Freshwater Ecology: Ponds and Mudflats - Placenta of Terrestrial Life
Dr Abe V Rotor
Living with Nature School on Blog
Paaralang Bayan sa Himpapawid with Ms Melly C Tenorio
738 DZRB AM 8 to 9 Evening Class, Monday to Friday
A pond is a transient environment.
Unlike a stream, river, or lake, it has feeble currents or none at all. It is
surrounded by thick vegetation which advances towards the center as it grows
older. As the pond fills up with sediments and muck, and its bottom gradually
drains, higher plants become progressively abundant.
In a shallow pond the forces of wind
and convection
keep the whole volume of water in circulation so that at any depth the
temperature is fairly uniform and the amount of gases, notably oxygen and
carbon dioxide is equally distributed.
"Stream in monsoon, pond in summer" detail of a wall mural by AVR 2010
The
relatively large ratio of surface to volume of ponds make them most susceptible
to weather and climatic changes than large bodies of water. Because of their
small size they are also susceptible to changes in physiographic conditions
like erosion and deposition.
Like
any community a pond grows, passes a relatively stable mature phase, and
ultimately dies. This basic ecological cycle is a result of interplay between
organisms and their environment. Organisms live in an environment where they
are adapted, and remain in the most stable area or niche which spells out their
success as a population and as members of an interacting ecosystem.
The
physical nature of the environment consequently determines what types of
organisms can settle successfully. Temperature, rainfall, altitude, soil
conditions and other environmental factors decisively influence the kinds of
plants that survive in a given place. Vegetation in turn, as well as the
animals, has selected effects on the kind of biotic community in that region. Organisms
gradually alter the local conditions. Raw materials are withdrawn from the
environment in large quantities, and metabolic wastes are returned together
with dead organisms, but of another form and in different place, thus resulting
to re-distribution and alteration of vast quantities of substances.
This
means that later generations of the original organisms may find the altered
local environment no longer suitable for themselves so that the members of the
community must resettle elsewhere or die out. Later a new community of
different plants and animals arrive and settle down. Again this new community
will alter the area according to its own specialization. Hence, it is said that
the living and non-living parts of the environment are vitally interlinked,
that changed in one produces change to the other.
Lotus (Nelumbium nelumbo) is common in tropical ponds
As a typical ecosystem, a pond relates
a classical story. Most ponds must have originated during the last ice age when
the moving glaciers scraped out giant sinks. Others have been known to originate
from a portion of a bay or lake that was isolated by a sandbar by the action of
waves and wind. Pirated rivers may also form into ponds. Most of the newly
formed ponds may be wiped out days, months or years later, by storm or silt
deposition. But a better-protected pond survives the drastic geologic fate. It
must somehow face the slow process of ecological succession through which
continuous dynamic processes take place that will ultimately lead to the
accumulation of organic matter and silt.
On
the functional aspect of ecological succession, like in any transient communities,
the progressive increase of organic matter which fills up the pond will lead
into a heterotrophic conditions which means that the dependent organisms
(heterotrophs) will increase in proportion to the increase of the producers
(autotrophs). These favor aquatic and semi-terrestrial organisms, and
therefore, biological diversity.
The living bed of terrestrial life is the
fertile bottom of the pond - the mudflat, which intermittently comes out to dry,
a cycle that incubates eggs of many organisms, allows spores and seeds to germinate,
and dormant organisms to become active.
The mudflats are exposed and submerged
at intervals depending upon the amount of water that enters the pond from the tributaries
upstream and from the surrounding watershed. As the remaining aquatic zone
further shrinks and the water flow meanders along the bottom, wider mudflats
are formed.
Closeup of a pond showing vegetation of Nymphaea lilies and cattails, AVR
No zone in the pond is richer in variety
and in number of living things, and no types of interrelationships could be
more complex, if not deceiving or unknown, than the aquatic zone where life
continues on in some most amazing and mystic ways. There are evidences that
these dynamic changes shall go on until the pond has completely transformed
into a terrestrial ecosystem, despite such threat of pollution which may have
already marked the face of the pond.
But nature proves flexible with
change. Normal changes would simply be dismissed by Nature’s own way of
adjusting the role of its own creatures. Changes shape the conditions of the
environment; that in turn, determine the organisms that fit better into it.
The
bottom of the pond is directly affected by the amount of water and by water
flow. It is the recipient of silt and other sediments from plant residues from
the surrounding watersheds and from the immediate shoulders of the pond. The
decreasing area occupied by water may indicate the age of the pond, and the
changes which, undoubtedly lead towards an irreversible transition from aquatic
to terrestrial state.
Typical
of old ponds and lakes, the aquatic zone considerably decreases with the lack
of water supply and by the steady deposition of silt and decomposing plant
remains- not to mention the garbage and other wastes thrown into the pond by
unscrupulous residents in the area. The black, spongy and fertile deposits are favorable to many plant species and consequently of the dependent animal organisms. From
time to time pioneer plants venture for a try to settle every time terrestrial
conditions begin to prevail.
