4.16-4.17 - Water pollution and eutrophication
Water pollution can damage an ecosystem even when the pollutant does not poison every organism directly. Sewage and fertiliser minerals can both change the growth of microorganisms, algae and aquatic plants. The key biological consequence is often a fall in dissolved oxygen, so animals and other aerobic organisms cannot respire normally.
Sewage as a water pollutant
Sewage is waste water that can contain faeces, urine, organic matter and microorganisms. If untreated or poorly treated sewage enters a river or lake, it adds living microorganisms and waste materials to the water.
Sewage
Sewage is waste water containing human waste and other organic material, often with microorganisms.
There are three biological consequences to keep separate.
| What sewage can add | Biological consequence |
|---|---|
| Pathogenic bacteria or other pathogens | Disease may spread to organisms that contact or drink the polluted water. |
| Organic waste | Decomposer bacteria break it down and respire, using dissolved oxygen from the water. |
| Nitrogenous waste and mineral ions, such as urea, nitrates or phosphates | Extra minerals can increase growth of algae or aquatic plants, leading to eutrophication. |
[DIAGRAM: asset_name: Water pollution and eutrophication - diagram 01; asset_slug: b60_water_pollution_and_eutrophication__diagram_01; recommended_method: deterministic_drawing; description: Monochrome flow diagram showing sewage entering a river, with separate branches for pathogens causing disease, organic waste being decomposed by bacteria using dissolved oxygen, and nitrogenous waste/mineral ions promoting algal growth.]
The oxygen link is exam-important. Decomposer bacteria use oxygen during aerobic respiration. If there is a lot of organic waste, the bacterial population and oxygen demand increase, so the concentration of dissolved oxygen in the water falls. Fish, invertebrates and many other aquatic organisms need dissolved oxygen for aerobic respiration, so they may die or move away, reducing biodiversity.
Fertiliser leaching and eutrophication
Fertilisers are added to soil to supply mineral ions for crop growth. Some fertiliser minerals are soluble, so rainwater can wash them out of the soil and into rivers, lakes or ponds. This movement of dissolved minerals away from the soil is called leaching.
Leaching
Leaching is the movement of soluble mineral ions out of soil and into water.
The specification phrase is "leached minerals from fertiliser". In answers, be more precise than just saying "nutrients" if the question allows it: nitrates and phosphates are useful examples because they can increase plant and algal growth.
Eutrophication
Eutrophication is the enrichment of water with mineral ions, causing excessive growth of algae or aquatic plants and leading to oxygen depletion.
The usual sequence is:
- Fertiliser minerals leach into a river, lake or pond.
- Algae or surface plants grow rapidly, forming an algal bloom or dense surface layer.
- The bloom blocks light from reaching plants lower in the water.
- Underwater plants photosynthesise less and may die.
- Dead algae and plants are decomposed by bacteria.
- Decomposer bacteria respire, using dissolved oxygen.
- Dissolved oxygen falls, so fish and other aquatic organisms die.
[DIAGRAM: asset_name: Water pollution and eutrophication - diagram 02; asset_slug: b60_water_pollution_and_eutrophication__diagram_02; recommended_method: deterministic_drawing; description: Monochrome seven-step eutrophication sequence from fertiliser mineral leaching to algal bloom, blocked light, plant death, bacterial decomposition, lower dissolved oxygen and death of aquatic organisms.]
The word "eutrophication" is useful, but it is not enough by itself for a high-mark answer. Pearson-style mark schemes reward the linked biology: mineral ions, algal growth, light blocked, less photosynthesis or plant death, decomposer respiration, less oxygen, and death of aquatic organisms.
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