What would happen if phytoplankton of an ocean is completely destroyed for some reason? 1) The ocean as a carbon sink would be adversely affected. 2) The food chains in the ocean would be adversely affected. 3) The density of ocean water would drastically decrease. Select the correct answer using the codes given below:

Phytoplankton are the primary producers at the base of the marine food web, and they also fix carbon through photosynthesis. So statements 1 and 2 are correct and statement 3 is wrong, which makes A the answer.

• Statement 1 (correct): Through photosynthesis, phytoplankton draw CO2 out of seawater and the atmosphere. When they die, part of that organic carbon sinks to the deep ocean. This is the biological pump, and losing it weakens the ocean's role as a carbon sink.
• Statement 2 (correct): Zooplankton, fish and other marine consumers depend on phytoplankton. Removing the base of the food chain collapses everything above it.
• Statement 3 (wrong): Seawater density depends mainly on temperature, salinity and pressure. Changes in phytoplankton biomass do not cause a drastic change in it.

Core concept: the biological pump and marine primary productivity. For Mains, this links to ocean acidification, the carbon budget in climate debates, and the iron-fertilisation controversy.

Trap: candidates sometimes reject statement 1 because phytoplankton look too small to matter for carbon. Their collective fixation is very large, which is why they matter globally.