Eutrophication (HL IB ESS OLD COURSE - IGNORE)

Revision Note

Eutrophication

What is eutrophication?

  • Eutrophication occurs when water bodies like lakes, estuaries and coastal areas receive large amounts of mineral nutrients, mainly nitrates and phosphates

    • This often results in the excessive growth of phytoplankton, a type of microscopic algae, as well as aquatic plants

  • Main nutrients involved:

    • Nitrates: often from agricultural run-off

    • Phosphates: commonly found in detergents and sewage that is discharged into waterways without proper treatment

The process of eutrophication

  1. Nutrient enrichment:

    • Excess nitrates and phosphates enter the water

    • This encourages rapid growth of phytoplankton, algae and aquatic plants

  2. Excessive aquatic plant growth:

    • Nutrient availability causes fast growth of aquatic plants (macrophytes) e.g. duckweed and water hyacinth

    • Dense plant growth nearer the surface can block sunlight reaching underwater plants

  3. Algal bloom formation:

    • Algae also use available nutrients to grow quickly

    • For example, when the mineral ions from excess fertilisers leach from farmland into waterways, they cause rapid growth of algae at the surface of the water

    • This is known as an algal bloom

    • Eventually, algae can completely cover the water surface

  4. Blocking of sunlight:

    • The algal bloom can completely block out sunlight and stop it from penetrating below the water surface

    • Aquatic plants below the water surface start to die as they can no longer photosynthesise

      • As this photosynthesis normally helps to oxygenate the water, dissolved oxygen levels begin to decrease

    • The algae also start to die when competition for nutrients becomes too intense Phytoplankton and excess aquatic plants die off

  5. Decay of phytoplankton and plants leading to oxygen depletion:

    • Bacteria decompose the dead plants and algae

    • As the bacteria respire aerobically, they use up the dissolved oxygen in the water

    • The amount of dissolved oxygen in the water rapidly decreases, so aquatic organisms such as fish and insects may be unable to survive

    • Dead zones in both oceans and freshwater can occur when there is not enough oxygen to support aquatic life

    • Hypoxia = low oxygen levels in water

    • Anoxia = severe or complete depletion of oxygen in water

  6. Impact on aquatic life:

    • Fish and other aquatic life die in large numbers due to lack of oxygen

    • This can eventually lead to a loss of species and imbalances in aquatic ecosystems

Diagram showing the sequence of events causing eutrophication in lakes, rivers and coastal waters
The sequence of events causing eutrophication in lakes, rivers and coastal waters

Positive feedback in eutrophication

  • Positive feedback amplifies changes, creating a reinforcing cycle in eutrophication:

  1. Increased nutrients:

    • Excess nitrates and phosphates from run-off or sewage

    • Promotes rapid growth of algae and aquatic plants

  2. Increased death:

    • Algae and plants die off in large numbers

    • Adds organic matter to the water

  3. Increased decomposition:

    • Bacteria decompose dead organisms, consuming oxygen

    • This decomposition releases more nutrients back into the water

  4. Cycle repeats:

    • Released nutrients promote further algal and plant growth

    • Each step reinforces the next, worsening eutrophication and its impacts on the aquatic ecosystem

Impacts of eutrophication

  • Eutrophication can greatly affect various ecosystem services:

  • Fisheries:

    • Fish kills: sudden losses of fish due to low oxygen

    • Reduced fish stocks: long-term depletion of fish populations in certain areas

  • Recreation and aesthetics:

    • Unpleasant odours: decaying algae and plants release unpleasant smells

    • Water quality: poor water conditions make swimming and boating unpleasant

    • Visual pollution: algal blooms create green or murky water

    • Foam and slime: algal blooms and decaying algae can cause foam and slimy water surfaces

  • Health:

    • Toxins: some algal blooms produce harmful toxins

    • Drinking water: eutrophication can lead to contamination of drinking water sources

Examiner Tips and Tricks

Pay attention to the difference between key terms such as eutrophication, decomposition, hypoxia and anoxia.

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