Explain aquatic effects of sewage and excess fertiliser
Untreated sewage and excess fertiliser can alter water conditions, disrupt aquatic ecosystems and reduce oxygen available to organisms.
Section 20.3 · Pollution
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Know that organisms interact with other organisms and their environment.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How can untreated sewage reduce oxygen available to aquatic organisms?
What you will learn
- Describe effects of untreated sewage on aquatic ecosystems
- Describe effects of excess fertiliser on aquatic ecosystems
Follow organic waste into water
Untreated sewage adds organic material and microorganisms to water. Decomposers break down this material and use oxygen during aerobic respiration. If oxygen is depleted, aquatic organisms that need dissolved oxygen can be harmed or die. Sewage may also introduce pathogens.
The size of the effect depends on the amount of sewage, dilution, water movement and the ecosystem. A description should connect the added material with changes in decomposer activity, water quality and organisms.
Follow excess mineral ions into water
Fertilisers washed into lakes or rivers add mineral ions, including nitrates. These nutrients can increase producer growth; when the producers die, decomposers break them down and use oxygen, potentially lowering dissolved oxygen. This sequence is eutrophication, explained in detail at Supplement level.
For the Core objective, describe the effects of excess fertiliser on aquatic ecosystems. You can connect nutrient enrichment with rapid producer growth, decomposition and oxygen loss, while keeping the step-by-step eutrophication chain precise.
Pause and explain
What can excess fertiliser do after entering freshwater?
Worked example
After fertiliser runoff, a pond develops dense algal growth and fish die. Give a likely link between these observations.
Show the worked solution
- Nutrients can increase producer growth.
- Decomposition can use dissolved oxygen.
Answer Fertiliser nutrients can increase producer growth. When producers die, decomposers break them down and use dissolved oxygen; oxygen depletion can kill fish that need it.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how untreated sewage can harm aquatic organisms that need oxygen.
Give me a hint
Connect sewage organic material, decomposition and oxygen use.
Compare my reasoning
- Sewage adds organic material to water.
- Decomposers respire aerobically and use dissolved oxygen.
Untreated sewage adds organic material. Decomposers break it down and use oxygen in aerobic respiration, reducing dissolved oxygen available to aquatic organisms.
Look for these in your work
- Organic input identified
- Oxygen pathway explained
Describe one way excess fertiliser can affect a lake ecosystem.
Give me a hint
Trace nutrient input through producer growth.
Compare my reasoning
- Fertiliser runoff adds mineral ions.
- Nutrients can increase producer growth and later decomposition.
Fertiliser runoff adds nutrients that can increase producer growth. When producers die, decomposition can use dissolved oxygen and harm organisms that need it.
Look for these in your work
- Nutrient input stated
- Ecosystem effect described
A student says fish died because nitrate ions poison them immediately. Improve the explanation using the supplied algal-growth evidence.
Give me a hint
Trace the indirect oxygen pathway.
Compare my reasoning
- Nutrients stimulate producer growth.
- Decomposition after death increases oxygen use by decomposers.
The evidence may fit nutrient enrichment: nutrients increase producer growth, then decomposers use oxygen while breaking down dead producers. Low dissolved oxygen can harm or kill fish; the nitrate is not necessarily an immediate poison in this account.
Look for these in your work
- Indirect chain followed
- Claim corrected
Common mistakes
- Saying fertiliser directly removes all dissolved oxygen
- Confusing fertiliser runoff with sewage pathogens
- Calling nutrient enrichment itself the final cause of fish death
What can you explain now?
How can untreated sewage reduce oxygen available to aquatic organisms?
Compare with the explanation
Decomposers use oxygen while breaking down organic material
Aerobic decomposition can lower dissolved oxygen.
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Make it stick
Compare the aquatic effects of untreated sewage and fertiliser runoff.
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