Question Constellation Sign up for free AQA · GCSE · Biology · Higher
Dissolved oxygen and otter populations
| Distance from where sewage was added in km | Number of bacteria × 1000/mm³ | Concentration of dissolved oxygen in mg/dm³ |
|---|---|---|
| 0.0 | 4 | 5.4 |
| 1.0 | 75 | 4.8 |
| 2.0 | 125 | 4.3 |
| 3.0 | 145 | 3.8 |
| 4.0 | 160 | 3.3 |
| 5.0 | 216 | 2.7 |
Otters:
- live in water and on the land
- eat mainly fish
The concentration of dissolved oxygen has decreased in a river where otters live.
Explain how the decrease in the concentration of dissolved oxygen in the river water will affect the population of otters.
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Marking points
- 1 Links lower dissolved oxygen to fewer/dead fish, or uses the accepted more-bacteria route.
- 2 Explains the intermediate biological reason: aquatic prey need oxygen for aerobic respiration, or fish die from bacterial infection.
- 3 Links reduced fish food to a smaller/less reproductive otter population or otter migration.
Full-mark answer
Fish need dissolved oxygen for aerobic respiration, so lower oxygen causes more fish to die and the fish population to decrease. Otters then have less food, so fewer survive or reproduce and the otter population decreases, or some otters migrate elsewhere.
Why this answer loses marks
Otters cannot breathe because there is less oxygen in the water.
Otters do not obtain oxygen from the water; trace the effect through fish respiration, prey abundance, and food availability.