Question

A lake near a village suffered heavy mortally of fishes within a few days. Consider the following reasons for this.

(A) Lots of urea and phosphate fertilizers were used in the crops in the vicinity.

(B) The area was sprayed with DDT by an aircraft.

(C) The lake water turned green and stinky.

(D) Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.

Which two of the above were the main causes of fish mortality in the lake?

• A. (A), (C)
• B. (A), (B)
• C. (B), (C)
• D. (C), (D)

Answer 1

Answer: (A)

(A), (C)

Dissolved salts such as nitrates, phosphates, and other nutrients, and toxic metal ions and organic compounds and domestic sewage which primarily contains biodegradable organic matter are readily decomposed. This is done by the bacteria and other micro-organisms, which can multiply using these organic substances as substrates and hence utilize some of the components of sewage. Micro-organisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result, there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. This causes mortality of fish and other aquatic creatures. The presence of large amounts of nutrients in waters also causes excessive growth of planktonic (free-floating) algae, called an algal bloom which imparts a distinct color to the water bodies. Algal blooms cause deterioration of the water quality and fish mortality.