QUESTION 55 (5 marks)
A fertiliser spill increases available nitrogen in a lake. Algal biomass rises sharply, then dissolved oxygen falls after much of the algae dies.
a) Explain the link between dead algal biomass and the fall in dissolved oxygen. [3 marks]
b) Predict one possible effect on the lake’s animal community and explain why. [2 marks]
Practice marking scheme
Answer
a) Decomposers break down dead algae; increased aerobic respiration consumes dissolved oxygen. b) Oxygen-sensitive animals may decline, changing community composition.
Working
The extra dead biomass supplies organic matter to decomposers. Their abundance or activity can rise, increasing aerobic respiration and oxygen consumption. Lower dissolved oxygen can reduce survival of fish or invertebrates with high oxygen requirements, favouring more tolerant species and altering the community. The prediction is conditional on oxygen becoming limiting.
Marking criteria
- Identifies decomposers breaking down dead algae. [1 mark]
- Links greater organic substrate to greater decomposer activity. [1 mark]
- Links aerobic respiration to dissolved oxygen consumption. [1 mark]
- Predicts a plausible change in oxygen-sensitive animals. [1 mark]
- Explains the prediction using oxygen limitation. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.