Topic 20.3 · Core
Water Pollution: Sewage, Fertiliser and Plastics
Three pollutants, three different mechanisms. Sewage and excess fertiliser end up killing aquatic organisms through the very same route, but they don't start the same way — and non-biodegradable plastic does something else again.
Pollution is the introduction of substances or agents by human activity into the environment that harm organisms or damage ecosystems. In water specifically, three pollutants are worth knowing in detail, and the exact route each one takes to cause harm is where the exam marks sit — not the general statement that pollution is harmful.
Untreated sewage
Sewage adds organic material and microorganisms directly into water. Decomposers break down that organic material, and their aerobic respiration uses up dissolved oxygen as they do it. The more sewage enters the water, the more organic material there is to decompose, the more aerobic respiration takes place, and the further dissolved oxygen falls — until fish and other organisms that depend on dissolved oxygen die or leave the area. Untreated sewage can also carry disease-causing microorganisms, which makes contaminated water a direct health risk on top of the ecological one.
Excess fertiliser
Fertiliser that isn’t taken up by crops can be washed or leached into rivers and lakes, raising the concentration of nitrate and other mineral ions in the water. Aquatic producers respond to that extra supply by growing rapidly, and it’s the later decomposition of all that extra producer biomass — once it dies — that drives up decomposer respiration and pulls dissolved oxygen back down, reaching the same end point as untreated sewage: oxygen-dependent organisms dying or leaving.
This is the same fertiliser that does real good on increasing food productionwhen it’s taken up by the crop it was applied to. The harm here comes specifically from the portion that escapes into water, and specifically through oxygen depletion rather than any direct toxic effect. For Supplement candidates, the complete step-by-step version of this sequence — with every intermediate stage named — is covered in full under eutrophication.
Non-biodegradable plastics
A non-biodegradable material isn’t broken down by decomposers at any useful rate, so plastic can remain in an ecosystem for a very long time. In aquatic ecosystems, animals can become entangled in plastic waste — restricting movement, causing injury, or trapping them outright — or they can swallow it because it resembles food, which can block or damage the digestive system and reduce feeding. On land, the same basic problems appear: animals can ingest or become entangled in plastic waste, and because it persists rather than breaking down, it can accumulate in a habitat and continue causing harm long after it was first discarded.
See the difference
Sewage harms aquatic ecosystems by adding organic material that decomposers respire, directly increasing oxygen use. Excess fertiliser harms them by first stimulating extra producer growth, with the oxygen problem arriving only once that extra biomass dies and decomposes. Both routes end in falling dissolved oxygen, but they begin in different places — and non-biodegradable plastic doesn’t touch dissolved oxygen at all, working instead through ingestion, entanglement and long-term persistence in the environment.