene-
mies, not by becom-
ing transparent, but
by taking on colors and forms in resemblance to their
environment. They escape capture, not alone by fleet-
ness, but also by development of defensive armor, by
shelter-building and by burrowing.

Large and small and all intermediate sizes occur
together along shore, and those that appear betimes in
open water make shifts innumerable for place and
food and shelter for their young.

There are many factors affecting the grouping of
littoral organisms into natural associations, most of
them as yet but little studied ; but the most important
single factor is doubtless the water itself. The density
of this medium and the consequent momentum of its
masses when in motion so profoundly affect the form
and habits of organisms that they may be roughly
divided into two primary groups for which are sug-
gested the following names:

3H



Lenitic Societies 315

I . Lenitic* or still- water societie s .

II. Lotic\ or rapid-water societies, living in waves or
currents.



LENITIC
SOCIETIES



|OKED together, less by
any common character
of their own than by
the lack of lotic charac-
teristics, we include
under this group name
those associations of
littoral organisms that dwell in the more quiet places
and show no special adaptations for withstanding the
wash of waves or currents. Wherever we draw the
line between lenitic and lotic regions, there will be
organisms to transgress it, for hydrographic conditions
intergrade. We have already seen how many organ-
isms transgress the boundary between limnetic and
littoral regions. Just as in that case we found a fairly
satisfactory boundary where the increasing depth of still
water is such as to preclude the growth of the higher
plants, so here the boundary between lenitic and lotic
regions may be placed where the movement of the
water is sufficient to preclude the growth of these same
plants.




*Lenis = calm, placid.
\Lotus = washed.



316 Aquatic Societies



The reason why lenitic societies include practically
the entire population of vascular plants has already
been stated (p. 145) : the plants have a complexity of
organization that cannot withstand the stress of
rapidly moving waters. They fringe all shoals, how-
ever, and they fill the more sheltered places with growths
of extraordinary density. In such places they pro-
foundly affect the conditions of life for other organisms :
the supplies of food and light and air, and the oppor-
tunities for shelter.

Streams and still waters, inhabited by lenitic societies,
may be divided roughly into three categories :

1. Those that are permanent.

2. Those that dry up occasionally.

3. Those that are only occasionally supplied with
water.

These so completely intergrade, and so vary with
years of abundance or scarcity of rainfall, that
there is no good means of distinguishing between them.
Perhaps for the humid Eastern States and for bodies of
still water the words pond and pool and puddle convey
a sense of their relative permanence. The population
of the pond is, like that of the lake, to a large extent
perennially active. It will be discussed in succeeding
pages. That of the pool is composed of those forms
that are adjusted to drouth: forms that can forefend
themselves against the withdrawal of the water by
migration, by encystment, by dessication, or by bur-
rowing, or by sending roots down into the moisture of
the bed. Some of these will be mentioned in the dis-
cussion of the population of the marshes. The puddles
have a scanty population of forms that multiply rapidly
and have a brief life cycle. The synthetic forms
among them are mainly small flagellates and protococ-



Lenitic Societies



317




FIG. 187. Lick Brook near Ithaca in spring. Its bed runs
dry later in the season. (Photo by R. Matheson.)



318



Aquatic Societies



coid green algae. The herbivores are such short-lived
crustaceans as Chirocephalus (see fig. 90 on p. 184) and
Apus, which have long-keeping, drouth -resisting eggs;
such rotifers as Philodina, remarkable for its capacity for
resumption of activity after dessication ; such insects as
mosquitoes. The carnivores are such adult water-bugs
and beetles as may chance to fly into them.

Whether a population shall be able to maintain itself
depends on the continuance of favorable conditions, at
least through the period of activity of its members.
In these pages we shall give attention only to the life
of relatively permanent waters.

Plants The shoreward distribution of plants in
natural associations is determined mainly by two
hydrographic factors: (i) movement and (2) depth of
the water. It is directly related to exposure to waves
and to currents. Everyone knows the difference in
appearance between plants growing immersed in a quiet




FIG. 1 88. The forefront of the Canoga marshes, where partly sheltered
from the waves of Cayuga Lake, clumps of the lake bulrush lead the
advance of the shore vegetation.



Lenitic Plants 319



pool and those growing on a wave- washed shore. The
former appear as if robed in filmy mantles of green, full-
fledged with leaves, and luxuriant. The latter appear
as if stripped for action, unbranched, slender and bare.
At one extreme are the finely-branched free-floating
bladderworts (see fig. 173 on p. 285) at the other are
such firmly rooted, slender, naked, pliant-topped forms
as the lake bulrush (figure 188) and eel-grass. These
latter anchor their bodies firmly and closely to the soil,
and send up into the moving waters overhead only soft
and pliant vegetative parts, that offer the lea^t possible
resistance to the movement of the water, and that, if
broken, are easily replaced. The long cylindric shoots
of the bulrush have their vessels lodged in the axis and
surrounded with a remarkable padding of air cushions.
They are not easily injured. The flat ribbon-like
leaves of eel-grass are marvels of adjustability to waves.

