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Aquatic Ecosystems & Eutrophication

  • Not called biomes
  • Differentiated on the basis of difference in
  • Salinity
  • Levels of dissolved nutrients
  • Water temperature & depth of sunlight penetration
Fresh Water Ecosystem
  • Classified as Lotic (Moving water) or Lentic (Stagnant water)
  • Salt content < 5ppt (part per thousand)
Marine Ecosystem
  • Approx. 71% of earth’s surface is covered with oceans
  • Average salinity 35ppt (90 % of which is NaCl)
Brackish water ecosystem
  • Salt content b/w 5 to 35 ppt
  • Example → Estuaries
  • Most important limiting factors of terrestrial ecosystem →  moisture & temperature
  • Most important limiting factors of aquatic ecosystem     →  sunlight & oxygen

Aquatic Organisms – Flora & Fauna

Neuston unattached organisms living at air-water interface
Periphyton organisms which remain attached to stem & leaves of rooted plants emerging above the bottom mud
Planktons Microscopic plants (phytoplankton like algae) & animals (Zooplankton like protozoan & crustaceans)
Nektons Animals which swim (from small insects to whales)
Benthos Organisms living in the bottom of water mass

Major factors affecting productivity of Aquatic habitats

Photic or euphotic zone

  • Depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur.
  • Depth of this zone depends on transparency of water & generally extends till 200 m from the surface (At 200 m approx 1 % of light compared to surface)
  • Both respiration & photosynthesis activity takes place

Aphotic or Profundal zone

  • Portion of a lake or ocean where there is little or no sunlight, formally defined as the depths beyond which less than 1% of sunlight penetrates
  • Bioluminescence is essentially the only light found in this zone
  • Plant growth is restricted as light level is too low for photosynthesis, Hence only respiration occurs



  • Standing water over a large area (more than 10 hectares)
  • At least 3 m of depth with little aquatic vegetation
  • Receives water from surface runoff & sometimes from ground discharge
  • Based on level of salinity lakes are known as Freshwater, Brackish or Saline lakes
  • Based on nutrient content lakes are classified as
  • Oligotrophic Very low nutrients
  • Mesotropic    Moderate nutrients
  • Eutropic        High nutrient content
  • Natural Eutrophication → Along with surface runoff various chemical substances & minerals mingle in its water leading to nutrient enrichment of lakes, which promotes growth of algae, aquatic life & various fauna
  • Cultural Eutrophication → Similar nutrient enrichment of lakes at an accelerated rate, caused by human activities (Discharge of wastewater or agricultural runoff) promotes algae bloom leading to depletion of aquatic life



  • Enrichment of an aquatic system by addition of nutrients, primarily phosphates & nitrates runoffs from chemical fertilizers
  • Nutrient enrichment accelerates the growth of algae which soon covers almost entire surface layer, known as algae bloom.
  • Algae bloom restricts the penetration of sunlight, which restricts process of photosynthesis among underwater plants, leading to depletion of oxygen >>> Death of aquatic organisms

Eutrophication process

  • Depletion of oxygen also occurs due to decomposition of dead algae by microorganisms which require oxygen for decomposition process
  • New anaerobic condition also promotes growth of bacteria (clostridium botulinum) which produces toxins deadly to aquatic organisms, birds & mammals
  • Harmful Algae bloom is commonly known as Red tide, which is basically a misnomer as blooms are not always red in color & certainly not associated with tides, Scientifically referred as Harmful algal blooms (HABs)
  • Common cause for algae blooms are nutrient enrichment of water (esp. of nitrates & phosphates) & rise in water temperature
  • Some HAB toxins can become airborne during a bloom & people can become ill by inhaling them
India Yearbook English India Yearbook Hindi Economic Survey 2017

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