• Light is a significant ecological factor.
  • Quality (wavelength or colour), duration (length of day) and intensity (energy input or actual intensity measured in gram calories) of light all influence plants and animals in various ways.
    • Both animals and plants respond to different wavelengths of light.
    • Colour vision occurs in majority of animals including arthropods, fishes, birds & mammals especially primates.
    • As light penetrates in deep waters, red and blue lights are filtered out thereby affecting occurrence of flora there. Example: Red algae due to presence of additional pigments (phycoerythrins) can be found in deep waters as compared to green algae.
    • Intensity of light affects the autotrophic layer or primary production and thus influence whole ecosystem.
  • It also controls structural and behavioural patterns of organisms.

Light In Relation To Terrestrial Environment:

  • Light intensity, quantity and quality are affected by various factors like atmosphere, layers of vegetation, layers of water, topographic factors like fog, clouds, suspended particles etc.
  • When angle of incidence is smaller, light rays travel longer distance through atmosphere causing reduction in intensity i.e. energy input.
  • Light intensity also decreases with increase in degree of latitude.
  • Water droplets in the atmosphere causes sunlight passing to scatter according to constituent wavelengths.
  • Clouds, suspended dust particles also scatter the radiation.
  • Also, atmospheric gases absorb and disperse small portions of light.
  • Canopy of forest affect the light intensity reaching the ground.

Light In Relation To Terrestrial Environment:

  • Before entering water, the light has already been subjected to changes caused due to atmosphere etc.
  • Light is further modified to in water in terms of intensity, spectral composition, angular and time distribution.
  • Water is very effective at light absorption and only 10% of light falling on water surface is reflected back.
  • Phytoplanktons, Zooplanktons and suspended particles contribute in absorption or reflection of light.
S.No.Name of RaysPenetration up to (depth) (in metres)
1Red10 – 20
2Orange20 – 40

Below 100 metres – only violet and ultra violet rays are present.

Below 1000 metres – No light penetrates and zone of total darkness occurs.

Zones depending upon light penetration in water bodies: 

Euphotic zone:

  • Upper, illuminated zone of aquatic ecosystem.
  • Corresponds to epipelagic zone.
  • Typical depth varies from only few centimetres in highly turbid eutrophic lakes to 200 metres in open oceans.
  • Receives sunlight & thus allow phytoplankton to carry put photosynthesis.
  • Home to majority of aquatic life.
  • Algae present here contributes significant levels of oxygen in atmosphere.
  • Also called Photic Zone, Epipelagic Zone or Sunlit Zone.

Disphotic zone:

  • Corresponds to mesopelagic zone.
  • Lies between 200 to 1000 metres.
  • Some light is present in this zone but could not support photosynthesis to occur.
  • Also called Twilight zone.

Aphotic zone:

  • Depths beyond which less than 1% light penetrates.
  • Majority of ocean water lies in this zone (majority of ocean biomass lies in photic zones i.e. euphotic and disphotic).
  • Also called Dark Ocean.
  • Aphotic zone is further divided into:
    • Mesopelagic Zone:
      • Larger than epipelagic zone.
      • Lies between 200 to 2000 metres depths.
      • Many organisms present here like fishes and invertebrates migrate to epipelagic zone to feed.
    • Bathyal Zone:
      • Larger than mesopelagic zone.
      • Lies between 2000 to 4000 metres depths.
      • Area can be interrupted by light from organisms themselves present here i.e bioluminescence.
      • Some species present here have lost ability to see.
      • Also called bathypelagic zone.
    • Abyssal Zone:
      • Lies between 4000 to 6500 metres depths.
      • Region relatively lack life.
      • Also called abyssalpelagic zone.
    • Hadal Zone:
      • Deepest region of ocean.
      • Lies between 6000 to 11000 metres depths.
      • Exists in long, narrow topographic V-shaped depressions.
      • Also called Hadopelagic Zone.
      • Characteristics: complete lack of sunlight, low temperatures, nutrient scarcity & extremely high hydrostatic pressures.
      • Most organisms here are scavangers and deterivores.
      • High levels of endemism occurs here – Gigantism in amphipods, mysids and isopods; Dwarfism in nematodes, copepods and kinorhynchs.
      • Wide range of metazoans mostly benthos occur here.
      • Examples of organisms present – Fish, Sea cucumber, bristle worms, bivalves, sea anemones, isopods, amphipods, gastropods etc.

(To be continued…)

Harjeet Kaur

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