Eutrophication (DP IB Environmental Systems & Societies (ESS))

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

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