Ecological Factor: Light

Introduction

• 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 Rays	Penetration up to (depth) (in metres)
1	Red	10 – 20
2	Orange	20 – 40
3	Yellow	50
4	Green	50-100
5	Blue	50-100

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…)