Q.Name three broad categories of substances that pollute water bodies (for example, disease-causing agents, oxygen-demanding wastes, and plant nutrients) and briefly state how each affects aquatic life.

Three broad categories of substance pollute water bodies, each affecting aquatic life through a different mechanism. Disease-causing agents (pathogens) -- bacteria, viruses and parasites from untreated sewage -- make water directly unsafe to drink or bathe in and can spread waterborne disease among both humans and animals that use the water. Oxygen-demanding wastes -- biodegradable organic matter from sewage and food-processing waste -- are broken down by aerobic bacteria that consume dissolved oxygen from the water as they decompose the waste, and a large enough load can deplete oxygen faster than it is replenished, suffocating fish and other oxygen-dependent organisms. Plant nutrients -- chiefly nitrogen and phosphorus compounds from sewage and fertiliser runoff -- over-stimulate algal and plant …

For each category, state the mechanism of harm explicitly -- disease transmission, direct oxygen consumption, or triggering eutro …

Treating nutrients and oxygen-demanding waste as the same category -- they harm aquatic life through related but distinct pathways (one directly depletes oxygen …