In a village, farmers started cultivating crops all around a lake which was always filled with water. They added lot of materials W to the soil in their fields to increase the yield of crops. Soon they found that the whole surface of water in the lake was covered with a green layer made up of tiny organisms X. After some time, the fish present in the lake started dying in large numbers because they could not get sufficient Y due to a process Z which had occurred in lake water and drained out all the Y present in the water of lake.
(a) What do you think materials W are?
(b) Name (i) X, and (ii) Y.
(c) What is the process Z known as?
(d) What happens during the process Z which uses up all the Y present in lake water?
(e) What does this example tell us about the use of materials like W?

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Solution

(a) W refers to the fertilisers.

(b) X is Algae and Y is oxygen.

(c) The process Z is known as eutrophication.

(d) During the process of eutrophication (Z), when the alga (Y) dies, the bacteria present in the water decay the alga.

(e) This example shows that excessive use of fertilisers (W) is bad for the environment.

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Question 27
In a village in Karnataka, people started cultivating crops all around a lake which was always filled with water. They added fertilisers to their field in order to enhance the yield. Soon they discovered that the waterbody was completely covered with green floating plants and fishes started dying in large numbers.
Analyze the situation and give reasons for excessive growth of plants and death of fish in the lake.

Read the passage and answer the following question.

In a village in Karnataka, people started cultivating crops all around a lake which was always filled with water. They added fertilisers to their field in order to enhance the yield. Soon they discovered that, the water body was completely covered with green floating plants and fishes started dying in large numbers.

What is the cause for excessive growth of plants and death of fish in the lake?

The farmers use large amounts of a substance P in the fields to increase the crop yield. The excess of P dissolves in water and runs into a lake. The substance P causes rapid growth of tiny green water plants Q in the lake which cover the whole lake like a green sheet. When the plants Q die, the organisms called R decompose them by utilizing S dissolved in lake water. The amount of dissolved S in water decreases too much due to which the fish living in lake suffocate and die. What are P, Q, R and S?

Fishes died within a few days in a lake near a village. Following are the possible reasons for this:

(a) Lot of urea and phosphate fertilisers used in crops in the vicinity

(b) Spraying agricultural fields with degradable pesticides

(d) Phytoplankton population declined in the lake

What are the two main causes of fish mortality in the lake?

The surface of the water in a lake appears green due to a layer of tiny free-floating organisms X on its surface. The lake water also contains organisms like water beetle, fish and tadpole. The sun shines over the lake water and provides energy for the functioning of this lake ecosystem.

(a) What could organisms X be?

(b) Write a food chain comprising of all the four organisms mentioned.

(d) Name (i) secondary consumer (ii) producer (iii) tertiary consumer, and (iv) primary consumer, in the above food chain.

(e) If the tertiary consumer gets 0.2 J of energy from the secondary consumer, then how much energy was radiated by the sun to the producer?

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