Between these two extremes are all gradations of
form and of fitness. Of the pool-inhabiting type are
the water crow-foot, the water milfoil, the water horn-
wort; of the opposite type are the long-leaved pond-
weeds and the pipeworts. Intermediate are the broader-
leaved pondweeds and Philotria.

These sometimes are found in running streams, but
they usually grow in the beds in dense mutually sup-
porting masses that deflect the current. If one place a
current meter among their tops he will find little move-
ment of the water there.

There is another place of security from waves, for
such plants as can endure the conditions there. It is
on the lake's bed, below the level of surface disturbance.
The stoneworts (see fig. 55 on p. 137) are branched and
brittle forms, very ill adapted to wave exposure, and
most of them live in pools, but a few have found this
place of security beneath the waves. There are
extensive beds of Chara on the bottom of our great



320



Aquatic Societies




FIG. 189. Shore-line vegetation.



Shore Plants 321



lakes, at a depth of 25 feet more or less, and within the
range of effective light. Associated with these, but
usually on the shoreward side, are beds of pond weeds.
Often there are bare wave-swept shores behind these
beds with no sign of aquatic vegetation that one can
see from the shore.

Depth of water determines the adjustment of aquatic
seed plants in three principal categories:

1. Emergent aquatics. These occupy the shallow
water, standing erect in it with their tops in the air,
and are most like land plants. They are by far the
most numerous in species.

2. Surface aquatics. These grow in deeper water,
at the front of (and oftentimes commingling with) the
preceding. The larger ones, such as the water lilies
are rooted in the mud of the bottom, and bear great
leaves that float upon the surface. The smaller ones
such as the duckweeds (see figs. 61 and 62, p. 149) are
free-floating.

3. Submerged aquatics. These form the outermost
belt or zone of herbage. They are most truly aquatic
in habits. Except for such forms as dwell in quiet
waters, they are rooted to the bottom. Depth varies
considerably within this zone. It extends from the
outer limits of the preceding (hardly more than five

FIG. 189.

A. Branches of four submerged water plants: (i) Philotria, (2) Cerato-
phyllum, (3) Ranunculus, (4) Nais.

B. Emergent aquatics, including a clump of arrow arum; two of the
pendulous club-shaped fruit-clusters are seen at (5) dipping into the water.

C. Zonal arrangement of the plants of the shore-line. The background
zone is cat-tail flag (Typha). Next comes a zone of pickerel- weed (Ponte-
deria) in full flower. Next, a zone of water lilies and such other aquatics
with floating leaves as are shown in D and E. In the foreground is a zone of
submerged plants a mixture of such forms as are shown in A above.

D. A closer view: Lemna, free-floating and Marsilia with four parted
floating leaves, and Ranunculus, in tufted sprays, submerged.

E. The floating leaves and emergent flower spikes of a pond weed,
Pctamogeton. (Photo by L. S. Hawkins.)



322 Aquatic Societies



feet at most) to the limits of effective light. Within
such a range of depth conditions of movement, pressure,
warmth and light find also a considerable range; hence,
the forms differ at the inner and outer margins of the
zone. Its forefront is usually formed by Chara as
stated above, and pondweeds follow Chara, with a
number of other forms usually commingled, in the
shallower part.

These groups are not free from intergradation since
some forms like the spatterdock (fig. 195 on p. 335) are
in part emergent, and some of the pondweeds have a
few floating leaves. But they are nevertheless con-
venient, and they represent real ecological differences.

Distribution of these plants in depth results in their
zonal arrangement about the shore line. When all
are present they are arranged in the order indicated.
It is an inviolable order; for the emergent forms cut off
the light from those that cannot rise above the surface,
and the latter overshadow those that are submerged.
The zones may vary in width and in their component
species, but when all are present and crowded for room
they can occur only in this order. The two accompany-
ing figures illustrate zonal arrangement ; figure 1 890, on
a low and marshy shore; figure 190, on a more elevated
shore, backed by a terrestrial flora.

The algce of littoral societies are those of the plancton
(practically all of which drift into the shoals) plus
numberless additional non-limnetic forms, many of
which are sessile. As with the vascular plants, algae
that are fragile (see fig. 198 on p. 338) and the larger
that float free (Spirogyra, etc.) develop mainly in pools
and quiet waters, while those having great pliancy of
body (Cladophora, see fig. 46 on p. 125) and protective
covering (slime-coat diatoms, etc.) are more exposed to
moving waters.



