Question

\Lambdaquatic animals die in water polluted by scwage due to (a) pathogens (b) clogging of gills by slit (c) reduction in oxygen (d) foul smell

Short answer

Reduction in oxygen (c).

Step-by-step solution

Understanding the Problem

Aquatic animals are dying in water that is polluted by sewage. Identify the main reason for their death given the options.

Option Analysis: Pathogens

Pathogens are harmful microorganisms that can cause diseases. While they can be a threat, they are not the primary cause of death in most cases of sewage pollution.

Option Analysis: Clogging of Gills by Silt

Clogging of gills can restrict the ability to breathe, but this is typically caused by sediment, not sewage.

Option Analysis: Reduction in Oxygen

Sewage pollution often leads to a reduction in oxygen levels due to the breakdown of organic matter, which consumes oxygen. This condition is called hypoxia.

Option Analysis: Foul Smell

While a foul smell is an indicator of pollution, it typically does not directly cause the death of aquatic animals.

Conclusion

Based on the analysis, the most likely cause of death for aquatic animals in water polluted by sewage is the reduction in oxygen levels.

Key concepts

Oxygen Depletion

When sewage enters a water body, it brings a high load of organic matter. Bacteria start breaking down this organic matter.
This process requires oxygen, leading to a significant decrease in dissolved oxygen levels in the water.
As oxygen levels drop, aquatic animals, like fish, struggle to survive.
They need oxygen for respiration. Without enough oxygen, they eventually die.
This phenomenon is known as oxygen depletion.
It disrupts the balance of the aquatic ecosystem, leading to the death of various species.

Sewage Pollution

Sewage pollution is a serious problem. It introduces a large amount of waste and contaminants into water bodies.
Common components of sewage include human waste, soaps, detergents, and various chemicals.
These substances cause multiple issues, including the emission of harmful pathogens that can cause diseases.
Moreover, toxic substances in sewage can poison aquatic life.
Industrial waste, often part of sewage, can contain heavy metals and other pollutants that are deadly to marine creatures.
In addition to pathogens and toxins, sewage increases the organic load in water, leading to oxygen depletion.

Hypoxia

Hypoxia is a condition where the level of dissolved oxygen in water is very low.
It is often caused by the over-enrichment of water bodies with nutrients like nitrogen and phosphorus, which are found in sewage.
These nutrients promote the excessive growth of algae, leading to a process called eutrophication.
When algae die, they sink and decompose, consuming a large amount of oxygen, which creates hypoxic conditions.
Such conditions are harmful to aquatic life because animals cannot breathe and thus, cannot survive in hypoxic waters.

Aquatic Animal Health

Healthy aquatic animals need clean water with sufficient oxygen levels.
Sewage pollution disrupts their environment in multiple ways, impacting their health severely.
When oxygen levels drop, animals become stressed and weak. They may fail to reproduce or grow properly.
Additionally, exposure to pathogens and toxic chemicals in sewage can cause diseases and infections.
Some aquatic animals might develop deformities due to prolonged exposure to pollutants.
In severe cases, the combined effects of sewage-derived toxins, pathogens, and low oxygen levels lead to mass die-offs of aquatic species.