AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-5)
Unit 8 · Topic 8.5
8.5 Eutrophication
Eutrophication happens when extra nitrogen and phosphorus enter water and fuel an explosion of algae. When the algae die, decomposing bacteria use up the dissolved oxygen, causing fish kills and dead zones. It is one of the most widespread water quality problems in the world.
Key terms
- eutrophication
- algal bloom
- dissolved oxygen
- hypoxia
- dead zone
- oligotrophic
Nutrient levels in water
Nitrogen and phosphorus are nutrients that plants and algae need to grow, and in most waters one of them is in short supply. That nutrient is the limiting factor: adding more of it lets growth take off. Phosphorus usually limits growth in fresh water, and nitrogen usually limits it in coastal ocean water.
An oligotrophic lake is low in nutrients: clear, with little algae, high dissolved oxygen and often deep. A eutrophic lake is rich in nutrients: green and murky, with lots of algae and low oxygen in deeper water. Lakes naturally become more nutrient-rich over thousands of years, but human inputs speed this up enormously, which is called cultural eutrophication.
The sequence of events
- 1. Excess nitrogen and phosphorus enter the water from fertilizer runoff, animal manure, sewage and detergents with phosphates.
- 2. Algae grow rapidly into an algal bloom, turning the water green.
- 3. The bloom blocks sunlight, so plants on the bottom can't photosynthesize and die.
- 4. The algae have short lives. As they die, they sink.
- 5. Bacteria decompose the dead algae and plants. Decomposition is aerobic respiration, which uses up dissolved oxygen (oxygen gas dissolved in the water).
- 6. Dissolved oxygen falls to hypoxic levels (very low, often defined as below about 2 mg/L) or even zero.
- 7. Fish and other animals that can't swim away suffocate, causing fish kills. Their decomposition uses even more oxygen, a positive feedback loop.
Dead zones and harmful blooms
A dead zone is an area of water so low in oxygen that most animal life cannot survive. The best-known US example forms each summer in the Gulf of Mexico at the mouth of the Mississippi River, which drains farmland across much of the central United States. It often covers several thousand square miles; the largest measured, in 2017, covered about 8,800 square miles.
Some algal blooms, called harmful algal blooms, also release toxins that can poison fish, pets and people and shut down drinking-water systems.
Solutions
Reduce nutrient inputs: apply fertilizer at the right time and amount, plant vegetated buffer strips along streams to catch runoff, restore wetlands that absorb nutrients, keep livestock waste out of streams, ban phosphates in detergents, and add tertiary treatment at sewage plants to remove nitrogen and phosphorus (topic 8.11).
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Explaining a dissolved oxygen graph
Dissolved oxygen (DO) is measured in a river at points above and below a farm drainage ditch. Upstream DO is 9.0 mg/L. About 2 km downstream, after a large algal bloom, it drops to 1.5 mg/L, then slowly recovers farther downstream. (a) Calculate the percent decrease in DO. (b) Explain why DO dropped.
Show the solutionHide the solution
- Step 1: (a) Percent change = (1.5 − 9.0) ÷ 9.0 × 100 = −7.5 ÷ 9.0 × 100 ≈ −83.3%, an 83% decrease.
- Step 2: (b) Fertilizer in the drainage water added nitrogen and phosphorus, causing an algal bloom.
- Step 3: When the algae died, bacteria decomposed them using aerobic respiration, which consumed dissolved oxygen faster than it could be replaced.
- Step 4: Farther downstream, nutrients are diluted and used up and the water mixes with air, so DO gradually recovers.
Answer: (a) About an 83% decrease. (b) Nutrients caused an algal bloom; decomposition of the dead algae by bacteria used up the oxygen.
- Example 2
Trap: who actually uses up the oxygen
A student writes: 'Algal blooms cause dead zones because the algae use up all the oxygen.' Improve this explanation.
Show the solutionHide the solution
- Step 1: Living algae photosynthesize and release oxygen during the day, so they are not the main oxygen drain.
- Step 2: The big oxygen loss happens after the algae die and sink: bacteria decomposing the dead algae use aerobic respiration, consuming dissolved oxygen.
- Step 3: Algae do use some oxygen at night through respiration, which can add to the low-oxygen problem, but decomposition is the main cause.
Answer: Decomposing bacteria breaking down dead algae use up the dissolved oxygen; that is the main cause of hypoxia, not the living algae themselves.
Common mistakes
- Leaving out the decomposition step. Exam scorers look for 'bacteria decompose the dead algae and use up dissolved oxygen.'
- Mixing up oligotrophic (low nutrients, clear, high oxygen) and eutrophic (high nutrients, murky, low oxygen).
- Naming only nitrogen. Phosphorus is often the limiting nutrient in fresh water.
On the exam
- Eutrophication is one of the most tested topics in the course. Practice writing the full chain from nutrient input to fish kill in order.
- When asked for a solution, name a specific one (buffer strips, reducing fertilizer, tertiary treatment) and explain how it cuts nutrient input.
Connected topics
Videos
Check yourself
4 questions on 8.5 Eutrophication. Pick an answer to see if you got it, and why.
Heavy spring rains wash fertilizer from surrounding cornfields into a small lake. Monitoring over the summer finds:
• June: nitrate and phosphate levels are high, and the water turns bright green with algae.
• July: large amounts of dead algae sink to the bottom; bacteria counts in the deep water rise sharply.
• August: dissolved oxygen near the bottom falls below 2 mg/L, and dead fish wash up on shore.
Described scenario
Which statement best explains the drop in dissolved oxygen in August?
Before the fertilizer runoff, the lake had clear water, few nutrients and high dissolved oxygen. A lake like this is described as
Which action would most effectively prevent this problem in future years?
A treatment plant discharges into a bay that suffers from algal blooms every summer. Which upgrade would most directly reduce the plant's contribution to the blooms?
0 of 4 answered