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Biota of Lentic Habitat
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 limnetic forms. Further, the aquatic organisms may also be classified into the following groups depending upon their sizes and habits.
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Neuston These are unattached organisms that live at the air-water interface. They may consist of microbial forms, plants and animals. Animals and microbial forms that spend their lives on top of the air-water interface, such as water striders are termed epineuston while others including insects such as beetles 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
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Plankton These are forms which are found in all aquatic ecosystems except fast moving rivers. They are small plants and microbial forms whose powers of self-locomotion are so limited that they cannot overcome currents. Thus their distribution is controlled largely by the currents in their ecosystems. The phytoplanktons and zooplanktons can move a bit, either to control their vertical distribution or to seize prey. Certain 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 plank tons are also called nektoplankton.
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Nekton Nektonic animals are swimmers and are found in all aquatic systems except for fast moving rivers. In order to overcome currents, these animals are relatively large and powerful.
Benthos The benthos include 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.
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The water surface of a lake or pond contains certain free floating hydrophytes such as Azolla, Lemma, Wolffa, etc. The microbial community lives associated with the surface film. The habitat is unusual in many ways since it is subjected to rapid temperature fluctuations, increasing to high light intensity and is very well aerated. Organic matter especially particulate and inorganic nutrients such as phosphates, accumulate on the surface of the film. The habitat is favourable for photoautotrophs. There are increased numbers of bacteria especially Pseudomonas and Caulobacter
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Neuston is a stressed habitat and microbial growth rate is low. Bacteria in neuston have hydrophobic surfaces and produce extracellular polysaccharides both of which are concerned with adsorption on the surface film.
Bubbles arising through the neuston layer and bursting play a major role in water-to-air transfer of bacteria and viruses. Characteristic autochthonous neuston microbiota include algae, fungi and protozoa. Cyanobacterial species include Aphanizomenon, Anabaena, Microcytis. Filamentous fungus include Cladosporium. Yeasts are also found. Among protozoan species include Diffugia, Arcella, Acinata, etc.
Aquatic life is most prolific in the littoral zone. The littoral zone of a lake
remains rich in pedonic flora especially up to the depth to which effective light penetration is possible facilitating the growth of rooted vegetation. Microorganisms exhibit different absorption spectra determining which
wavelength of light can be utilised for photosynthesis. Green and purple
sulphur bacteria grow at the sediment-water interface below the layers of
short wavelength absorbing algal and cyanobacterial growth by utilising wavelength of light not absorbed by the overlying phytoplankton because the purple and green sulphur bacteria obtain electrons from hydrogen sulphide at lower energy loss than water splitting photoautotrophs and thus require lower light intensities for carrying out photosynthesis. Conditions in the euphotic zone (area of light penetration) are favourable for the growth of photoautotrophs.
The bottom of the lake (benthos) represents the interface between hydrosphere and lithosphere. Sedimented organisms in the profundal zone are largely secondary producers and are dependent on the transport of organic compounds from the overlying zone. Particulate nutrients sedimented by gravitational forces concentrate on the surface of the sediment. Growth occurs on the sand grains (sediment) if they are relatively undisturbed by currents and wave action.
Flora consists of bacteria and small diatoms (Fragilaria, Opephora). There are also some motile organisms like diatoms (Nitzhia), cyanobacteria and bacteria. In a fresh water lake, mud found at the sediment, the acidity, water type and the nutrient status controls the microorganisms found there. All muds have diatoms as the major group of microflora. Bacteria are more on the surface and anaerobes are found for some depth in the mud. Oxygen can diffuse only very slowly in the waterfilled pore spaces of sediments. Concentration of inorganic nutrients (nitrogen and phosphorus) are important in determining the ability of the habitat to support microbial growth and metabolism. Salt concentration of water and pH also influence the characteristic autochthonous microbes of some lakes with high salt concentration (28%) to develop halophilic (organisms which can grow at high salt concentrations) population (archaea and some algae, e.g. Dunaliella).
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