The excessive growth of algae in a water body due to enrichment with nitrates and phosphates is called:

The correct answer is B, eutrophication. When fertilisers, detergents and sewage add nitrates and phosphates to a lake or a slow-moving river, algae multiply into a bloom that covers the surface. Light is cut off from plants below, and when the bloom dies the decomposers use up the dissolved oxygen, so fish and other aquatic animals suffocate. Where human activity is the cause it is called cultural eutrophication. Option A is wrong because biomagnification is the increase in concentration of a persistent poison such as DDT at each higher level of a food chain, a different process altogether. Option C is wrong because salinisation is the build-up of salts in soil or water, usually through over-irrigation in dry regions. Option D is wrong because sedimentation is the settling of suspended solids, which is in fact a step in treating sewage rather than a form of nutrient pollution.