Describe lake as an ecosystem giving examples for the various zones and the biotic components in it ?

Lake Ecosystem: 

Lakes are large inland water bodies containing still water. They are deeper than ponds. Most lakes contain water throughout the year. There are 3 zones in deep water lakes in relation to light, temperature, pressure etc. The three zones are:

• Littoral zone 
• Limnetic zone
• Profundal zone

1.Littoral zone

It is the shallow part of the lake closer to the shore. Light penetrates up to the bottom. It is euphotic. It has rich vegetation and higher rate of photosynthesis hence rich in oxygen.
Biota of littorial zone:
Producers: Littoral zone is rich with pedonic flora and phytoplankton.
Rooted Vegetation: It includese mergent vegetation (Typha, Sagittaria), floating vegetation (Water lilies) and submerged plants (Hydrilla).
Phytoplankton: Diatoms, green algae, Volvox, Spirogyra etc.
Consumers:

• Zooplankton: They float in water. They are water fleas- Daphnia
• Neuston: There are animals living at the air water interface constitute the neuston. Example: Water beetles, larvae of mosquito
• Nekton: They are capable of swimming. Example: Fishes, amphibians, waters snakes
• Periphyton: The animals that are attached to the aquatic plants. Example: Water snails, Hydra
• Benthos: The animals that rest on (or) move on the soil of the lake. Example: Red annelids

2. Limnetic zone: 

It is the open water zone away from the shore. It is the zone of effective light penetration. Here the rate of photosynthesis is equal to the rate of respiration. It is the region of rapid variation of the level of water, temperature, oxygen.
Biota of limnetic zone:
Consumers: Zooplankton, fishes, frogs, etc.

3) Profundal zone:

It is the deep-water area present below the limnetic zone. Light is absent. Producers are absent.
Biota of profundal zone: 

It includes the organisms such as decomposers, red annelids clams etc. They release nutrients which are used by the biotic communities of both littoral and limnetic zones.
 

The lake ecosystem performs:

• conversion of inorganic substances into organic material with the help of the radiant solar energy.
• consumption of autotrophs by the heterotrophs.
• decomposition and mineralisation of dead matter to release them back for reuse by the autotrophs.