Essay on Lentic Ecosystem (2629 Words)

Here is your essay on Lentic Ecosystem !

Lentic ecosystems include all standing fresh water habitats such as lakes, ponds, marshes, swamps, bogs, meadows, etc. Some of the important lentic ecosystems or wetlands have been tabulated in table 20.1.

Lakes and Ponds:

Lakes are inland depressions containing standing water. They may vary in size from small ponds of less than acre to large seas covering thousands of square miles. They may range in depth from a few feet to over 5,000 feet.

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Ponds, however, are considered as small bodies of standing water so shallow that rooted plants can grow over most of the bottom. Most ponds and lakes have outlet streams and both are more or less temporary features on the land­scape because filling no matter how slow, is inevitable.

The aquatic habitats of lake and pond remain vertically strati­fied in relation to light intensity, wave length absorption, hydrosta­tic pressure, temperature, etc. In a lake, for example, there are three to five well recognized horizontal strata namely: (i) Shallow water near the shore forms the littoral zone.

It contains upper warm and oxygen rich circulating water layer which is called emption. The littoral zone includes rooted vegetation, (ii) Sublittora zone-extends from rooted vegetation to the non-circulating cold water with poor oxygen zone, i.e., hypolimnion. (iii) Limnetic zone is the open water zone away from the shore. It is the zone upto

Table 201. Classification of wetland or lentic ecosystems (excluding lakes and ponds) (Smith. 1974):

the depth of effective light penetration where rate of photosynthesis is equal to the rate of respiration, (iv) Profundal zone is the deep- water area beneath limnetic zone and beyond the depth of effective light penetration, (v) Abyssal zone is found only in deep lakes, since it begins at about 2,000 meters from the surface. Ponds have little vertical stratification. In them littoral zone is larger than the limne­tic zone and profundal zone. In a small pond the limnetic and profundal zones are not found. Lakes thus differ ponds in having relatively larger limnetic zone and profundal zone than littoral zone.

Fig. 20.2 Different zone of a deep freshwater lake

In fact, the warm top layer, the epilimnion is heated by the sun and hamogenised by the wind and other currents, while, the deep cold layer; the hypolimnion is not heated by sun and not cir­culated by wind (Fig. 20 4). The transition between the two is the metalimnion or thermo cline.

The basis upon which the layers are maintained is strictly thermal and is caused by the fact that the warmer water is lighter than the colder water. Once a thermo cline is formed, no exchange of water occurs between the epilmnion and hypolimnion.

Further, lentic water of a lake or pond is also classified on the basis of the depth of light penetration enabling photosynthesis into trophogenic zone (includes littoral plus sublittoral zones), and is tropholytic zone (upper part of profundal zone).

The former is often distinguished by abundant plant growth and dependent fauna, while the latter denotes a general absence of vegetation and har­bours mostly saprobes. In between the two zones is the compensa­tion level which forms a boundary between two zones (Fig. 20 3). It exibits perfect equilibrium between respiration and photosyn­thesis.

Fig. 20.4. Patterns of wind-induced circulation in hypolimnion and epilimnion of a thermally stratified lake. Circulations remain greater in epilimnion than in hypoli­mnion (after Clapham, Jr., 1973).

Physicochemical properties of lakes and ponds:

Lakes have the tendency to become thermally stratified during sum­mer and winter to undergo definite seasonal periodicity in depth distribution of heat and oxygen. Light too penetrates only to a cer­tain depth, depending upon turbidity. These gradations of oxygen, light and temperature profoundly influence life in the lake, its distri­bution and adaptation.

Kinds of lakes:

Different classifications of lakes on the basis of physical factors, productivity, etc, exist. Based on temperature Hutchinson (1957) classified lakes into dimictic, monomictic and polymictic lakes. The dimictic lakes exhibit two over­turns every year while monomictic lakes present only a single overturn per year.

The monomictic lakes may be cold monomictic and warm monomictic, the former being characterized by a circu­lation only during summer, while the latter has a complete circu­lation in winter as well.

Polymictic present circulations throughout the year based on the humic acid contents the lakes of world have been classified into clear water lakes and brown water lakes; brown water contains high humus content.

Clear water lakes may be divided into two types—the type, whose water is poor in nutritive plant material and shows nearly equal distribution of oxygen during summer and winter months and whose mud bottom contains little organic material ; and the entropic type, which is rich in nutrients.

At greater depth below the thermo cline in summer Atrophic lakes show a considerable reduction in oxygen content and their mud bottom is composed of typical muck. In ponds stratification is of minor significance.

