Eutrophication (HL IB ESS OLD COURSE - IGNORE)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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
Nutrient enrichment:
Excess nitrates and phosphates enter the water
This encourages rapid growth of phytoplankton, algae and aquatic plants
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
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
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
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
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:
Increased nutrients:
Excess nitrates and phosphates from run-off or sewage
Promotes rapid growth of algae and aquatic plants
Increased death:
Algae and plants die off in large numbers
Adds organic matter to the water
Increased decomposition:
Bacteria decompose dead organisms, consuming oxygen
This decomposition releases more nutrients back into the water
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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