Algal blooms do not cause :-

reduction in BOD

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

An algal bloom is a sudden increase in the population of algae in a freshwater or marine habitat. An algal bloom is caused by an enrichment in the nutrient content of the water known as eutrophication. High levels of nitrogen and phosphorus in atrophic waters encourage the growth of algae.

Eutrophication, and the accompanying algal blooms, are often a sign of water pollution, occurring when detergents, fertilizers, or sewage enter a river, lake, or sea.

Algal blooms make water cloudy and create unpleasant odors. Some algae produce toxins that poison fish and other aquatic organisms.

The photosynthetic activity of the algae and their eventual death encourages the growth of oxygen consuming bacteria called decomposers. Thus, the biochemical oxygen demand (BOD) of the water tends to increase where algal blooms are present.