Biology Today - February 2018

Light Zones in Aquatic Habitats

• Zones in lake water are determined by gradients of light, oxygen and temperature from the water surface to lake bottom.

Limnetic zone

Vegetation

Euphotic zone

Disphotic zone

Benthic zone

Aphotic / Profundal zone

Fig.: Light zones in deep lake and ocean

Soil

Low

Littoral

zone

It is open water zone where water

is very deep. Amount of oxygen

and light decreases with depth.

High

It is the lower part

of the photic zone,

(twilight zone), receives

light at or below the

compensation point.

It is upper part of

limnetic zone to which

light can penetrate.

It is shallow coastal region. Light is

able to pass through shallow water

and reach the bottom. Therefore,

producers occur throughout from

surface to bottom.

Light and oxygen levels

Lowermost zone, include sediment surface.

It is the upper part of photic

zone, receives light more than

compensation point.

Photic zone

It is the zone below photic

zone. Producers do not occur,

only consumers are found here.

3. Water

• Water forms about 71% of the earth’s surface and represents the most extensive habitat for the organisms.


• Productivity and distribution of land plants depends on availability of water.

Xerophytes

Plants of dry habitats with the problem of more water loss through transpiration than the water available from soil, HJ, $FDFLD&DVXDULQD etc.

On the basis of moisture availability

Mesophytes

Plants of moist habitats with luxuriant vegetative growth. Spines

and thorns are absent, HJ, most crop plants, vegetable and fruit

plants.

Hygrophytes

Plants of wet areas with soft stems of moderate height, large thin leaves

with hydathodes for excreting excess water in guttation, HJ$SOXGD,

5DQXQFXOXVVFOHUDWXV, etc.

Terrestrial plants

Aquatic animals

In rivers and streams, animals obtain

most of their food from organic materials

coming from outside the water.

Desert animals

Animals of arid areas either conserve water or depend on food as major source

of water. Some of these animals eat only seeds, HJ, kangaroo rat. Desert

animals conserve water by producing solid faeces and excreting solid urine.

Animals

Stenohaline

Organisms restricted to narrow range

of salinity tolerance, HJ, shark.

Euryhaline

Organisms tolerant to wide

range of salinity, HJ, 6DOPRQ

Types of organisms based on salinity tolerance

4. Soil

• The fertile surface layer of the earth capable of supporting plant growth is called soil.


• Humus is the most important part of the soil that makes the soil porous, thereby increasing its air and water-holding capacity.


• Total soil water content or holard consists of two parts - chresard (water available to plants) and echard (water not available
  to plants).


• A soil profile consists of several soil horizons.