Aquatic Ecosystem, Aquatic Organisms, Lake Ecology, Eutrophication

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Ecosystems consisting of water as the main habitat are known as aquatic ecosystems. Aquatic ecosystems are classified based on their salt content.

  • Fresh water ecosystems- The salt content of fresh bodies is very low, always less than ppt (parts per thousand). E.g. lakes, ponds, pools, springs, streams, and rivers

  • Marine ecosystems - the water bodies containing salt concentration equal to or above that of sea water (i.e., ppt or above). E.g. shallow seas and Open Ocean

  • Brackish water ecosystems — these water bodies have salt content in between ppt. e.g. estuaries, salt marshes, mangrove swamps and forests.

Aquatic Organisms

  • The aquatic organisms are classified on the basis of their zone of occurrence and their ability to cross these zones.

  • The organisms (both flora and fauna) in the aquatic ecosystem are unevenly distributed but can be classified on the basis of their Life form or location into five groups


  • These are unattached organisms which live at the air-water interface such as floating plants, etc.

  • Some organisms spend most of their lives on top of the air-water interface such as water striders, while others spend most of their time just beneath the air-water interface and obtain most of their food within the water.

  • E.g., beetles and back-swimmers.


These are organisms which remain attached to stems and leaves of rooted plants or substances emerging above the bottom mud such as sessile algae and their associated group of animals.


  • This group includes both microscopic plants like algae (phytoplankton) and animals like crustaceans and protozoans (zooplankton) found in all aquatic ecosystems, except certain swift moving waters.

  • The locomotory power of the planktons is limited so that their distribution is controlled, largely, by currents in the aquatic ecosystems.


  • This group contains animals which are swimmers.

  • The nektons are relatively large and powerful as they have to overcome the water currents.

  • The animals range in size from the swimming insects (about mm long) to the largest animals, the blue whale.


  • The benthic organisms are those found living in the bottom of the water mass.

  • Practically every aquatic ecosystem contains well developed benthos.

Factors Limiting the Productivity of Aquatic Habitats


  • Sunlight penetration rapidly diminishes as it passes down the column of water. The depth to which light penetrates a lake determines the extent of plant distribution.

  • Based on light penetration and plant distribution they are classified as photic and aphotic zones

Photic zone

  • It is the upper layer of the aquatic ecosystems, up to which light penetrates and within which photosynthetic activity is confined.

  • The depth of this zone depends on the transparency of water.

  • Both photosynthesis and respiration activity takes place.

  • Photic (or “euphotic”) zone is the lighted and usually well-mixed portion that extends from the lake surface down to where the light level is of that at the surface.

Aphotic Zone

  • The lower layers of the aquatic ecosystems, where light penetration arid plant growths are restricted forms the aphotic zone.

  • Only respiration activity takes place.

  • Aphotic zone is positioned below the littoral and photic zones to bottom of the take where light levels are too low for photosynthesis. Respiration occurs at all depths so the aphotic zone is a region of oxygen consumption. This deep, unlit region is also known as the profundal zone.

Dissolved oxygen

  • In aquatic ecosystems oxygen is dissolved in water, where its concentration varies constantly depending on factors that influence the input and output of oxygen in water.

  • In fresh water the average concentration of dissolved oxygen is per cent (also expressed as parts per million or ppm) by weight, which is times lower than the concentration of oxygen in an equivalent volume of air.

  • Oxygen enters the aquatic ecosystem through the air water interface and by the photosynthetic activities of aquatic plants.

  • Therefore, the quantity of dissolved oxygen present in an ecosystem depends on the rate at which the aforesaid two processes occur.

  • Dissolved oxygen escapes the water body through air-water interface and through respiration of organisms (fish, decomposers. zooplanktons. etc).

  • The amount of dissolved oxygen retained in water is also influenced by temperature. Oxygen is less soluble in warm water. Warm water also enhances decomposer activity. Therefore, increasing the temperature of a water body increases the rate at which oxygen is depleted from water.

  • When the dissolved oxygen level falls below ppm, many aquatic organisms are likely to die.

Other limiting factors which influence on aquatic productivity are


  • Transparency affects the extent of light penetration.

  • Suspended particulate matters such as day, silt, phytoplankton, etc make the water turbid.

  • Consequently it limits the extent of light penetration and the photosynthetic activity in a significant way.


