Pollution

Dissolved oxygen falls because aerobic decomposer bacteria, feeding on the mass of dead algae, respire in enormous numbers — that respiration is what consumes it. Answers stopping at "the dead algae block the light" or "the algae use up the oxygen" never reach the marking point. Name the decomposers, and name aerobic respiration.

The natural greenhouse effect is not the problem — it keeps the planet habitable. What causes climate change is the enhanced greenhouse effect: the extra heat trapped because human activity has pushed carbon dioxide and methane above natural concentrations. Writing "the greenhouse effect" on its own loses the mark.

Examiners want the process, not the object. Methane comes from the anaerobic decomposition of organic matter — in cattle digestion, in flooded rice paddy fields, and in landfill sites. Naming only "cows" or "rubbish" says where it happens without saying what happens, and rarely earns the mark on its own.

Check which target the question names. Sewage in a river kills aquatic organisms via eutrophication; sewage in a water supply spreads water-borne disease to the people who drink or wash in it. Candidates asked for the effect on HUMANS describe dying fish and polluted rivers instead, and score nothing.

Only recycling, reusing and switching to biodegradable alternatives are credited as ways to reduce non-biodegradable plastic pollution. Burning plastic releases toxic gases — you have swapped one pollution problem for another. Fines for littering change behaviour but do not reduce the plastic already produced.

The last step of eutrophication is death by oxygen starvation. Fish and aquatic insect larvae die because the dissolved oxygen they need for aerobic respiration has been used up — not because the fertiliser has poisoned the water, and not because the algal bloom shaded them. Say "lack of dissolved oxygen".

Biodegradable waste — sewage, dead algae, food waste — can be broken down by decomposers, and that decomposition is exactly what drives eutrophication and generates landfill methane. Non-biodegradable plastic cannot be broken down by any decomposer, so it accumulates instead of being recycled into the ecosystem.