Eutrophication (College Board AP® Environmental Science): Study Guide

What is eutrophication?

• Eutrophication occurs when a body of water is enriched in nutrients, typically nitrates and phosphates

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

• There are two types of eutrophication

  • Natural eutrophication, which is a slow, organic process occurring due to the build-up of sediment and nutrient enrichment

  • Cultural or anthropogenic eutrophication, however, is a fast, human-induced water pollution typically caused through:

    • agricultural run-off

    • detergents and sewage that are discharged into waterways without proper treatment

    • nitrogen from animal breeding and fuel combustion

Methods of reducing cultural eutrophication

• The aim should be to reduce nutrient inputs from point and non-point sources , particularly phosphorus and nitrogen

• Methods include:

  • improving sewage treatment to remove phosphorous and nitrogen

  • reducing fertilizer use to minimize nutrient runoff

  • recycling animal manure to reduce nutrient leaching

  • riparian buffer strips

  • improving wetlands as they act as natural filters

  • public awareness and education to reduce consumption of protein-rich foods

The process of eutrophication

• The process of eutrophication occurs in stages

• Nutrient enrichment from excess nitrates and phosphates enters the water and increases phytoplankton, algae, and aquatic plant growth

• This causes dense aquatic plant growth to occur near the surface that blocks sunlight reaching underwater plants

• An algal bloom forms that quickly uses available nutrients and eventually, the bloom completely covers the water's surface

• Aquatic plants below the water surface begin to die as they are unable to photosynthesize

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

• When competition for nutrients becomes too intense, the algae, phytoplankton and plants begin to die

• The decay of phytoplankton and plants leads to further oxygen depletion because

  • decomposing bacteria respire aerobically, using up the already depleted dissolved oxygen in the water

  • aquatic organisms such as fish and insects are then unable to survive, resulting in large die-offs

Stages of eutrophication

Hypoxic waterways

• Hypoxic waterways are those bodies of water that are low in dissolved oxygen

• Caused through:

  • excess nutrients forming algal blooms and depleting dissolved oxygen

  • the breakdown of organic matter like sticks, grass, and crops by bacteria, particularly following floods

  • water stratification due to temperature or salinity variations can cause hypoxia

• The effects of hypoxia are

  • large-scale die-off within the waters

  • illness from eating shellfish poisoned by harmful algal blooms

  • on-shore death of marine mammals and birds from eating the algal bloom

  • the lack of aquatic life in hypoxic environments causes 'dead zones'

• Estuaries, coastal waterways, lakes, rivers, and streams can become hypoxic

• Chesapeake Bay, Lake Erie, the Gulf of Mexico, and Long Island Sound are all hypoxic

Oligotrophic waterways

• Oligotrophic waters are usually seen in upland areas with hard, acid rocks

• Compared to eutrophic waterways, oligotrophic water has

  • very clear, deep and cold water

  • firm sandy beds

  • low nutrient concentrations

  • stable algae populations

  • high levels of dissolved oxygen

  • low numbers of aquatic organisms

• Lake Superior is considered the most oligotrophic lake in the world, although it still has enough nutrients for plants and animals to survive

• Oligotrophic waters tend to stratify into three layers in summer, with the highest temperatures in the first 5–24 meters of water due to warming by the sun

  • Within in the upper layer, the epilimnion, water is mixed by wind and wave movement

    • Photosynthesis and atmospheric diffusion keep dissolved oxygen levels high in summer

  • The metalimnion or thermocline, acts as a temperature barrier

    • This prevents mixing and heat exchange between the layers

  • The hypolimnion is the lowest and coldest

    • Organisms rely on detrital fallout from the epilimnion and oxygen levels are quickly used

    • Oxygen is replenished when thermal stratification breaks down and the water mixes again

    • This is called ‘overturn’ and occurs in autumn or winter

Anthropogenic causes of eutrophication

• Humans increase the rate and incidence of eutrophication by releasing high levels of nitrogen and phosphorus into nature

• These levels occur from:

  • Rain washing fertilizers from farms, lawns, and golf courses into rivers and lakes

  • Wastewater from sewage treatment plants can cause nutrient contamination

  • Waste from industry

  • Burning fossil fuels

  • Aquaculture

  • Livestock rearing

  • Transporting livestock feed and manure can release excess nutrients

• In developing countries, untreated sewage and industrial wastewater are the main cause of eutrophication

• Typically, sewage facilities and manufacturers are less regulated; therefore, wastewater is discharged directly into waterways