IB Diploma Environmental Systems & Societies HL · Water and Aquatic Food Production Systems
Mini-Lesson
Water and Aquatic Food Systems
This mini-lesson follows water through the Earth system and onto our plates: the hydrological cycle and ocean circulation, the uneven distribution of freshwater and the meaning of water security and scarcity, how humans use and pollute water (including eutrophication), and how we harvest the oceans — fisheries, maximum sustainable yield, overfishing and aquaculture.
Systems lens: the water cycle is driven by solar energy and gravity, moving water between stores (oceans, ice, groundwater, atmosphere) through flows such as evaporation, transpiration and precipitation.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Water · the hydrological cycle
Stores and flows of the water cycle
The hydrological cycle is a closed system for matter, powered by the Sun and gravity.
Stores: oceans (by far the largest), ice caps and glaciers, groundwater (aquifers), lakes and rivers, soil water, and atmospheric vapour.
Flows: evaporation and transpiration (together, evapotranspiration), condensation, precipitation, surface run-off, infiltration and percolation into groundwater.
Human interference alters flows: deforestation reduces transpiration and increases run-off and flooding; abstracting groundwater faster than recharge lowers water tables.
Quick check
Naming the flow
?Water vapour is released into the atmosphere from the leaves of a rainforest. Which flow of the water cycle is this?
Water · the oceans
Ocean circulation
Oceans store most of the planet’s heat and drive climate. The thermohaline circulation (the global ocean conveyor) is driven by differences in temperature and salinity: cold, salty water sinks near the poles and flows deep toward the equator, while warm surface water flows back poleward.
This conveyor redistributes heat (e.g. the Gulf Stream warms NW Europe) and nutrients (upwellings feed the world’s richest fisheries). Melting freshwater from ice sheets could slow it, with major climate consequences.
Water · security and scarcity
Freshwater distribution and water security
Only a small fraction of Earth’s water is usable freshwater, and it is very unevenly distributed.
Physical (absolute) scarcity — not enough water exists in a region (e.g. arid North Africa).
Economic scarcity — water exists but people lack the infrastructure or money to access it safely (much of sub-Saharan Africa).
Water security means reliable access to enough safe water for health, livelihoods and ecosystems. It is threatened by population growth, irrigation demand, pollution and climate change.
Calculate
Your turn — how much is fresh?
1About 97.5% of all the water on Earth is salt water in the oceans. Calculate the percentage of Earth’s water that is freshwater.
%
Hint: freshwater % = 100 − 97.5.
Quick check
Which kind of scarcity?
?A region receives plenty of rain, but most people cannot get clean water because there are no pipes, treatment plants or money to build them. This is an example of...
Water · human use
Using and sharing freshwater
Agriculture is by far the largest user of freshwater (~70% globally), mostly for irrigation.
Aquifers supply groundwater but many are being mined faster than they recharge (fossil water).
Dams and reservoirs store water and generate power but flood habitats, trap sediment and displace people.
Transboundary rivers (Nile, Tigris–Euphrates, Mekong) can spark water conflict between nations sharing them.
Water · pollution
Water pollution and eutrophication
Excess nutrients (nitrates and phosphates from fertiliser run-off and sewage) cause eutrophication:
Nutrients trigger an algal bloom that blocks light, so plants below die.
Bacteria decomposing the dead matter multiply, raising the biochemical oxygen demand (BOD).
Dissolved oxygen falls, suffocating fish and invertebrates — a "dead zone".
Measuring water quality: dissolved oxygen and BOD, turbidity, nitrate and phosphate levels, and indicator species (e.g. stonefly larvae indicate clean, well-oxygenated water).
Quick check
Tracing eutrophication
?A river turns green and fish die downstream of farmland. What is the first cause in the chain that leads to the fish deaths?
Water · aquatic food
Fisheries and maximum sustainable yield
Fish are a vital protein source, but stocks are finite. The maximum sustainable yield (MSY) is the largest catch that can be taken each year without reducing the stock — roughly the population’s natural surplus (growth).
Harvesting at or below MSY is sustainable; the stock replaces what is taken.
Overfishing beyond MSY shrinks the breeding stock, so future yields fall — sometimes to collapse (e.g. the Newfoundland cod fishery).
Calculate
Your turn — maximum sustainable yield
2A fish stock of 500 000 tonnes grows by 12% each year. Calculate the maximum sustainable yield — the surplus that can be harvested annually without depleting the stock.
Costs: nutrient and antibiotic pollution, escapees and disease spreading to wild fish, loss of mangroves for shrimp ponds, and feeding carnivorous farmed fish on wild-caught fishmeal.
Managing fisheries: catch quotas, minimum net-mesh and fish sizes, closed seasons, and marine protected areas where stocks can recover.
Quick check
Weighing aquaculture
?Which of these is a genuine environmental drawback of intensive fish farming?
Sort it
Sort the water terms
Tap a term, then tap whether it is a water store, a water flow, or a pollutant.
🧊 Water store
💧 Water flow
⚠️ Pollutant
Match it
Match the water and fisheries terms
Tap a statement on the left, then its matching answer on the right.
Statement
Answer
Recap
The big ideas to know
Water cycle: stores (oceans, ice, groundwater, atmosphere) linked by flows (evapotranspiration, precipitation, run-off, infiltration)
Security: only ~2.5% of Earth’s water is fresh; scarcity can be physical or economic
Pollution: eutrophication: nutrients → algal bloom → high BOD → oxygen crash → dead zone
Fisheries: harvest at or below the maximum sustainable yield; overfishing collapses stocks
Solutions: quotas, mesh sizes, marine protected areas and carefully managed aquaculture
You can now trace water from cloud to catch and explain how to use it sustainably. Press Finish to see your score.
🏆
Mini-lesson complete!
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