Question

What is the reason for a region of a lake turned into a dead zone?
(A) Eutrophication
(B) Acid rain
(C) Washed off fertilizers in the lake
(D) All of the above

Answer 1

In order to answer this question, you must be aware of the meaning of the dead zone. The term "dead zone" refers to hypoxic zones in oceans and large lakes that contain low oxygen levels. It is caused by washed-away lake nutrients and the introduction of pollutants from the soil or man-made nutrients into these lakes.

Complete answer:
Eutrophication occurs when excess fertilisers from the soil enter rivers and lakes, promoting the growth of algae and other aquatic plants. As a result of the overcrowding, there is a competition for sunlight space and oxygen, causing the lakes to become dead zones with low oxygen content.
Acid rain occurs when industrial waste gases such as carbon dioxide, nitrogen dioxide, and sulphur dioxide combine with atmospheric water to form acids. These acids combine with rain and fall on lakes and forests, polluting them. The reactions can be written as follows:
$S{O_2} + {O_2} \to S{O_3}$
$S{O_3} + {H_2}O \to {H_2}S{O_4}$(Sulphuric Acid)
${N_2} + {O_2} \to NO$
$NO + {O_2} \to N{O_2}$
$N{O_2} + {H_2}O \to HN{O_3} + NO$(Nitric Acid)
All of these acids in the rain end up polluting the lakes and turning them into dead zones.
Similarly, fertilisers washed off the land end up in the lakes. Aquatic plant and algae growth occurs there. As a result of the competition for sunlight and oxygen, the lakes become dead zones.
As a result, option (D) is correct.

Note:
Dead zones are reversible if their causes are reduced or eliminated. The application of fertilizers can vary in amount, timing, and method with varying impacts on water quality. Planting of certain grasses, grains or clovers, called cover crops, can recycle excess nutrients and reduce soil erosion, keeping nutrients out of surface waterways.