Key Learning Objectives
- Describe the causes and sequence of eutrophication in lakes and rivers.
- Explain how algal blooms block sunlight and trigger deep aquatic plant death.
- Explain how decomposer respiration leads to anoxia and fish asphyxiation.
Topic Summary & Core Notes
Trace the step-by-step ecological process of eutrophication caused by agricultural fertiliser runoff, algal blooms, decomposer oxygen depletion, and aquatic death.
Key Rules, Formulas & Definitions
Eutrophication 5-Step Cascade
Fertiliser Runoff → Algal Bloom → Light Blocked & Plants Die → Decomposers Multiply & Respire → Oxygen Depleted → Fish Suffocate
Decomposer Respiration Equation
Glucose + Oxygen → Carbon Dioxide + Water + Energy (consuming dissolved oxygen in water)
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: The Step-by-Step Eutrophication Process
Question: Explain why fish in a river die several weeks after excessive nitrate fertilisers wash in from nearby farmland.
Step-by-Step Solution:
- Step 1: Algal bloom: Excess nitrates cause rapid growth of surface algae (algal bloom).
- Step 2: Light starvation: The thick algal layer blocks sunlight, preventing underwater plants from photosynthesising, causing them to die.
- Step 3: Bacterial decomposition: Aerobic bacteria/decomposers multiply rapidly as they feed on dead plant matter.
- Step 4: Oxygen depletion: Decomposers consume dissolved oxygen during aerobic respiration, causing oxygen levels to plummet and fish to suffocate.
Final Answer: Decomposing bacteria multiply to break down dead plants and use up all dissolved oxygen during respiration.
Senior Examiner Insight: Examiners award key marks for stating bacteria RESPIRE aerobically to use up oxygen (do not say the algae use up all the oxygen).
Worked Example 2
Worked Example 2: Biomagnification vs Bioaccumulation
Question: Distinguish between bioaccumulation in an individual organism and biomagnification along a food chain.
Step-by-Step Solution:
- Step 1: Bioaccumulation: The buildup of a persistent non-biodegradable toxin (e.g. microplastics, heavy metals) inside a single organism over time.
- Step 2: Biomagnification: The increasing concentration of that toxin at progressively higher trophic levels in a food chain.
Final Answer: Bioaccumulation is toxin buildup within one organism; biomagnification is increasing concentration up trophic levels.
Senior Examiner Insight: Top predators (apex predators) always suffer the highest toxin concentration due to biomagnification.
Common Pitfalls & Examiner Warnings
Common Mistake: Saying algae directly kill the fish
Algae do not kill fish directly; it is the DECOMPOSING BACTERIA using up dissolved oxygen through respiration.
Common Mistake: Claiming plants stop photosynthesising because of cold water
Underwater plants die because surface algal blooms physically block sunlight.
Common Mistake: Confusing Eutrophication with Biomagnification
Eutrophication is nutrient enrichment leading to oxygen loss; Biomagnification is toxin accumulation up food chains.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
Which agricultural ions primarily trigger eutrophication when washed into waterways?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Nitrates and Phosphates
Detailed Explanation: Nitrates and phosphates in fertilisers stimulate explosive algal and plant growth.
Mark Scheme & Scoring Points: 1 mark for Nitrates / Phosphates.
Question 2 of 3
Why do dissolved oxygen levels plummet during the later stages of eutrophication?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) Aerobic bacteria multiply and consume oxygen during respiration as they decompose dead matter.
Detailed Explanation: Decomposers use dissolved oxygen for aerobic respiration, creating anoxic dead zones.
Mark Scheme & Scoring Points: 1 mark for aerobic respiration by decomposing bacteria.
Question 3 of 3
Which trophic level suffers the highest concentration of toxins in biomagnification?
Click to Reveal Answer & Mark Scheme
Correct Answer: C) Apex (top) predators
Detailed Explanation: Non-biodegradable toxins concentrate as biomass is transferred up each successive trophic level.
Mark Scheme & Scoring Points: 1 mark for Apex / Top predators.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
Accelerate Your Academic Progress
Join tailored 1-to-1 or masterclass revision programmes with senior examiner insights.
Book a Free Consultation