Ecological Classification of Freshwater Organisms

Introduction to Freshwater Ecosystem

Freshwater habitats occupy a relatively small part of Earth’s surface but have great importance. They are the economical, cheapest, and convenient source of water for domestic and agricultural usage.

The components of freshwater are “bottle neck” in the water cycle. Freshwater environments are subjected to variations in the factors such as salinity, dissolved gases, light, turbidity, salinity, etc. as compared to marine waters.

Freshwater can be of two types. These are either lentic or lotic water. the lentic water types include all standing freshwater habitats. For example, lakes, swamps, marshes, ponds, meadows, etc.

Lotic water includes all types of flowing water. It can be rivers, streams, mountain brooks, or springs. The water that falls on Earth, some is absorbed by the soil or percolated downward to form groundwater and some are evaporated.

Freshwater ecosystems more evidently show seasonal and diurnal temperature fluctuations. The diurnal variation of maximum 5 ᵒC has been observed in some ponds having a depth of 3.0 m. however, flowing waters as in rivers, streams, etc. do not show much wider fluctuations.

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Light in water is a factor of profound importance for its role in photosynthetic processes. It is often regarded as a limiting factor in the distribution of organisms specifically the planktons. Also, lotic waters have a high percentage of oxygen.

Life forms of Freshwater Ecosystem

On the basis of life forms or life habitats, freshwater organisms can be divided into the following categories:

1. Planktons


Those tiny and immobile organisms which because of their size live at the mercy of movements of water are called planktons. The plankton community is further divided into two categories.

  • Phytoplankton

The planktons of plant origin are called phytoplankton. These include unicellular algae, diatoms, desmids, etc.

  • Zooplankton

The planktons of animal origin are called zooplanktons. These include copepods, cladocerans, protozoans, rotifers, and larval stages of many invertebrates.

2. Neuston


The organisms resting or swimming on the surface of the water are called neuston. They may include floating algae, macrophytes such as ‘duckweeds, and many types of animals as well.

  • Epineuston

Animals that spend their lives on the upper surface of the water are called Epineuston. For example, hydrometra, Ranatra, etc.

  • Hyponeuston
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Animals that live beneath the surface film are known as Hyponeuston. These include many snails, larvae, and pupae of mosquitoes and other insects, protozoans like Arcella, etc.

3. Periphyton


In 1953, Ruttner proposed the German word ‘AUFWUCH’ which is more appropriate than the English word ‘periphyton’. The term aufwuch refers to any community that develops on the surface of the submerged substrate.

  • Epilithic: The community developing on rocks or stones is called epilithic.
  • Epiphytic: The community developing on plants is called epiphytic.
  • Epissamic: The community developing on the sand.
  • Epizoic: The community developing on hard parts of living organisms and shells is called Epizoic.

These mainly consist of an assemblage of microorganism such as diatoms, algae, rotifers, etc.

4. Benthos


Organisms attached or crawling or resting on the bottom or living in the bottom sediments are called Benthos. The bottom community includes larvae of several insect species, gastropods, crustaceans, bivalves, planarians, annelids, worms, etc.

Organisms that live on the surface of the substrate are collectively called Epifauna and those living in sediment or burrowing in the substrate are called Infauna.

5. Nekton


Nektons are defined as those organisms which are able to navigate at will. These include large, powerful swimming animals such as shrimps, crabs, fish, prawns, large insects, and many other higher vertebrates.

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