But in many parts of the old exposed bottom left
by the receding water, terrestrial plants can not settle down because time and
again the water immediately submerges the previously baked flats to become once
more a slosh of mud that readily shallows a wader to his knees. And so the
outcome of the battle turns to the advantage of the aquatic plants- Eichhhornia (water hyacinth), Alternanthera, Jussiaea,
Nymphaea and Pistia
(kiapo) and of course to the ever-present thick scums of blue-greens and green
algae with their co-dependents. Ipomea,(kangkong), the adventuresome Brachiaria (para grass) and other grasses. Other plants hand are pushed back to safer limits where they wait for conditions to favor
another invasion, that is when the mudflats shall come out to the sun again.
The
story of competition between the two groups continues indefinitely and all the
while the sluggish water meanders against the shoulders of the pond and etches
the old bottom. But all along, sediments pile on
the bottom until small isolated “islands” are formed in the middle of the water
zone. The isolation of these islands can not be for long, so their barrenness,
for the dormant seeds under the warm rich soil suddenly come to life and
together with air borne seeds and spores, and the stranded shoots and tillers,
which make these islands “small worlds” themselves.Cyperus is a common pond plant. It is a relative of the Papyrus, the source of the first paper.
No
place in the aquatic zone is absolutely for a particular species. However the
dominance of a species can be noted from one place to another. For example, the
pseudo-islands in the middle of the aquatic zone may be dominated by Brachiaria, while the lower part
of the pond where water is usually deeper, harbors the remnants of the once
dominant Eichhornia. At the
headend, the old bottom may be covered up with grass, except in places that may
be occupied by Jussiaea repens, a succulent broad-leaf and a
water-loving species.
Any
decrease in area of the true aquatic zone a corresponding increase of the
immediate zone. Terrestrial plant species continuously pursue the reclaimed
flats. Ipomea and Alternanthera species appear at
the front line of the invasion while the grasses stand by. The logic is that
the former can better withstand the conditions of the waterline. Their roots
bind the particles of silt and humus, which are suspended in the water, and
when the plants die, organic matter is added, thus favoring the terrestrial
species take over. It is as if these benefactors are robbed at the end by their
own beneficiaries.
The
aquatic and shore zones are more or less homogeneous as far as their principal
plant species are concerned. This could be explained by the fact that the newly
established zone (aquatic zone invaded by plants) is but an extension of the
shore zone, and was it not that the shore zone a part of the aquatic zone?
Mountain Pond, on-the-spot painting by AVR;
Pristine Pond, painting by Anna Rotor-Sta Maria
Hence,
the close relationship of the two zones can be readily noted, although they can
be divided by alterne. This demarcation
is not steady as shore vegetation spreads out into the water zone.
The
phytoplanktons composed of countless green algae, flagellates, diatoms, desmids
and a multitude of bacteria are the precursors of the food pyramid. They form the
broad base of a pyramid structure. Simplified, the phytoplanktons make up the
larger group, on which the zooplanktons depend.
Insects and other arthropods lead the third group of organisms, while
amphibians fish and reptiles make up the fourth. The farthest link is made up
of the decomposers, which ultimately produce organic matter and humus upon which phytoplanktons and plants
depend live on. The food chain web is characterized by mutualism, parasitism,
predatism, saprophytism, commensalism, and decomposition – all of which link all
organisms into a greater whole, the ecosystem.
In
the pond, the rooted as well as the floating plants and the phytoplanktons are
the “producers”. They support the herbivores (insects and fishes), and they add
organic matter when parts or the whole of their bodies die. Zooplanktons
generally feed upon the phytoplanktons, although some are dependent upon
organic matter and humus. Small fishes, crustaceans and insects eat the
zooplanktons in turn,, and these will be eventually eaten by carnivores. If not
eaten, every plant and animal eventually die and decompose, its protoplasm
reduced to the basic materials that green plants needed for growth.
The
shores progressively widen following the drying of the mudflats. This area is
usually dominated by grass, followed by crawling and viny plants, such as those
belonging to the morning glory family (Convulvolaceae). Shrubs on the farther
edge of the pond join annuals. During the rainy season the shores are
waterlogged. The soil is black and it emits methane and ammonia gases, which
show that anaerobic decomposition is taking, place. Muck is the product of this
slow process. The soil is rather acidic but many plants tolerate it. High
ferrous content can also be noted as rusty coloration, a characteristic of
waterlogged soil.
Towards
the end the shore becomes dry. Vegetation changes follow a dynamic pattern, the
grass producing numerous secondary stalks, which become thick and bushy. The
broad-loaf species tend to grow in clumps or masses. Some plants in the slope
zones descend to join some plants in the shore zone, some are forced into prostate
growth. Along the water line the grass is tall and verdant green. Meantime the
trees close in. The tree line advances
to the edge of the pond a soon the pond will die.
Old Pond, painting in oil by AVR
The Dying Pond
"Death be not proud," this dreaded fate defied;
In death something rises at its side,
As on a dying pond, a swamp in its place
Grows, dying in peace and grace.
And the watery grave dries into grassland
Where roam the hooves and paws in band;
And the winged sweep the air retreating
On the trees nearby and advancing.
One by one the trees come when the wind blows,
They ride on furs, beaks and claws;
A woodland soon rises from the trees' breath
And hides the pond, the grass and death. ~
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