Zonal, Arrangement of Plants



323




FIG. 190. Zonal arrangement of plants. At the front is a zone of Marsilea
extending down into the water. Next is a zone of bur-reed, with the
spiny seed-heads showing near the center of the picture. Back of this
is a zone of tall composites. The flower clusters of the joe-pye-weed
show above the bur-reed tops. In the background is a zone of trees.



324



Aquatic Societies



The animal population of the shores is likewise dis-
tributed largely in relation to water movement, or to
conditions resulting therefrom. There is a zonal
arrangement of animal life along shores that is only a
little less definite than that of plants. It is much less
obvious, for plants are fixed in position and come out
more into the open and into view. Nevertheless, even
the most free-roving animals, the fishes, as we have
already seen (p. 233), keep in the main to certain shore-
ward limits.

Distribution in relation
to depth and to character
of bottom comes out
clearly in Headlee's stud-
ies of the mussels of Win-
ona Lake. In that lake
the play of the waves on
shore yields a clean beach
line of sand and gravel,
and sifts the finer mater-
ials into deeper water.
The succession is gravel
and sand, marly [sand,
sandy marl, coarse white
marl, marly mud and very
soft black mud. The last
named, beginning at a
depth of some 20 feet,
covers a very large central portion of the lake bottom.
Mussels cannot live in it for they sink too deeply
and the fine sediment clogs their gills. Hence the
mussels are restricted to the strip along shore. With-
in this strip they are arranged according to hard-
ness of bottom and exposure to waves. The accom-
panying diagram illustrates the distribution of four of
the common species. The two Anodontas, having



CRAWL,


MARL


\

MUD


Sf(HD









10


20 rr


3


2'?'










*J *


* >


4








)
3


2'

2 /


B



FIG. 191. Diagram of distribution of
mussels in Winona Lake, Indiana

A, outline of lake with the mussel zone
stippled and marked out by two ten-foot
contours.

B, shows the relation of four of the common-
est species to depth and character of bottom:

1. Anodonta edentula. 3. Unio rubigniosa.

2. Anodonta grandis. 4. Lampsilis luteolus.



Plancton Animals 325

lighter shells less prone to sink, live in the deeper zone
of mixed marl and mud, and so are able to forage farther
out on the bottom. On account of their thinner shells
they are excluded from residence near the shore line,
where the waves would crush them. The heavier
shelled Unio requires a more solid bottom for its sup-
port, and is uninjured by the beating of heavy waves.
Hence, its shoreward distribution. Lampsilis, however,
is a more freely ranging form, having a rather light shell.
It overspreads the range of all the others, coming in the
less exposed places rather close to shore.

Plancton animals The animals of the shoreward
plancton are less transparent than those of the lake.
They are also far more numerous. They show more
color. The color is often related to situation. In
small ponds and marshes they are darker as a rule than
in large ponds. They include forms of very diverse
habits among which are the following:

1. Forms that swim freely and continuously in the
more open places. These only are common to both
littoral and limnetic regions.

2. Forms that are free swimming, but that rest
betimes on plants; Cladocerans with adherent "neck
organs" ; Copepods with hooked antennae, etc.

3. Forms that can and that do swim betimes, but
that more habitually creep on plants; many ostracods,
copepods and rotifers.

4. Forms that live on or burrow in the slime that
covers stems or other solid supports, and that swim
but poorly and but rarely in the open water; Leeches
and oligochete worms, rhizopods and midge larvae.

5. Sessile forms that cannot swim, but that become
detached and drift about passively in the open water,
at certain seasons; hydras, statoblasts of fresh-water
sponges and of bryozoans, resting eggs of rotifers and of
cladocerans, etc.



326 Aquatic Societies



Few of these can thrive in the waters of the limnetic
region of a lake ; but there is at least one member of the
first group that takes advantage of an abundant supply
of food in lake waters, migrates out, and develops
enormously, overshadowing in numbers sometimes the
truly limnetic forms. It is Chydorus sphcericus. It is
rather a littoral than a limnetic species, yet it often
abounds in the open lakes, following a rich development
there of blue-green algae suitable for its food.

SPATIAL RELATIONS

A large part of the animal life of the littoral region is
disposed in relation to upper and lower surfaces of the
water. This grouping by levels is due to gravity.
Where the air rests upon the water, making available
an unlimited supply of oxygen, there at the surface are
aggregated forms that require free air for breathing.
Where the water rests upon the solid earth, there at the
bottom are the forms that hide or burrow in the ground.


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Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 18 of 26)