AP® Environmental Science · Unit 8 · Topic 8.5
Dissolved Oxygen, BOD, and Dead Zones
Dissolved oxygen is the oxygen gas fish and aquatic organisms breathe; biochemical oxygen demand measures how much of it decomposers will use up breaking down organic pollution, and heavy pollution can drain so much oxygen that it creates a dead zone.
Water holds oxygen as a dissolved gas, and warm water holds less of it than cold water because gas solubility drops as temperature rises. That is a cruel timing problem: warm summer water holds the least oxygen right when fish, with faster metabolisms in the heat, need the most. Add organic pollution like sewage, manure, or dead plant matter, and the problem compounds. Decomposer bacteria swarm the organic matter and respire, consuming dissolved oxygen as they break it down. Biochemical oxygen demand, BOD, is a direct measurement of this: it is the amount of dissolved oxygen that decomposers remove from a water sample over five days at a fixed temperature. High BOD means lots of food for bacteria, which means lots of oxygen about to disappear.
Picture a sewage pipe emptying into a river. Right at the outfall, dissolved oxygen is still near normal, but organic waste is flooding in. A short distance downstream, bacteria have multiplied and are consuming oxygen faster than the river can replace it, so dissolved oxygen plunges to a minimum. This dip is the bottom of the oxygen sag curve. Farther downstream, the organic load has mostly been consumed, bacterial activity falls off, and the river re-absorbs oxygen from the atmosphere and from photosynthesizing algae, so oxygen climbs back toward normal. Fish kills cluster right at the sag's low point.
A faster-flowing river reshapes that curve: more turbulence mixes in atmospheric oxygen faster, so the sag is shallower and the river recovers in a shorter distance. A sluggish river or one choked with extra nutrients fueling algae sags harder and recovers more slowly, which is exactly the mechanism behind eutrophication and dead zones downstream.
Key terms
- Dissolved oxygen (DO)
- Oxygen gas molecules dissolved in water, available for fish and other aquatic organisms to breathe.
- Biochemical oxygen demand (BOD)
- The amount of dissolved oxygen decomposers consume breaking down organic matter in a water sample over five days.
- Oxygen sag curve
- The pattern of dissolved oxygen dropping sharply below a pollution source then gradually recovering farther downstream.
- Dead zone
- An area of water so depleted of dissolved oxygen that most aquatic life cannot survive there.
- Eutrophication
- Nutrient enrichment of water that fuels excess algae growth, whose decomposition later drives oxygen levels down.
Watch it explained
- How it worksHow does biochemical oxygen demand measure what decomposers will take out of the water?
- Worked exampleFind the dissolved oxygen after 5 days starting at 9 mg per liter with a BOD of 6 mg per liter.
- How it worksWalk me through an oxygen sag curve forming downstream of a sewage outfall.
- Going furtherWhat happens to that sag curve when the same outfall discharges into a faster river?
Check yourself
End of topic.