Learning focus
Develop precise chemical explanations, connect observations to reactivity and structure, interpret industrial and environmental processes, and apply evidence to unfamiliar contexts.
Sources
Nitrates enter water from fertiliser runoff. Phosphates may come from fertilisers and detergents. These ions are plant nutrients.
Growth and decay
High nutrient levels can cause rapid growth of algae and aquatic plants. When this material dies, decomposing microbes increase in number.
Oxygen depletion
Microbial respiration uses dissolved oxygen, so oxygen concentration falls. Fish and other aerobic organisms may die or leave the area.
Scope and evidence
The syllabus does not require every stage of eutrophication in detail, but learners must connect nitrates and phosphates with deoxygenation and harm to aquatic life.
High-value recall and connections
Essential recall: Where may phosphates come from? Fertilisers and detergents. Why does dissolved oxygen fall? Decomposer microbes respire while breaking down dead material. State one effect on aquatic life. Fish and other aerobic organisms may die. Practical connection: Interpret a graph of nitrate concentration, algal abundance and dissolved oxygen over time. Describe correlations without assuming causation beyond the evidence. Examination connection: The immediate oxygen loss is largely caused by increased microbial respiration during decomposition.
Practical or data skill
Interpret a graph of nitrate concentration, algal abundance and dissolved oxygen over time. Describe correlations without assuming causation beyond the evidence.
Examination tip
The immediate oxygen loss is largely caused by increased microbial respiration during decomposition.
Review questions and suggested answers
Question 1
Where may phosphates come from?
Suggested answer
Fertilisers and detergents.
Question 2
Why does dissolved oxygen fall?
Suggested answer
Decomposer microbes respire while breaking down dead material.
Question 3
State one effect on aquatic life.
Suggested answer
Fish and other aerobic organisms may die.
