Phytoplanktons are found on illuminated surface of water. On the other hand, benthic organisms are found at the sediments of a lake. Three zones in a lake can be identified which are determined by gradients of light, oxygen and temperature from the water surface to bottom.

Littoral zone: It is shallow water zone present at the edge of lake. It bears rooted plants.

Limnetic zone: It is present behind littoral zone and contains often water. Here phytoplankton’s are found in plenty. Light can penetrate from 20-40m.depthdepending on clarity of water.

Profundal zone (dark zone): In this zone light fails to reach. Bottom of lake constitutes benthic region where micro-organisms, snails and sluge etc. are found.

most important role of atmosphere

Role of atmosphere

Air functions as a medium for Locomotion of insets, birds etc. Ozone layer of atmosphere protects the living organisms from harmful radiations of sun.

Air is the source of oxygen, carbon dioxide and nitrogen required for various metabolic activities of living beings. It helps in dispersal of spores, pollen, seeds etc. Air transmits sound for communication. Burning of fire takes place in presence of oxygen.


The gases present in the atmosphere are chiefly oxygen, carbon dioxide and nitrogen which greatly influence the living organisms. Atmosphere is the thick gaseous envelope around the earth. The composition of Atmospheric air is: Oxygen—20.84%, Nitrogen—-78.64%, Carbon dioxide—0.03%, Water vapors—0.1%


Light is a very important of the climatic factors which is directly responsible for the growth, development and differentiation of plants. Similarly, light as a complex physical factor, also affects the colour vision, eye-size, skin pigmentation, Migration, reproduction and biological periodicity of the animals. Thus, light is a Factor of physical and physiological importance.


Size of eye: Marine animals of 500-3,000 meters depth have larger eyes than those found in surface water.

Photo-periodism: It refers to the response of organisms to the duration of sunlight in a day (photoperiod).In many animals’ migration, hibernation and Reproductive behaviors are controlled by relative lengths of day and time. Metabolism: Light increases the enzymatic activities and general metabolic Rate. It also increases photo oxidation and respiration rates. Animals found in Caves have low rate of metabolism.

is spider an insect?

Although a spider looks like an insect, it is not. Spiders have eight legs whereas the insect have six. Spiders do not have the feelers and Wings which the insect have. An insect’s body is divided into three distinct parts head, Thorax & abdomen whereas a spider’s body has only two parts combined head and thorax and the abdomen.

Infect spiders belong to a different group of animals called arachinds which also include mites, ticks, Scorpions etc.

हालांकि मकड़ी एक कीट की तरह दिखती है, लेकिन ऐसा नहीं है। मकड़ियों के आठ पैर होते हैं जबकि कीट के छह। मकड़ियों के पास कीट के फीलर्स और पंख नहीं होते हैं। एक कीट के शरीर को तीन अलग-अलग भागों में विभाजित किया जाता है, सिर, थोरैक्स और पेट जबकि एक मकड़ी के शरीर में केवल दो भाग होते हैं जो सिर और छाती और पेट को मिलाते हैं। वास्तव में मकड़ियाँ जानवरों के एक अलग समूह से संबंधित होती हैं जिन्हें अरचिन्ड कहा जाता है जिसमें घुन, टिक, बिच्छू आदि भी शामिल हैं।

what is ecosystem (part – 2)


The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.

  • It is expressed in terms of weight (g-2)  or energy (kcal m-2)
  • The rate of biomass production is called productivity.

Gross primary productivity: (GPP) is the rate of production of organic matter during photosynthesis.

Net primary productivity: A considerable amount of energy is utilized by plants in respiration.

Gross primary productivity minus respiration losses (R) is the net primary productivity.

GPP – R = NPP.

Net primary productivity is the available biomass for the consumption to heterotrophs (herbivore and decomposers.

Secondary productivity: is defined as the rate of formation of new organic matter by the consumer.

DECOMPOSITION: Earthworm is said to be ‘friends’ of farmer:

  • Breakdown the complex organic matter.
  • Loosening of the soil helps in aeration and entry of root.

The decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, called decomposition.

Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute the detritus.

The process of decomposition completed in following steps:

Fragmentation:  Break down of detritus into smaller   particles by detritivore   (earthworm).

Leaching: Water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

Catabolism:  Bacterial and fungal   enzymes degrade detritus into simple inorganic   substances.

Humification: Accumulation of dark coloured amorphous substances called humus.