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.
Understanding Environmental Science: Algal Blooms and Dead Zones
Nutrients act like plant food. In many fresh lakes, phosphorus is the nutrient that most strongly limits growth. In many coastal waters, nitrogen is often more important.
This pattern is not absolute, because each watershed has its own geology, soils, land use, and water flow. A small amount of extra nutrient can have a large effect when it removes the main limit on growth. Rain is often the trigger.
It washes loose soil, lawn chemicals, animal waste, and leaking wastewater into streams. Those streams carry the nutrients downstream, sometimes over many kilometres.
A bloom does not make oxygen conditions simple. During daylight, algae use sunlight to make sugars and release oxygen. At night, algae respire just like other living things.
They use oxygen and release carbon dioxide. Fish, plants, bacteria, and other organisms are doing the same. Oxygen can therefore be highest in late afternoon and lowest just before sunrise.
A single daytime measurement can make a pond look healthy when it becomes stressful overnight. Scientists measure dissolved oxygen at several depths and times because the pattern matters as much as one reading.
The deepest water is often the most vulnerable. Sun-warmed water near the surface is less dense than cold water below it. A layer can form between them that slows mixing.
Wind may stir the surface without reaching the bottom. Organic particles settle down from above and give bacteria a large food supply near the sediment. As bacteria process this material, they draw oxygen from water that is poorly connected to the air.
Some animals can swim away from low oxygen, but clams, worms, and many small bottom animals cannot. Their loss removes food for fish and reduces the normal recycling of nutrients in sediments.
Low oxygen can create a feedback loop. In oxygen-poor sediments, chemical forms of phosphorus may be released more easily into the water. That phosphorus can support later algal growth when conditions improve.
Dead zones can therefore persist or return even after a single pollution event has ended. Reducing the problem usually starts on land. Farmers can match fertilizer amounts to crop needs, keep vegetation beside streams, and avoid spreading manure before heavy rain.
Cities can repair sewer leaks, manage stormwater, and reduce fertilizer use on lawns. When learning this topic, track the links from land to water, nutrients to growth, growth to decay, then decay to oxygen loss. That chain explains why prevention is usually more effective than trying to remove a bloom after it has formed.
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.