There is a concern over the increase in harmful algal blooms in the seawaters of India. What could be the causative factors for this phenomenon? Select the correct answer from the codes given below:

Answer: (d) 1, 2 and 3

All three add nutrients that feed blooms.

Statement by statement

1. ✓ YES Estuaries bring fertiliser and sewage nutrients into the sea.

2. ✓ YES Monsoon run-off washes nutrients off the land.

3. ✓ YES Upwelling brings nutrient-rich deep water to the surface.

The idea

Algae in the sea are normally held back by a shortage of nutrients, mainly nitrogen and phosphorus. Give them a sudden supply and they multiply into a bloom. All three items in the question are simply different routes by which nutrients reach the surface water.

Nutrients arrive: rivers, run-off or upwelling → Algae multiply fast → Some kinds release toxins (“red tide”) → The bloom dies and rots → Rotting uses up oxygen → Fish die; a dead zone forms

The trap

Upwelling is natural, so it may seem unrelated to “pollution”.

Also know

Enrichment of water with nutrients is called eutrophication. In the Arabian Sea, winter blooms of the green Noctiluca have become common. Upwelling is natural and also useful: the upwelling off Kerala during the monsoon supports the oil sardine fishery.