Origin of lakes and ponds:

Lakes and ponds arise in many ways. For example, North American lakes were formed by glacial erosions and deposition and a combination of the two. Glacial abrasions of slopes in high mountain valleys engraved basins, which filled with water from rain and melting snow to pro­duce tarns.

Retreating valley glaciers left behind crescent-shaped ridges of rock debris, which dammed up water behind them. Lakes are also formed by the deposition silt, driftwood, and other debris in the beds of slow-flowing streams. Craters of extinct volcanoes may fill with water and landslides can block off streams and valleys to form new lakes and ponds.

Further man intention­ally creates artificial lakes by damming rivers and streams for power, irrigation, and water storage, or by constructing small ponds and marshes for water, and wild life. Manmade lakes are often called impoundments. They may be a ‘close’ type or ‘open’ type depending on the discharge of water into it.

Sandy- nulla is a closed impoundment, while Pykara is an open impound­ment (Srecnivasan, 1957). Impoundments differ from natural lakes in thermal and oxygen variations and are often characterized by a low percentage of bottom dwellers or benthos.

Ponds occur in most regions of adequate rainfall. Temporary ponds, which are often shallow and quick drying, contain a charac­teristic Lentic environment. Permanent ponds contain water throughout the year, while flood-plain ponds are formed when a stream shifts its position, leaving the formed bed isolated as a body of standing water.

Biotic Communities of Lakes and Ponds:

Different organisms of the lentic environment can be ecologi­cally classified based on whether they are dependent on the subs­tratum or free from it. Organisms depending on the substratum are called pedonic forms and those that are free from it are the limonitic forms. Further, the aquatic organisms may also be classified into following groups depending upon their sizes and habits:

1. Neuston:

These are forms which are found in all aquatic ecosystems at the air-water interface. They may include floating plants such as duckweed, as well as many types of animals. Ani­mals that spend their lives on top of the air-water interface, such as water striders, are termed epineuston, while others, including insects such as diving beetles and back swimmers, which spend most of their time on the underside of the air-water interface and obtain much of their food from within water, are termed hyponeuston.

2. Plankton:

These are forms which are found in all aquatic ecosystems except for fast-moving rivers. They are small plants and animals whose powers of self-locomotion are so limited that they cannot overcome currents in their ecosystems. Most plankton (phytoplankton’s and zooplanktons) can move a bit, however, either, to control their vertical distribution or to seize prey.

Certain ani­mal planktons or zooplanktons are exceedingly active and move relatively great distances considering their small size, but they are so small that their range is still controlled largely by currents and such planktons are also known as nektoplankton.

3. Nekton:

Nektonic animals are swimmers and are found in all aquatic systems-except for the fastest-moving rivers. In order to overcome currents, these animals are relatively large and power­ful; they range in size from the swimming insects of quiet water, which may be only about 2 mm long, to the largest animal that has ever lived on earth, the blue whale.

4. Benthos:

The benthos includes the organisms living at the bottom of the water mass. They occur virtually in all aquatic ecosystems. The benthos organisms living above the sediment-water interface are termed benthic epifauna and those living in the sedi­ment itself are termed infauna.

Neustons of Lentic aquatic environment:

The water surface of lake or pond contains certain free-floating hydrophytes such as Wolffia, Lemna, Spirodella, Azolla, Salvinia, Pistia, and Eichhornia. These plants remain in contact with water and air, hut no soil.

Biota of littoral zone:

Lentic aquatic life is most prolific in the littoral zone. The littoral zone of a lake remains rich in pedonic flora especially upto the depth to which effective high penetration is possible facilitating the growth of rooted vege­tation.

At the shore proper is the emergent vegetation which remains firmly rooted in the shore substratum but their tops with ‘”in chlorophyll bearing regions are exposed. Certain rooted cogent plant species of littoral zone are Ranunculus, Monochoria; yyperus and Rumex.

Interspersed with these plants are the cattails), bulrushes (Scirpus), arrowheads (Sagitaria) and pickerel- weeds. Out slightly deeper are the rooted plants with floating, eaves such as the water lilies lymphoma, Nelumbo, Aponogeton, Trapa, Marsilea, and Potamagetons.

Still deeper in the littoral zone of lake are the fragile thin-stemmed water weeds, rooted but totally submerged. Such submerged vegetation includes plants like Elodea, Vcllisneria, Myriophyllum, Isoetes, Hydrilla, Chara, Potamogeton, etc.

