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Water Pollution and Eutrophication
Water Pollution and Eutrophication
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Water pollution happens when harmful substances or excess nutrients enter rivers, lakes, groundwater, or oceans and change water quality. One major form is eutrophication, a process in which too much nitrogen and phosphorus causes rapid growth of algae and aquatic plants. This matters because polluted water can harm drinking supplies, fisheries, recreation, and entire ecosystems. Eutrophication is especially important because it can begin with common human activities such as farming, sewage discharge, and fertilizer runoff.
In a healthy lake, nutrients are present at low to moderate levels and oxygen stays available for fish and other organisms. When nutrient input becomes too high, algal blooms spread across the surface and block sunlight from reaching deeper water. As algae die, decomposers break them down and use dissolved oxygen, which can create hypoxic or anoxic conditions. The result is a chain reaction that reduces biodiversity, kills aquatic life, and shifts the lake from a balanced system to a stressed one.
Key Facts
- Eutrophication is nutrient enrichment of water, mainly by nitrogen (N) and phosphorus (P).
- A common pathway is fertilizer runoff + sewage input -> algal bloom -> decomposition -> oxygen depletion.
- Dissolved oxygen is often abbreviated DO and is measured in mg/L.
- Photosynthesis in algae and plants can be summarized as 6CO2 + 6H2O -> C6H12O6 + 6O2.
- Cellular respiration and decomposition use oxygen: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy.
- Low oxygen water is called hypoxic, and near-zero oxygen water is called anoxic.
Vocabulary
- Eutrophication
- Eutrophication is the enrichment of a water body with excess nutrients that leads to heavy plant and algal growth.
- Algal bloom
- An algal bloom is a rapid increase in algae population that can discolor water and disrupt aquatic ecosystems.
- Dissolved oxygen
- Dissolved oxygen is the amount of oxygen gas mixed into water and available for aquatic organisms to use.
- Hypoxia
- Hypoxia is a condition in which dissolved oxygen levels are too low to support many aquatic animals.
- Nutrient runoff
- Nutrient runoff is water flow from land that carries fertilizers, manure, or other nutrient sources into streams and lakes.
Common Mistakes to Avoid
- Thinking all plant growth in water is harmful, which is wrong because aquatic plants and algae are normal parts of healthy ecosystems at balanced nutrient levels.
- Assuming eutrophication means toxic chemicals were added directly, which is wrong because ordinary nutrients like nitrogen and phosphorus can trigger major ecological damage when present in excess.
- Believing algal blooms increase oxygen for fish all the time, which is wrong because after the algae die, decomposition often removes large amounts of dissolved oxygen from the water.
- Ignoring the source of nutrients on land, which is wrong because preventing eutrophication usually requires reducing runoff from farms, lawns, and sewage systems before it reaches the water.
Practice Questions
- 1 A lake receives phosphorus-rich runoff after heavy rain. If algal coverage rises from 15% of the lake surface to 60%, by what factor did the coverage increase?
- 2 A water sample has dissolved oxygen of 9 mg/L in spring and 3 mg/L after a summer algal bloom. What is the decrease in dissolved oxygen, and what percent decrease is this?
- 3 Explain why fish deaths may occur several days after an algal bloom begins rather than at the exact moment nutrients enter the lake.