K325 / 2027

Lesson 6 of 9 / Organisms and their environment

Why sewage can remove oxygen from water

How can added organic matter harm fish?

I can explain the effects of sewage using decomposition, respiration and dissolved oxygen.

  • I can explain sewage, marine plastic and insecticide biomagnification impacts.Syllabus

    Syllabus K325, Topic 11(h). Explain sewage, marine plastic and insecticide biomagnification impacts.

About 5 min

Make a guess. You are not marked.

Sewage leaks into a river and fish die downstream. Sewage is not poisonous to fish. What killed them?

  1. Lack of oxygen, used up by decomposers.
  2. Too much food made the fish overeat.
  3. The sewage blocked their gills.
Show the answer

Lack of oxygen, used up by decomposers.

Decomposers multiply on the sewage and their respiration uses dissolved oxygen faster than it is replaced.

The key idea

Microorganisms breaking down sewage respire and use dissolved oxygen. If demand exceeds replacement, oxygen falls and aquatic organisms can be harmed.

Dissolved oxygen is oxygen present in water and available to aquatic organisms. Biological oxygen demand describes the oxygen used by microorganisms breaking down organic material under specified conditions.

Explore the idea

What changes downstream?

Original river dissolved-oxygen dataUpstream 8 mg/L, below discharge 3 mg/L, farther downstream 6 mg/L. Selected location: Below discharge.813263Dissolved oxygen (mg/L)Locations along the flow

2. Below discharge: 3 mg/L. Added organic material supports decomposer respiration. Oxygen use can exceed replacement, lowering dissolved oxygen.

Original practice data, not a real river survey. Lines guide the eye between three samples; distance and time are not measured on the horizontal axis. Temperature, flow and further inputs can change the pattern.

Explanation

Untreated sewage introduces organic matter into water. Microorganisms feed on and decompose this material. As aerobic decomposers become more active and numerous, their respiration uses more dissolved oxygen.

If oxygen use is faster than replacement from air exchange and photosynthesis, dissolved oxygen concentration falls. Fish and other aerobic aquatic organisms may then be unable to obtain enough oxygen for respiration. The causal link is increased biological oxygen demand, not simply that sewage physically blocks all oxygen.

Sewage can also add mineral nutrients that stimulate algal growth. Dense growth can shade submerged plants. When algae and plants die, decomposers break them down, increasing oxygen demand further. This sequence is part of eutrophication; each stage should be connected rather than stated as a memorised list.

Treatment reduces organic loads and harmful contamination before discharge. Interpreting river data also requires care: oxygen can recover downstream as organic material is broken down and water is aerated, but recovery depends on flow, temperature and continued inputs. Not every river follows one identical curve.

Step by step
  1. 1

    Start with the added material

    Sewage adds organic matter and may add nutrients.

  2. 2

    Name the biological response

    Decomposers use organic matter and respire, increasing oxygen demand.

  3. 3

    Connect oxygen to organisms

    Lower dissolved oxygen limits aerobic respiration in fish and other aquatic organisms.

Worked example

An original downstream pattern

Dissolved oxygen is 8 mg/L upstream, 3 mg/L shortly below an organic sewage discharge, and 6 mg/L farther downstream. Explain the initial fall without assuming sewage directly poisons every fish.

One way to explain it

The added organic material supports increased microbial decomposition. Aerobic microbial respiration uses dissolved oxygen, causing concentration to fall when use exceeds replacement. Fish may suffer because less oxygen is available for their respiration. The later rise is consistent with reduced organic demand and re-aeration, but further data are needed to identify their contributions.

Why this answer works
  • Connect the discharge to decomposers.
  • Connect respiration to the oxygen fall.
  • Treat downstream recovery as evidence to interpret, not proof of one cause.

Watch out for this

A student says: "Algae alone are the whole explanation for every sewage-related oxygen decline." What is wrong with this?

Show the answer

Decomposition of sewage organic matter can directly raise oxygen demand. Nutrient-driven algal growth and later decomposition can add another route to oxygen depletion.

Check your understanding

What directly uses dissolved oxygen when organic sewage is decomposed aerobically?

  1. Respiration by decomposer microorganisms.
  2. Photosynthesis by all decomposers.
  3. The formation of a cell wall around the whole river.
Show the answer

Respiration by decomposer microorganisms.

Greater oxygen demand can lower dissolved oxygen and harm other aerobic organisms.

Exam-style questions

Check your understanding

Untreated sewage enters a river. Which sequence explains why fish die downstream?

  1. Fish eat the sewage and are killed by the extra protein in it
  2. Algae in the sewage release too much oxygen, which poisons the fish
  3. Sewage blocks out light, so the fish cannot see to find their food
  4. Bacteria decompose the sewage and use up oxygen, so fish cannot respire
Show the answer

Bacteria decompose the sewage and use up oxygen, so fish cannot respire

Decomposers respire aerobically as they break down the sewage, lowering dissolved oxygen.

Quick recall

Card 1 of 3. Answer in your head, then check.

How can added organic matter harm fish?

You can return to this lesson any time.