The phytoplankton of littoral zone of lake is chiefly composed of diatoms (e.g., Navicula, Cyclotella), blue green algae (e.g., Microcystis, Oscillatcria), green algae (e.g., Cosmarium, S aurastrum) and holophytic flagellates. Sreenivasan (1970) has observed the abundance of marine dinophycean flagellate Ceratium hirudinella in some impoundments of India.

Fig. 20.5. Flora of the lentic habitat (after Smith, 1977).

The littoral zone also contains great concentration of animals, which remain distributed in recognizable communities. In or on the bottom are various dragonfly nymphs, crayfish, isopods, worms, snails and clams. Other animals live in or on plants and other objects projecting up from the bottom. These include protozoans like Vorticellth.

Stentor; larvae of Dytiscus, Laccotrophes, leech- like Glossnphnnia, climbing dragonfly, damsel fly nymphs, rotifers (Rrachiunus), flat worms, Bryozoa, hydra, snails (Lymnaea) and other.

With the exception of a few rotifers like Keratella, Brachi- onus, Diu’rella and Trichocerca and crustaceans like Daphnia and Moina, most freshwater animals of lentic habitat depend in one way or other on the aquatic vegetation. Larvae of Chironomus are found to inhabit the interior of rotting vegetation- Plananans are found underneath the leaves of floating plants.

The phytoplankton of littoral zone of lake is chiefly composed of diatoms (e.g., Navicula, Cyclotella), blue green algae (e.g., Microcystis, Oscillatcria), green algae (e.g., Cosmarium, S auras- trum) and holophytic flagellates. Sreenivasan (1970) has observed the abundance of marine dinophycean flagellate Ceratium hirudinella in some impoundments of India.

Zooplankton of littoral zone consists of water fleas such as Daphnia, rotifers and ostracods. The free swimming fauna (Nekton) includes Paramecium, Euglena, Rbnaira, Corixa, Dytiscus, larvae of Culex and of Chaoborus, Gerris, Gyrinus, etc. Among the verte­brates are frogs, salamanders, snakes and turtles are the nektons of littoral zone.

Floating members of the community (Neuston) are whirliging beetles, water spiders and numerous Protozoa. Many pond fishes such as sun-fish, top minnows, bass, pike and gar spend much of their time in the littoral zone.

Biota of limnetic zone:

Limnetic zone is the region of rapid variation, with the water level, temperature and oxygen content varying from time to time. In certain Himalayan lakes which have a glacial origin, change in water level is a conspicuous phenomenon, the onset of summer bringing forth a rapid and sudden rise in water level.

Fig. 20.6. Fauna of littoral zone of lentic habitat (after Odum, 1971).

Various protozoans which are capable of encystment during adverse ecological conditions, tardigrades like Macrobiotus, and rotifers like Rotaria, Philodina, copepods, snails and frogs occupy the limnetic zone. Sedentary and slow moving forms are excluded from this zone because of predators and lack­ing of permanent substratum for attachment. Many microscopic plants (Volvox, Euglena) and fishes also occupy this zone. The limnetic zone has autotrophs in abundance.

Biota of profundal zone:

The deep (profundal) life consists of bacteria, fungi, clams, blood worms (larvae of midges), annelids and other small animals capable of surviving in a region of little light and low oxygen. In the profundal zone, autotrophy cannot produce food, and the main source of energy is detritus that rains out of the limnetic zone.

All the organisms of this zone are heterotrophs either as detritus feeder or as carnivores. Most of our largest lake fishes inhabit the dark waters of hypolimnion most of their lives. In addition to detritus from the limnetic zone food washed into lakes by rivers can settle out and serve as food for profundal animals.

Benthos of Lake Bottom:

The lake bottom in young lakes may be of the original rock; in older lakes, it would have been covered with sediment to form a uniform substrate of mud or sand. The benthos community includes several species of insect larvae, including those of small mosquito like midges, burrowing mayflies, clams snails, and tubeworms.

Macrobenthic fauna of Loni reservoir of Rewa district of Madhya Pradesh (India) was found to include annelids like oligochaetes (Aulodrilus pluriseta and Branchiura sowerbyi) and Hirudinea (Hirudinaria sp.) ; molluscs such as bivalves (Pisidium clarkeannum, Parreysia favidens, P. corrugata, Indonaia caerulea and Laniellidens corrianus) and gastro­pods (Melanuides tuberculatus, Viviparus bengalensis, Gyraulus sp., Lymnea sp., and Indoplanorbis exustus); crustaceans (Prawn Macro- brachium lamarrei); insect larvae of Diptera (Chaoborus sp., and Chironomus sp.), Odonata, Trichoptera and Plectoptera, and a few fresh-water sponges (Gupta, 1976).