Deep water lakes contain three distinct zones namely
(i) Littoral zone
(ii) Limnetic zone
(iii) Profundal zone.
(i) Littoral Zone : It is the shallow part of the lake closer to the shore. Light penetrates up to the bottom. It is 'euphotic' (having good light) has rich vegetation and higher rate of photosynthesis, hence rich in oxygen.
(ii) Limnetic Zone : It is the open water zone away from the shore. It extends up to the effective light penetration level, vertically. The imaginary line that separates the limntic zone from the profundal zone is known as 'zone of compensation/compensation point' light compensation level. It is the zone of effective light penetration. Here the rate of photosynthesis is equal to the rate of respiration. Limnetic zone has no contact with the bottom of the lake.
(iii) Profundal Zone : It is the deep water area present below the limnetic zone and beyond the depth of effective light penetration. Light is absent. Photosynthetic organisms are absent and so the water is poor in oxygen content. It includes mostly the anaerobic organisms which feed on detritus.
The organisms living in lentic habitat are classified into pedonic forms. Which live at the bottom of the lake and those living in the open waters of lakes, away from the shore vegetation are known as limnetic forms.
Biota (animal and plant life of a particular region) of the littoral zone :
Littoral zone is rich with pedonic flora (especially up to the depth of the effective light penetration). At the shore proper emergent vegetation is abundant with firmly fixed roots in the bottom of the lake and shoots and leaves are exposed above the level of water. These are amphibious plants. Certain emergent rooted plants of littoral zone are the cattails (Typha) bulrushes (scripus) arrowheads (sagitlaria). Slightly deeper are the rooted plants with floating leaves, such as the water lilies (Nymphaea) Nelumbo, Trapa etc., still deeper are the submerged plants such as Hydrilla - Chasa, potamogeton etc. The free floating vegetation includes pista, wolffia, Lemna (duck weed), Azolla, Eichhornia etc.
The phytoplankton of the littoral zone composed of diatoms (coscinodiscus), Nitzschia etc) green algae (volvax spirogyra etc), euglenoids (Euglena, phacus etc) and dinoflagellates (Gymnodinium, cystodinium etc.)
Animals the consumers of the littoral zone, are abundant in this zone of the lake, these are categorized into zooplankton, neuston, nekton, periphyton and benthos. The Zoo-plankton of the littoral zone consists of 'water fleas' such as Daphnia, rotizers and ostracods.
The animals living at the air-water interface constitute the neuston.They are of two types. The epineuston and hyponeuston. Water striders (Gerris), beetles, water bugs (Dineustes) from the epineustan/supraneuston and the hydoneustan/infraneuston includes the larvae of mosquitoes.
The animals such as fishes amphibians, water-snakes, terrapins, insects like water scorpion (Ranatra) back swimmer (Notonecta), driving beetley (Dytiscus), capable of swimming constitute the nekton.
The animals that are attached to / creeping on the aquatic plants, such as the 'water snails' nymphs of insects, bryozoauns, turbellarians, hydras etc., constitute the periphyton.
The animals that rest on or move on the bottom of the lake constitute the benthos e.g: red anielids, chironomid, larvae, cray fishes, some isopods amphipods clams etc.
Biota of the Limnetic Zone : Limnetic zone is the largest zone of a lake. It is the region of rapid variations of the level of the water, temperature, oxygen availability, etc., from time to time. The limnetic zone has autotrophs (photosynthetic plants) in abundance. The chief autotrophs of this region are the phytoplankton such as the euglenoids, diatoms, cyanobacteria, dinoflagellates and green algae.
consumers of the limnetic zone are the zooplanktonic organisms such as the copepods, Fisher frogs, water snakes etc., form the limnetic nekton.
Biota of the Profundal Zone : It includes the organisms such as decomposers (bacteria),chironomid larvae, Chaoborus (phantom larva), red annelids, clams etc., that are capable of living in low oxygen levels. The decomposers of this zone decompose the dead plants and animals and release nutrients which are used by the biotic communities of both littoral and limnetic zones.
The lake ecosystem performs all the functions of any ecosystem and of the biosphere as a whole. i.e., conversation of inorganic substances into organic material, with the help of the radiant solar energy by the autotrophs, consumption of the autotrophs by the heterotrophs; decomposition and mineralization of the dead matter to release them back for reuse by the autotrophs (recycling of minerals).
