IB DP Environmental Systems and Societies – Water systems, security, aquatic food production and pollution Study Guide

This study guide teaches Topic 4, Water, of IB Diploma Programme Environmental Systems and Societies (ESS). It covers syllabus sections 4.1 to 4.4: water systems, water access, use and security, aquatic food production systems, and water pollution. Topic 4 is studied at both SL and HL; the IB recommends 12 teaching hours at SL and 25 at HL, so HL students study some topics in extra depth. It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The public brief lists only the topic and its hours, so check your school’s subject guide for the exact subtopic wording.

Use it with the Topic 4 revision notes and the Topic 4 practice questions. For the course overview, see the IB DP ESS course hub and the printable ESS checklist.

What this unit covers

Section	What you must be able to do	Level
4.1 Water systems	Model the water cycle as storages and flows; explain human impacts; outline ocean circulation	SL and HL
4.2 Water access, use and security	Calculate and interpret water per person; evaluate supply and demand strategies	SL and HL
4.3 Aquatic food production systems	Explain MSY; interpret catch data; evaluate fisheries management and aquaculture	SL and HL
4.4 Water pollution	Measure pollution; explain eutrophication and biomagnification; evaluate management	SL and HL

HL students also apply the three HL-only lenses (see the end of this page). This unit builds on the Topic 1 Foundations study guide and the Topic 2 Ecology study guide.

4.1 Water systems

The hydrological cycle as a system

The hydrological cycle is a system powered by solar energy and gravity. Water is held in storages and moves by flows. Oceans hold about 97% of all water, as salt water. Freshwater is under 3%: most is in ice caps and glaciers, and groundwater is the largest store of liquid freshwater. Lakes, soil, the atmosphere, rivers and organisms are small stores.

Flows are of two kinds:

• Transformations change the state of water: evaporation, transpiration, condensation, freezing, melting, sublimation and deposition.
• Transfers move water without changing its state: advection (wind carrying vapour), interception, infiltration, percolation, surface runoff, throughflow, groundwater flow and river flow.

Residence time

Residence time is the average time a water molecule spends in a storage. For a storage in steady state:

residence time = size of storage ÷ rate of flow out (or in)

Oceans and deep groundwater hold water for thousands of years; the atmosphere, for days. A store with a long residence time refills slowly, so over-use makes it effectively non-renewable.

Worked example. Lake Arven (fictional) holds 6.3 km³ of water. In a normal year 0.9 km³ flows out, and the same amount flows in.

residence time = 6.3 km³ ÷ 0.9 km³ per year
               = 7 years

A pollutant entering the lake takes years to flush out, so lakes recover more slowly than rivers.

Human impacts on flows

• Urbanisation. Impermeable surfaces cut infiltration, so more water becomes fast surface runoff, piped straight to rivers. Lag time shortens and peak discharge rises, raising flood risk.
• Deforestation. Less interception and transpiration; more runoff and soil erosion.
• Irrigation. Moves water from rivers and aquifers onto fields, raising evaporation.
• Dams. Store water, raise evaporation and even out flow downstream.
• Abstraction. Pumping from aquifers faster than recharge lowers the water table.

Ocean circulation

Ocean currents move heat around the planet. Wind drives surface currents. In the thermohaline circulation, cold, salty, dense water sinks near the poles and flows slowly through the deep ocean. Where deep water rises to the surface (upwelling), it brings nutrients that support high primary productivity and rich fisheries. Warm currents raise the temperature of nearby coasts; cold currents lower it.

4.2 Water access, use and security

Uneven supply and rising demand

Freshwater is not spread in line with where people live. Demand is rising with population, irrigated farming, industry and living standards. Agriculture is the largest user of freshwater worldwide.

Two kinds of scarcity:

• Physical scarcity: there is not enough water in the region to meet demand.
• Economic scarcity: water exists, but people lack the money or infrastructure to reach it.

