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.
Understanding Water Pollution and Eutrophication
The nutrient that limits growth is important. In many freshwater lakes, phosphorus is the main limiting nutrient because it is naturally scarce. A small added amount can therefore produce a large biological response.
In many estuaries and coastal waters, nitrogen is more often the limiting nutrient. Nutrients do not disappear when rain stops. Phosphorus can attach to soil particles and settle into lake sediment.
Later, low oxygen near the bottom can release some of that stored phosphorus back into the water. This internal loading can keep a lake polluted for years after outside inputs have been reduced.
Oxygen conditions change through the day and through the seasons. During daylight, algae and underwater plants produce oxygen through photosynthesis. At night, they respire, as do fish, bacteria, and other organisms.
Oxygen levels can fall sharply before sunrise, especially after a large bloom. Warm water holds less dissolved oxygen than cold water, which makes summer heat especially dangerous. Lakes can also form layers in summer.
Warm surface water may not mix well with colder deep water. Dead algae sinking into the deep layer then feed bacteria there, while little new oxygen reaches it. Scientists use dissolved oxygen measurements at different depths to find these hidden low oxygen zones.
Point sources come from one identifiable outlet, such as a wastewater pipe or factory discharge. They can often be monitored at the outlet and controlled with treatment equipment. Nonpoint sources are spread across a landscape.
Rain can carry fertilizer from fields, pet waste from yards, oil from roads, soil from construction sites, and leaking septic waste into drains or streams. This makes nonpoint pollution harder to manage because no single pipe is responsible.
A stream may look clear after a storm, yet still carry dissolved nitrogen or phosphorus that cannot be seen. Muddy runoff matters too because soil particles can transport phosphorus.
Prevention works best when it happens before pollutants reach water. Farmers can match fertilizer amounts to crop needs, avoid spreading before heavy rain, and plant cover crops that hold soil in place. Vegetated strips beside streams slow runoff and trap sediment.
Towns can improve sewage treatment, repair leaking pipes, and use rain gardens or permeable pavement to reduce fast storm flow. Students should pay attention to the difference between concentration and total load.
A low nutrient concentration carried by a huge flood can still deliver a large amount of pollution. They should also connect land use to water quality, since activities far upstream can affect lakes, rivers, and coastal waters many kilometres away.
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.