question_answer

Study the graph given below and answer the questions that follow.

(i) What does BOD stand for? Explain the term.

(ii) What is the relationship between dissolved oxygen and BOD?

(iii) Mention the effect of higher BOD value on aquatic life in the river.

Or

(i) Draw a pyramid of numbers of a situation, where a large population of insects feed upon a very big tree. The insects inturn are eaten by small birds which inturn are fed upon by large birds.

(ii) Differentiate giving reasons between the pyramid of biomass of the above situation and the pyramid of number that you have drawn.

Answer:

(i) BOD is Biochemical Oxygen Demand. It is the amount of oxygen required for microbial breakdown of biodegradable organic matter.
(ii) The amount of dissolved oxygen in water body and the value of BOD are inter - related. As the BOD increases, the amount of dissolved oxygen in the water body decreases sharply from the point of sewage discharge. Hence, higher the BOD value, more polluted is the water body.
(iii) The effects of higher BOD value on aquatic life are as follows
(a) It causes high mortality of aquatic animals.
(b) Algal blooms occur due to excessive growth of algae as the amount of nutrients is too high.
Or
(i) Structure of a pyramid of number is given below
(ii) Differences between the pyramid of biomass of the given situation and pyramid of number drawn above are as Follows
Pyramid of biomass	Pyramid of number
The biomass of the single tree is much more than the biomass of insects.	The base is much smaller with a single tree compared to the thousands of insects.
The pyramid looks upright with these two levels.	The pyramid looks inverted with these two levels.
The biomass of birds is much more than those of insects.	The number of birds feeding on the insects is less than that of insects.