Aquatic systems as natural capital
Big idea: Oceans, lakes, and rivers are natural capital — they provide food, regulate climate, and support billions of livelihoods. Fish stocks are renewable natural capital that can regenerate IF we harvest them sustainably.
Ecosystem services from aquatic systems
- Provisioning — fish, shellfish, seaweed, freshwater
- Regulating — climate regulation, carbon storage, water purification
- Supporting — nutrient cycling, habitat for biodiversity
- Cultural — recreation, tourism, spiritual value
Renewable but not unlimited
Fish populations CAN regenerate, but only if we harvest at or below the maximum sustainable yield (MSY). Overfishing pushes stocks below this point, causing collapse.
Renewable does not equal infinite! Fish are renewable natural capital, but overfishing can make them effectively non-renewable if populations collapse below recovery threshold.
IB-style question — energy transfer in a lake food chain [1]
A freshwater lake food chain is: algae → water fleas → roach (a small fish). Energy stored is 9000 kJ in the algae and 720 kJ in the water fleas.
Calculate, as a percentage, the efficiency of energy transfer from algae to water fleas. [1]
How to answer it, step by step
- Write the formula, then put the numbers in
• Efficiency = (energy at next level ÷ energy at level below) × 100
• = (720 ÷ 9000) × 100 - Work it out
• 720 ÷ 9000 = 0.08
• 0.08 × 100 = 8%
Final answer
Always divide the smaller (higher-level) energy by the larger (lower-level) energy, then ×100 — and add the % sign.
IB-style question — why coastal waters are so productive [2]
The waters off a cold-current coastline support some of the world's richest fishing grounds.
Explain why marine primary and secondary productivity is so high in these coastal waters. [2]
How to answer it, step by step
- Where do the nutrients come from?
• Upwelling brings deep, nutrient-rich water up to the surface
• More nutrients = more phytoplankton growth (high primary productivity) - Why does this boost the animals too?
• Shallow, sunlit water lets phytoplankton photosynthesise fast
• Lots of phytoplankton feeds zooplankton and fish — high secondary productivity
Final answer
Link nutrients (upwelling) to plant growth, then plant growth to animal growth — a productivity chain, not just 'lots of fish live there'.