(HAO) Cultural Eutrophication Lesson

Cultural Eutrophication

Cultural eutrophication occurs when inorganic nutrients, especially nitrogen and phosphates present in fertilizers, runoff from the land into waterways. Cultural eutrophication is a positive feedback loop that continues to exacerbate the problem.

Eutrophication includes: Nitrogen, Phosphorus: These nutrients cause an increase in phytoplankton Sediments from land block sunlight Phytoplankton growth on Sedgegrass Oxygen Algae Bloom Algae Die Sedgegrass Decay Lose: Food, Habitat & Oxygen Production

First, nutrients enter the water. Nitrogen and phosphorous are often limiting in aquatic ecosystems, so when they enter the water, phytoplankton tend to grow out of control, known as an algae bloom, and exhaust all of their resources.

image of Phytoplankton off of coastline

Picture: An aerial view of Lake Okeechobee in Florida shows an algal bloom. A third of all lakes studied by the USGS contained toxins produced by similar blooms.

This bloom also causes submerged vegetation to die because sunlight cannot penetrate through the algal bloom for them to photosynthesize. The phytoplankton also begin to die because they've exhausted their resources. The dead submerged plants and algae accumulate as detritus on the floor of the waterway. This causes a bloom of decomposing bacteria, which use oxygen and end up actually depleting the oxygen in the water, causing other organisms to die, which increases the amount of detritus, an, thus, increases the amount of decomposing bacteria. Eventually, the area completely runs out of oxygen, which is known as a dead zone.

Dead Zone

Aquatic organisms need oxygen to survive. They "breathe" dissolved oxygen (DO), or oxygen that is dissolved in water. Dissolved oxygen can be reduced when sediments, especially clay, are present in runoff or if fertilizers are present in runoff, which can catalyze cultural eutrophication, which reduces the amount of oxygen in the water. Dead zones are places in water where there is no dissolved oxygen, making it difficult or impossible for life to survive.  

image of dissolved oxygen in Gulf of Mexico

This is what has happened in parts of the Gulf of Mexico because the Mississippi has dumped so many nutrients into the Gulf of Mexico.

graph of BOD and DO concentration compared to distance

When pollutants enter an aquatic ecosystem, they are high near the source of the pollutant, and gradually decrease in concentration the farther away you get from the source. This means that eutrophication and biological oxygen demand (BOD) are highest near the source and dissolved oxygen is lowest near the source.

When you graph this phenomenon of low DO and high BOD near the source of the pollutant and low BOD and high DO the farther you get from the source, it is known as an Oxygen Sag Curve.

Image reads Oceanography

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