Question

Water hyacinth is termed as the ‘terror of Bengal’. It causes death of fish because it __________.
(a) Covers water surface and inhibits sunlight to pass through
(b) Drains oxygen from water that causes oxygen deficiency
(c) Absorbs nutrients from water that causes malnutrition
(d) Releases \[{\text{C}}{{\text{O}}_2}\] in huge amounts which is lethal to fish.

Answer 1

Hint: If a particular plant or weed is growing invasively in the given habitat, it might impose some serious threats to other organisms in the surroundings. From the name, we get a clue that the habitat of Water hyacinth is water bodies and it is growing invasively thereby utilizing nutrients of other organisms like fish.

Complete step by step answer:
Water hyacinth or Eichhornia crassipes is a weed found in the tropical desert to the subtropical or warm temperate desert and in rainforest zones as in Bengal. It is a very fast-growing variety of weeds that grows on the surface of water bodies. In a very less time period, it occupies the entire surface of the water and hence blocks the passage of atmospheric oxygen and water surface. As a result, the oxygen levels of the water body start depleting rapidly. When this plant dies, it sinks to the bottom of the water body which also causes a fall in oxygen levels of the water. Dissolved oxygen levels can reach dangerously low concentrations for fish that are sensitive to such changes. 
Water hyacinth is an exotic shrub mainly found in the parts of Bengal. The hot and humid climate of Bengal favours the growth of this plant. It strongly competes with the native species and also has destroyed many of them in Bengal. Fish which is the supplement food in Bengal, has some serious threats due to the invasive growth of Water hyacinth and hence it is called as “Terror of Bengal”.
So, the correct answer is ‘(b) Drains oxygen from water that causes oxygen deficiency’.

Note: Further, decreased oxygen levels in a water body also catalyze the release of excess phosphorus from sediments in the water body which eventually speed up the process of eutrophication. Death and decay of this plant in large masses deteriorate water quality and the quantity of potable water.