Question

Which of the following is absent in polluted water:-
A. Hydrilla
B. Water hyacinth
C. Larvae of stonefly
D. Blue green algae

Answer 1

Hint: Polluted water contains toxic substances which make it unfit for human consumption. It also disturbs the aquatic flora and fauna. This leads to growth of algal bloom and eutrophication.

Complete Answer:
The polluted water is rich in phosphates and nitrates. Due to the availability of excess nutrients and reduced oxygen, the algae like blue green algae use the nutrients and grow in the water. This results in formation of algal blooms.
1. Hydrilla- Hydrilla grows best in the carbon dioxide rich water. The polluted water is rich in carbon dioxide. It shows changes in physiology and morphology.
2. Water hyacinth- It is an invasive species that grows and forms a thick layer over the water. It depletes the amount of dissolved oxygen in water.
3. Larva of stone fly- It is a sensitive insect that produces larvae in water. It requires well aerated water and does not grow in polluted water. It acts as a bio-indicator of water pollution.

The correct answer is (C). Larva of stone fly is absent in polluted water.

Note: When the environment becomes rich in nutrients, the aquatic environment faces a problem of increase in the algae. Some of the algae produce toxins which are harmful for other organisms. These toxins can enter the food chain and can be fatal.