Water security means reliable access to enough water, of acceptable quality, for health, livelihoods and ecosystems, at an acceptable level of risk from floods and droughts.

Measuring availability per person

A widely used indicator divides a country’s renewable freshwater by its population. Common thresholds:

m³ per person per year	Category
1,700 or more	No stress
1,000 to 1,699	Water stress
500 to 999	Water scarcity
Below 500	Absolute scarcity

Worked example. The fictional country of Veloria has 42 km³ of renewable freshwater a year and 24 million people. Its population is projected to reach 30 million by 2040.

1 km³ = 1,000,000,000 m³ = 10⁹ m³

now:   42 × 10⁹ m³ ÷ 24 × 10⁶ people = 1,750 m³ per person  → no stress
2040:  42 × 10⁹ m³ ÷ 30 × 10⁶ people = 1,400 m³ per person  → water stress

Population growth alone moves Veloria into water stress. The indicator ignores quality, seasons and access.

Water footprints and virtual water

A water footprint is the freshwater used to produce everything a person or country consumes. Virtual water is water embedded in traded goods: a dry country importing grain imports the water used to grow it.

Strategies for water security

Increase supply	Reduce demand
Dams and reservoirs	Drip irrigation instead of flooding fields
Desalination of seawater	Metering and pricing water by volume
Rainwater harvesting	Fixing leaks in supply pipes
Transfers between river basins	Recycling greywater
New wells into aquifers	Growing less water-hungry crops

Evaluate each one. Desalination is reliable but energy-hungry and returns salty brine to the sea. Dams flood land, displace people and trap sediment. Coastal aquifer pumping can cause saltwater intrusion. Shared rivers can cause tension between upstream and downstream users.

4.3 Aquatic food production systems

Fisheries and maximum sustainable yield

Fish stocks are renewable natural capital; their natural income is each year’s growth. Maximum sustainable yield (MSY) is the largest catch that can be taken year after year without reducing the stock. In the simple logistic growth model, a population grows fastest at about half its carrying capacity, so MSY is taken there.

If catches go beyond MSY, the stock shrinks and catches can only be held steady by more effort. This is overfishing. Better technology (sonar, bigger nets) can keep catches high while the stock collapses.

Catch per unit effort (CPUE) is a better guide to stock health than total catch:

CPUE = catch ÷ fishing effort

Worked example. A fictional fishery landed 48,000 t in 2016 using 12,000 boat-days, and 45,000 t in 2024 using 18,000 boat-days.

CPUE 2016 = 48,000 ÷ 12,000 = 4.0 t per boat-day
CPUE 2024 = 45,000 ÷ 18,000 = 2.5 t per boat-day
change    = (2.5 − 4.0) ÷ 4.0 × 100 = −37.5%

The total catch fell by only 6.25%, but each boat-day now catches 37.5% less. The stock is very likely shrinking.

Other impacts: bycatch of unwanted species, seabed damage from bottom trawling, and food-web change when predators are removed. Open-ocean stocks are a common resource, so the tragedy of the commons applies: each fleet gains by catching more, while the cost of collapse is shared.

Managing fisheries

• Quotas (a total allowable catch).
• Minimum mesh size, so young fish escape and breed.
• Closed seasons during spawning.
• Marine protected areas and no-take zones, which restock nearby waters.
• Gear rules that cut bycatch.
• Eco-labels that let consumers choose sustainable fish.

Enforcement is the weak point: quotas fail if fleets misreport or fish illegally.

Aquaculture

Aquaculture is the farming of fish, shellfish and seaweed. It can reduce pressure on wild stocks. Its impacts:

• Clearing mangroves for shrimp ponds.
• Waste feed and faeces adding nutrients to nearby water.
• Disease and parasites spreading to wild fish; antibiotic use.
• Escaped fish competing or interbreeding with wild ones.
• Carnivorous species eating feed made from wild-caught fish.

