Interactions, food webs and nutrient cycles
Analyse organism interactions, energy transfer, productivity, ecological pyramids and the movement of nutrients through marine systems.
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 3.1, 3.2, 3.3 for examinations in 2025–2030. AS candidates study Topics 1–5; A Level candidates study all nine topics and are expected to know the AS content. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or grades. The syllabus requires core practical activities and scientific-method experience. Online explanation and simulated data questions can rehearse methods but do not count as completing practical work or demonstrate safe, competent use of equipment. Learners need suitable teacher-supervised laboratory and field opportunities.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: What does a food-chain arrow point from and to?
What you will learn
- Classify competition, predation, parasitism, mutualism and commensalism
- Construct and interpret food chains, food webs and ecological pyramids
- Calculate productivity and energy transfer with stated units
- Trace carbon, nitrogen and mineral nutrients through marine food webs
Organism interactions
Intraspecific competition occurs within a species; interspecific competition occurs between species. Predators consume prey. Parasitism benefits a parasite while harming its host; mutualism benefits both partners; commensalism benefits one with no measurable effect on the other. Use a specific marine example and state the effect on each organism. Interactions may change when resource levels or habitat conditions change.
Feeding relationships and energy
A producer fixes energy, usually by photosynthesis; some use chemosynthesis. Food-chain arrows point from food to consumer. Producers and primary, secondary, tertiary and quaternary consumers occupy trophic levels; decomposers recycle matter. Energy is lost between levels through respiration, heat, waste and uneaten material. Productivity is biomass produced per unit area or volume per unit time. Pyramids of energy are not inverted; pyramids of numbers and biomass can be affected by parasites or plankton blooms.
Nutrients and cycles
Nutrients include gases, ions and organic molecules; examples of dissolved ions include Mg2+, carbonate, phosphate and nitrate. Nitrogen supports amino acids, chlorophyll and DNA; carbon forms organic molecules; magnesium is in chlorophyll; calcium contributes to bones, shells and coral skeletons; phosphorus is in DNA and bones. Dissolved ocean nutrients are a reservoir for producers and consumers. Trace carbon and nitrogen through dissolved forms, producers, consumers, decomposers and exchange with the atmosphere, sediments and ocean. Distinguish nutrient cycling from one-way energy flow.
Sources for this lesson
Pause and explain
Why is energy transfer between trophic levels usually below 100%?
Worked example
A food chain has producers with 12 000 kJ m−2 y−1 and primary consumers with 1 800 kJ m−2 y−1. Calculate transfer efficiency and one possible reason for the loss.
Show the worked solution
- Use efficiency = energy at next level ÷ energy at previous level × 100%.
- Calculate 1 800 ÷ 12 000 × 100 = 15%.
- Energy can be lost through respiration, heat, waste or material not eaten.
Answer Transfer efficiency is 15%. The remaining energy is not all destroyed: some is used in respiration and released as heat, while some biomass is uneaten or lost as waste.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A producer fixes 20 000 kJ m−2 y−1. The next trophic level stores 2 400 kJ m−2 y−1. Calculate the energy transfer efficiency.
Give me a hint
Divide the energy stored at the next level by that at the first level, then multiply by 100.
Compare my reasoning
- Write efficiency = 2 400 ÷ 20 000 × 100%.
- Calculate 0.12 × 100%.
- Retain the percentage sign and compare like units.
Efficiency = 12%. The calculation compares the energy stored at the next trophic level with energy fixed by producers over the same area and time.
Look for these in your work
- Uses the correct ratio
- Calculates 12%
- Keeps units comparable
A reef fish population declines after a predator is removed. Give a food-web explanation and one alternative factor to investigate.
Give me a hint
Use indirect effects rather than assuming a one-link chain.
Compare my reasoning
- Consider how removing a predator may change a prey species.
- Trace how the changed prey abundance can affect another food-web member.
- Name another factor such as habitat change, fishing or disease.
Removing a predator could allow its prey to increase; that prey may then consume more of the declining fish’s food or compete with it. The pattern does not prove this pathway: habitat change, fishing, disease and environmental conditions should also be checked.
Look for these in your work
- Describes an indirect trophic effect
- Uses a plausible pathway
- Avoids claiming cause from correlation
Phytoplankton take up dissolved nitrate and are eaten by zooplankton. Explain how nitrogen can return to dissolved forms after the zooplankton die.
Give me a hint
Include decomposition and recycling, not energy transfer.
Compare my reasoning
- State that nitrogen in organism tissues is in organic molecules.
- Explain that decomposers break down dead material and waste.
- Describe inorganic nitrogen compounds returning to water for producer uptake.
Nitrogen in tissues and waste is organic. Decomposers break these compounds down and mineralise nitrogen to dissolved inorganic forms such as nitrate or ammonium, which producers can take up again. This recycles matter; energy is still lost between trophic levels.
Look for these in your work
- Identifies organic nitrogen
- Includes decomposer action
- Distinguishes matter cycling from energy flow
Common mistakes
- Drawing food-chain arrows from consumer to food
- Calling energy a recycled resource
- Reporting productivity without a time unit
- Assuming correlation between two populations proves predation
What can you explain now?
Why is energy transfer between trophic levels usually below 100%?
Compare with the explanation
Organisms use energy and some biomass is not consumed or assimilated
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How is a nutrient cycle different from the transfer of energy through a food web?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.