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Unlock This CourseEutrophication is one of the most serious environmental problems caused by human activities. It happens when too many nutrients, especially nitrogen and phosphorus, enter water bodies like lakes, rivers and coastal seas. This might sound harmless, but it actually creates a chain reaction that can kill fish, destroy ecosystems and make water unsafe for humans to use.
Think of it like over-fertilising your garden - too much of a good thing becomes harmful. When water gets "over-fertilised" with nutrients, it leads to massive algae growth that can suffocate aquatic life.
Key Definitions:
Natural eutrophication happens very slowly over thousands of years as nutrients gradually accumulate. Human-caused eutrophication happens much faster - sometimes in just a few decades - because we add huge amounts of nutrients through pollution.
Human activities are the primary drivers of modern eutrophication problems. The main culprits are activities that release nitrogen and phosphorus compounds into water systems.
Farming is the biggest contributor to eutrophication worldwide. When farmers use fertilisers containing nitrogen and phosphorus to help crops grow, not all of these nutrients stay in the soil.
Rain washes excess fertilisers from fields into nearby streams and rivers. This is especially bad during heavy rainfall or when fertilisers are applied just before storms.
Livestock produce huge amounts of manure rich in nitrogen and phosphorus. When not properly managed, this waste can leak into water systems.
Poor farming practices can cause nutrient-rich topsoil to wash away into waterways, carrying fertilisers and organic matter with it.
Cities and industries also contribute significantly to eutrophication through various pollution sources.
Human waste contains high levels of nitrogen and phosphorus. Even treated sewage often releases these nutrients into water bodies. Untreated sewage is even worse, directly dumping nutrients into rivers and seas.
Factories, especially those processing food or chemicals, can release nutrient-rich wastewater. Paper mills, food processing plants and chemical factories are common sources.
Eutrophication follows a predictable sequence of events. Understanding this process helps explain why it's so damaging to aquatic ecosystems.
The eutrophication process can be broken down into several key stages, each leading to the next in a destructive cycle.
➡ Excess nitrogen and phosphorus enter the water body through runoff, sewage discharge, or atmospheric deposition. These nutrients act like fertiliser for aquatic plants and algae.
🌱 With abundant nutrients available, algae multiply rapidly, forming thick green mats on the water surface. This explosive growth is called an algal bloom and can happen within days or weeks.
☀ The dense algae layer blocks sunlight from reaching underwater plants. Without light, these plants cannot photosynthesise and begin to die off.
🐛 Dead algae and plants sink to the bottom where bacteria decompose them. This decomposition process uses up large amounts of dissolved oxygen from the water.
🐟 Oxygen levels drop dramatically, creating hypoxic (low oxygen) or anoxic (no oxygen) conditions. Fish and other aquatic animals struggle to breathe and may die in large numbers.
The impacts of eutrophication extend far beyond just making water look green and smelly. It creates serious problems for both aquatic ecosystems and human communities.
Massive fish deaths occur when oxygen levels drop too low. Entire fish populations can be wiped out, disrupting food chains and destroying fishing industries.
Many species cannot survive in eutrophic conditions. Native plants and animals are replaced by pollution-tolerant species, reducing overall biodiversity.
Important habitats like seagrass beds and coral reefs are destroyed when they cannot get enough light or oxygen to survive.
Eutrophic water often smells bad, tastes unpleasant and may contain harmful toxins produced by certain types of algae. This makes water unsafe for drinking, swimming, or other recreational activities.
Tourism, fishing and water treatment costs increase dramatically. Property values near affected water bodies often decline and businesses dependent on clean water may be forced to close.
Lake Erie, one of North America's Great Lakes, experiences severe eutrophication problems. Agricultural runoff from surrounding farmland, particularly from Ohio, Michigan and Ontario, delivers massive amounts of phosphorus to the lake each year. This creates algal blooms so large they can be seen from space. In 2014, the city of Toledo had to shut off water supplies to 400,000 people because toxic algae made the water unsafe to drink. The dead zone in Lake Erie now covers thousands of square kilometres during summer months, devastating fish populations and the local fishing industry.
Preventing eutrophication requires reducing nutrient inputs from human activities. This involves changes in farming practices, better waste treatment and stronger environmental regulations.
Using GPS and soil testing to apply fertilisers only where and when needed, reducing waste and runoff.
Planting vegetation along waterways to filter nutrients before they reach water bodies.
Applying fertilisers when plants can best use them, avoiding application before heavy rains.
Upgrading wastewater treatment plants to remove more nitrogen and phosphorus before releasing water back into the environment.
Building rain gardens, permeable pavements and constructed wetlands to filter stormwater runoff naturally.
The River Thames in London was once severely polluted and eutrophic, with very little aquatic life. Through strict pollution controls, improved sewage treatment and reduced industrial discharge starting in the 1960s, the Thames has made a remarkable recovery. Today, over 125 species of fish live in the Thames, including dolphins and seals that occasionally visit from the North Sea. This shows that eutrophication can be reversed with proper management and political commitment.