Algal blooms do not cause :-

The correct answer is Option (3) → reduction in BOD

An algal bloom is the excess growth of free-floating algae when a water body receives too many nutrients. It colours the water.

Option (1) imbalance in ecosystem dynamics — An effect of algal blooms. The bloom upsets the balance of the aquatic community.

Option (2) deterioration of the water quality and fish mortality — An effect of algal blooms. The book names this effect directly: the water spoils and fish die.

Option (3) reduction in BOD — Not an effect of algal blooms. The effect is reversed. When the algae die, microbes decompose them using oxygen, so the BOD (biochemical oxygen demand) rises.

Option (4) increase in organic matters in water body — An effect of algal blooms. The huge mass of algae, alive and dead, adds organic matter to the water.

Note: This question is from Chapter 16, Environmental Issues, of the pre-rationalised NCERT book. This chapter is not in the present NCERT book (2026-27). The extract below is from the pre-rationalised edition.

NCERT Extract (Pre-rationalised edition, Chapter 16, Environmental Issues):
Domestic Sewage and Industrial Effluents – “… Presence of large amounts of nutrients in waters also causes excessive growth of planktonic (free-floating) algae, called an algal bloom (Figure 16.4) which imparts a distinct colour to the water bodies. Algal blooms cause deterioration of the water quality and fish mortality. Some bloom-forming algae are extremely toxic to human beings and animals.”