Algal blooms happen when algae grow rapidly near the surface of a lake, estuary, or coastal bay. They matter because they can block sunlight, release toxins, and change the chemistry of the water. The most serious danger often comes after the bloom dies, when decomposers use up dissolved oxygen.
When oxygen falls too low, fish, crabs, and many bottom-dwelling organisms cannot survive.
Key Facts
- Eutrophication is nutrient enrichment of water, usually by nitrogen and phosphorus.
- Common nutrient sources include fertilizer runoff, sewage, manure, and stormwater from cities.
- Photosynthesis by algae: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- Decomposition uses oxygen as bacteria break down dead algae and organic matter.
- Hypoxia often means dissolved oxygen is below about 2 mg/L, which stresses or kills many aquatic animals.
- Dead zones are common in stratified waters where warm, fresh, or less dense surface water does not mix well with deeper water.
Vocabulary
- Algal bloom
- A rapid increase in algae or cyanobacteria in water, often caused by excess nutrients and warm, sunny conditions.
- Eutrophication
- The process in which a body of water becomes enriched with nutrients, leading to excessive plant or algal growth.
- Dissolved oxygen
- Oxygen gas mixed into water that aquatic organisms use for respiration.
- Hypoxia
- A condition in water where dissolved oxygen is so low that many organisms become stressed or die.
- Watershed
- An area of land where water drains into the same river, lake, estuary, or ocean area.
Common Mistakes to Avoid
- Thinking algae directly remove all the oxygen during photosynthesis, which is wrong because algae produce oxygen in sunlight but oxygen is heavily consumed later during decomposition.
- Assuming clear water is always healthier than green water, which is wrong because some harmful nutrients or toxins may be invisible and clear water can still have low oxygen at depth.
- Forgetting that nitrogen and phosphorus come from land, which is wrong because fertilizer, sewage, manure, and storm drains often drive blooms far downstream.
- Treating dead zones as permanent everywhere, which is wrong because many expand seasonally when warm stratified water limits mixing and shrink when storms or cooling restore oxygen.
Practice Questions
- 1 A bay has dissolved oxygen of 7 mg/L in spring and 1.5 mg/L in late summer. By how many mg/L did the oxygen decrease, and is the late summer water hypoxic if hypoxia is below 2 mg/L?
- 2 A farm field loses 12 kg of nitrogen in runoff after a storm. If 25 percent reaches a nearby lake, how many kilograms of nitrogen enter the lake?
- 3 Explain why a thick algal bloom can lead to dead fish even though algae can produce oxygen during photosynthesis.