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.

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Solution

Since people used excessive fertilizers in the fields, they were carried down to the lake during rains. As many fertilizers contain phosphates and nitrates, the water body became enriched with these chemicals. These chemicals promote excessive growth of aquatic plants and the surface of water was completely covered with plants. Eutrophication (depletion of light) in the water body and insufficient availability of dissolved oxygen and nutrients resulted in the death of fishes.

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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?

Q. 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?

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 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?

A lake near village suffered heavy mortality of fishes within a few days. Consider the following reasons for this.

(i) Lots of urea and phosphate fertiliser were used in the crops in the vicinity.

(ii) The area was sprayed with DDT by an aircraft.

(iii) The lake turned green and stinky.

(iv) Phytoplankton population in the lake declined initially thereby greatly reducing photosynthesis.

Which two of the above were the main causes of fish mortality in the lake?

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