Saturday, July 22, 2017

Freshwater Ecology: Ponds and Mudflats (Placenta of Terrestrial Life)

Dr Abe V. Rotor
Living with Nature School on Blog 

  Hut by a Pond 
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 pond 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.

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 population and 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 Raw materials are withdrawn from the environment in large quantities, and metabolic wastes are returned together with dead organisms, but of another form and in different place, thus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.
mudflat picturesThis 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.

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.

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 are an envy of 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 on the other 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.

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 homogenous 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?

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.~

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