  • The water temperature changes less rapidly than the temperature of air because water has a considerably higher specific heat than air, i.e larger amounts of heat energy must be added to or taken away from water to raise or lower its temperature.

  • Since water temperatures are less subject to change, the aquatic organisms have narrow temperature tolerance limit.

  • As a result, even small changes in water temperature are a great threat to the survival of aquatic organisms when compared to the changes in air temperatures in the terrestrial organisms.

The classification of organisms and limiting factors discussed here apply in general to all aquatic ecosystems - lakes, Ponds, Rivers, Streams, Estuaries, oceans and seas.

Lake Ecology

Any - body of standing water, generally large enough in area and depth, irrespective of its hydrology, ecology, and other characteristics is generally known as lake.

Ageing of Lakes

  • Like any organism, lakes are born as they originate by various geological and geomorphic events, and ‘grow’ with time to change in their various morphological and functional characteristics and eventually die.

  • They receive their water from surface runoff (sometimes also groundwater discharge) and along with it various chemical substances and mineral matter eroded from the land.

  • Over periods spanning millennia, ‘ageing’ occurs as the lakes accumulate mineral and organic matter and gradually, get filled up.

The nutrient-enrichment of the lakes promotes the growth of algae, aquatic plants and various fauna. This process is known as natural eutrophication.

Similar nutrient enrichment of lakes at an accelerated rate is caused by human activities (discharge of wastewaters or agricultural runoff) and the consequent ageing phenomenon is known as ‘cultural eutrophication’.

In India

  • In India, natural lakes (relatively few) mostly lie in the Himalayan region, the floodplains of Indus, Ganga and Brahmaputra.

  • In the semi-arid and arid regions of western and peninsular India, tens of thousands of water bodies have been constructed over millennia.

  • Lake ‘Sudarshan’ in Gujarat’s Girnar area was perhaps the oldest man-made lake in India, dating back to 300 BC.

  • Lakes are also classified on the basis of their water chemistry. Based on the levels of salinity, they are known as Freshwater, Brackish or Saline lakes (similar to that of classification of aquatic ecosystem).

  • On the basis of their nutrient content, they are categorized as Oligotrophic (very low nutrients), Mesotrophic (moderate nutrients) and Eutrophic (highly nutrient rich).

  • Vast majority of lakes in India are either eutrophic or Mesotrophic because of the nutrients derived from their surroundings or organic wastes entering them.

General Characteristics of Oligotrophic and Eutrophic Lakes

General Characteristics of Oligotrophic and Eutrophic Lakes
General Characteristics of Oligotrophic and eutrophic Lakes

Type of lake





Aquatic plant production




Aquatic animal production




Aquatic plant nutrient flux




Oxygen in the hypolimnion (bottom layer)





Tend to be deeper

Tend to be shallower


Water quality for domestic & industrial uses




Total salts or conductance

Usually lower

Sometimes Higher


Number of plant and animal species



Removal of the Nutrients from a Lake

  • Flushing with nutrient-poor waters.

  • Deep water abstraction.

  • On-site P-elimination by flocculation/flotation with water backflow, or floating Plant NESSIE with adsorbents.

  • On-site algae removal by filters and P-adsorbers.

  • On-site algae skimming and separator thickening.

  • Artificial mixing / Destratification (permanent or intermittent).

  • Harvest of fishes and macrophytes.

  • Sludge removal.


  • Greek word — Eutrophic means adequate & healthy nutrition.

  • Eutrophication is a syndrome of ecosystem, response to the addition of artificial or natural substances such as nitrates and phosphates through fertilizer, sewage, etc that fertilize the aquatic ecosystem.

  • The growth of green algae which we see in the lake surface layer is the physical identification of an Eutrophication.

  • Eutrophication is the enrichment of an aquatic system by the addition of nutrients.

  • It is primarily caused by the leaching of phosphate and or nitrate containing Fertilisers from agricultural lands into lakes or rivers.

  • Some algae and blue-green bacteria thrive on the excess ions and a population explosion covers almost entire surface layer is known as algal bloom. This growth is unsustainable, however.

  • As Algal Bloom covers the surface layer. It restricts the penetration of sunlight. Perhaps because another nutrient becomes limiting, death of aquatic organisms takes place.

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