Feed conversion ratio (FCR) = mass of feed ÷ increase in mass of the farmed animals. Lower is better. Farming filter feeders such as mussels needs no feed at all.

4.4 Water pollution

Types and sources

Point sources release pollution from one identifiable place, such as a sewage outfall. Non-point sources are diffuse, such as runoff from many farms, and are much harder to control.

Pollutant	Example sources
Organic matter	Sewage, food-processing waste
Nutrients (nitrates, phosphates)	Fertilisers, sewage, detergents
Toxic metals, pesticides	Mining, industry, farming
Plastics and microplastics	Litter, fishing gear, synthetic clothing
Pathogens	Untreated sewage
Heat	Cooling water from power stations
Suspended solids	Soil erosion, construction

Measuring water pollution

Direct measures sample the water: dissolved oxygen, nitrate, phosphate, pH, temperature, turbidity and faecal coliforms. Biochemical oxygen demand (BOD) is the oxygen that microorganisms use to break down organic matter in a sample, usually over five days at 20 °C. High BOD means lots of organic pollution.

Indirect measures use indicator species. Stonefly and mayfly larvae need clean, oxygen-rich water; sludge worms tolerate low oxygen. A biotic index scores a site from the species present. Direct readings are a snapshot; indicator species reflect weeks or months.

Below a sewage outfall, bacteria break down organic matter and use oxygen: BOD rises and dissolved oxygen falls. Further downstream the organic matter is used up and oxygen recovers from the air and photosynthesis.

Eutrophication

1. Nitrates and phosphates enter a lake or coastal water.
2. Algae and plants grow fast (an algal bloom).
3. The bloom shades plants below, which die.
4. Decomposers break down the dead matter and use up dissolved oxygen.
5. Fish and invertebrates die. Their decay releases more nutrients.

Step 5 feeds back into step 2: a positive feedback loop. Severe cases produce dead zones with almost no oxygen.

Biomagnification

Some pollutants, such as mercury and certain pesticides, are persistent and fat-soluble. They build up in an organism’s tissues (bioaccumulation) and become more concentrated at each higher trophic level (biomagnification).

Worked example. A fictional estuary study found a pesticide at 0.04 ppm in zooplankton, 0.3 ppm in small fish, 2.4 ppm in large fish and 18 ppm in fish-eating birds.

zooplankton → birds: 18 ÷ 0.04 = 450 times more concentrated

Top predators receive the highest doses.

Managing pollution at three levels

1. Change the human activity that produces the pollutant: use less fertiliser, switch to phosphate-free detergents, cut single-use plastics.
2. Control the release of the pollutant: treat sewage to remove nutrients, plant buffer strips beside rivers, set legal discharge limits.
3. Clean up and restore: dredge nutrient-rich sediment, pump air into lakes, remove plastic from beaches.

Level 1 deals with the cause and is usually cheapest in the long run. Level 3 treats symptoms and often has to be repeated.

HL: applying the lenses to water

• Environmental law: agreements that share a river between states, and discharge permits.
• Environmental and ecological economics: pricing water by volume; making polluters pay.
• Environmental ethics: whether clean water is a right, and who pays to protect it.

Common errors

• Calling infiltration or runoff a transformation.
• Forgetting to convert km³ to m³ (× 10⁹) before dividing by population.
• Judging a fishery on total catch alone and ignoring rising effort.
• Saying algae “use up the oxygen”: decomposers cause the oxygen crash.
• Mixing up bioaccumulation and biomagnification.
• Proposing only clean-up measures when asked to evaluate management.

Next steps

Use the Topic 4 revision notes, then the Topic 4 practice questions. For exam format, see the ESS exam preparation guide and the ESS syllabus guide.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. This guide covers Topic 4, Water: sections 4.1, 4.2, 4.3 and 4.4. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.

The brief gives Topic 4 Water 12 teaching hours at SL and 25 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.