HL only: Topic 4.4 Water pollution: Anoxic and hypoxic “dead zones” are becoming more common because nutrient pollution, sewage disposal, global warming and stronger water stratification interact to reduce dissolved oxygen. These pressures increase oxygen consumption while limiting oxygen replacement.
The Mechanism
Hypoxic water contains very little dissolved oxygen, while anoxic water contains effectively none. The main process producing these conditions is eutrophication:
- Agricultural fertilizer runoff and sewage add nitrates and phosphates to water.
- Increased nutrient availability causes rapid phytoplankton growth, sometimes producing an algal bloom.
- Algae eventually die and become organic matter.
- Aerobic decomposers break down this material and consume dissolved oxygen through cellular respiration.
- Oxygen concentrations fall, killing or displacing fish and other aerobic organisms. Their decomposition can consume still more oxygen, intensifying the problem.
Several interacting factors make this sequence increasingly frequent:
| Factor | How it promotes dead zones |
|---|---|
| Fertilizer runoff and sewage | Increase nutrient loading and the production of decomposable organic matter. |
| Global warming | Warmer water holds less dissolved oxygen and can increase respiration and decomposition rates. |
| Stronger stratification | Density differences reduce mixing, preventing oxygen-rich surface water from reaching deeper water. |
| Expanding coastal settlements and agriculture | Increase sewage discharge and nutrient runoff into rivers, lakes and coastal waters. |
A named example is the seasonal dead zone in the Gulf of Mexico, strongly associated with nutrient inputs carried by the Mississippi River from its agricultural watershed. Its impacts include reduced fish habitat, ecological damage and economic risks for coastal fisheries, raising environmental justice concerns for communities dependent on marine resources.
A common misconception is that algae directly “use up” all the oxygen. The major oxygen decline usually occurs when microorganisms decompose dead algal biomass.
Exam Technique
For an explain question, give the complete causal chain from nutrient input to decomposition and oxygen depletion. For higher-mark responses, connect eutrophication with warming and stratification, then support the explanation using a named example.