Question

A lake receiving domestic sewage shows
A. Drying up due to algal bloom
B. Increased fish production due to higher nutrient availability
C. Death of fish due to oxygen depletion
D. Increased food web organization

Answer 1

The natural surroundings are association between the living and nonliving components and they are present in a certain equilibrium. When the balance between the different compositions are disrupted by increase in the harmful components causes disruption of the normal system. Any disruption in the normal environment is bound to affect the flora and fauna falling under it.

Complete step by step answer: Eutrophication is a process that can be defined as a water body like a lake or pond that becomes overly enriched with nutrients and which process causes the growth of simple plant life in a huge number. The excessive growth of simple plants that are algae and plankton in a water body refers to the process of eutrophication.
Due to several harmful effects, eutrophication is considered a serious environmental concern. It often results in a downfall of water quality and the depletion of dissolved oxygen in water bodies. In which water bodies eutrophication takes place, that water is known as eutrophic waters which finally become a dead zone to its organism living in the water.
Eutrophication can be explained as the enrichment of natural waters with inorganic nutrients. There are many lakes in which the water is naturally eutrophic and in some lakes, the process of eutrophication takes place gradually as the lake matures. The process of eutrophication occurring more widely nowadays due to various human activities. In some cases, the plant nutrients are introduced artificially which causes community changes in the ecosystem of water. It is also responsible for the downfall of water quality in many freshwater systems.
When a lake receives domestic sewage, it becomes eutrophic and biological oxygen demand in the water gets an increase. This refers to the fact that organic matter is present in the water body. Bacteria and other microorganisms that present in the organic matter of sewage start to consume the dissolved oxygen of water. This causes an exhaust of the oxygen in the water body. Due to the decrease in oxygen leads the aquatic organism to death. Fishes that totally depend on the dissolved oxygen for their respiration. Due to oxygen depletion in the water, which is caused by the eutrophication, the fishes can die.
Therefore, the correct option is option C.

Note: A lake or any other water bodies of freshwater receiving the domestic sewages or inorganic nutrients leads to the eutrophication of water bodies. Due to this quality decreases can take place in the water. The bacteria present in the sewages consumes the dissolved oxygen from the water which leads to a depletion of oxygen in the water that affects the lives of other organisms living in the water and causes death of the fishes. When a lake is receiving domestic sewage, this is known as eutrophication. At present eutrophication has become a worldwide environmental problem. It leads to the damages of many freshwaters and coastal marine ecosystems in the world.