Which of these are the two major sources of nitrate pollution in rivers?

a. The burning of fossil fuels by factories and cars.

b. Animal wastes and the burning of fossil fuels by cars.

c. Animal wastes and fertilizers.

d. Fertilizer runoff and the burning of fossil fuels by cars.

e. Animal wastes and the burning of fossil fuels by factories.

Photo of a very eutrophic pond

The correct answer is c. animal wastes and fertilizers.

The cycling of nitrogen through the environment is important since it is an important substance that is used by living organisms to make the biological molecules, amino acids, and nucleic acids.

Bacteria in the soil fix large amounts of atmospheric nitrogen. The bacterial species convert nitrogen into nitrates and nitrites which can then be taken up by the root hairs of plants. Animals take in nitrogen ions when they then feed on the animals.

When plants and animals die and decay, bacteria break down the tissues and transform the nitrogenous products into ammonium in the soil and water. This then is converted by other bacteria into usable nitrites and nitrates, and the cycle continues.

Humans have had a large impact on the nitrogen cycle through agricultural and forestry practices. The farming of livestock has resulted in large quantities of animal wastes entering wetlands when it rains.

In addition, the extensive use of fertilizers has resulted in even more nitrogen ions, along with phosphorous, and potassium, being added to soils. These nutrients also find their way into aquatic ecosystems when it rains and soil washes off into water bodies.

The accumulation of excess nutrients, including nitrates, results in eutrophication of wetlands. The problem with eutrophication is that it favors the growth of certain blue-green algae, which results in vast blooms of these algal species.

The consequence of these algal blooms is that it reduces oxygen and light levels in the water, resulting in some organisms dying out as a result. A further problem is that some species of blue-green algae produce cyanotoxins which kill animals.

The nitrogen cycle

Nitrogen is an important part of molecules, including nucleic acids and proteins. Different forms of nitrogen are found in the atmosphere and in the soil and water.

Nitrogen gas in the atmosphere has to first be fixed before it can be used by most organisms. Bacteria and archaea in the soil do most of the biological nitrogen fixation, and in fact, many plants have symbiotic bacteria called Rhizobium associated with their roots.

These bacteria help the plants by fixing the nitrogen into a form that the plants can then use. Any nitrates taken up by plants are converted into ammonia and nitrites.

Ammonia is a toxic product for plants so bacteria convert this into either nitrites or nitrates, which can then be taken up by the root hairs of plants. Animals then take in the nitrogenous products when they feed on the plant matter.

The nitrogen ions are then used to form biological molecules in the tissues of the animal. When living organisms die and decay the nitrogenous products are released back into the surroundings, bacteria convert these waste materials into the form of ammonium.

Nitrates are relatively soluble and therefore can easily enter the water. The problem is that human activities add excess amounts of nitrates to the environment.

Livestock such as cattle and sheep contribute large amounts of waste that every year runs off the land and into aquatic systems.

The other issue is that people use fertilizers, which adds additional nitrogen ions to the soil. Many farmers use fertilizers that contain nitrogen, phosphorous and potassium in various proportions. When it rains these nutrients find their way into small streams and rivers and lakes.

Nutrients and eutrophication

The input into wetlands of excess quantities of phosphorous, nitrates and nitrites causes the condition of eutrophication. Besides fertilizers and animal wastes, deforestation and human wastes also add to the problem of eutrophication.

Even though a certain number of nutrients are needed by all living organisms, too many of any particular nutrients can cause a problem for the aquatic ecosystem.

Usually, the nutrients that enter a wetland are taken up and used by plants living in the wetland. These nutrients are passed on to animals when they feed on these plants.

It is crucial though that there not be too little or too much of any substance since this will destabilize the ecosystem. If too many nitrates enter the water it can make the water of the aquatic ecosystem become eutrophic.

While it may seem beneficial to have lots of nutrients added to the water, the problem is that it favors the development of particular types of blue-green algae (cyanobacteria).

The problem with this is that these algae grow so fast as to form a bloom. Such algal blooms cause the oxygen levels in the water to drop very low and they cut off the light to lower levels of water.

A lack of oxygen seriously impacts aquatic life such as fish and the reduction in light levels causes problems for animals that hunt by sight.

This can cause entire species to be eliminated from the ecosystem which then causes a drastic reduction in biodiversity in the wetland. This is not only a problem in freshwater but also a concern in marine ecosystems.

Eutrophication and toxic species

Some of the algae that become overly abundant are actually toxic to other organisms. Species of cyanobacteria such as Anabaena, Microcystis, and Cylindrospermopsis can form large blooms in response to eutrophication.

The problem is that these species have been found to be toxic to animals. These toxins are known as cyanotoxins and they tend to attack the liver of animals that consume the water.

This can cause large numbers of animals to die when they drink water that contains large toxic cyanobacterial blooms.

This is a concern for human health as well and many countries have instituted monitoring programs to determine how badly water is impacted by toxic cyanobacteria.


  1. Editors of Encyclopedia Britannica (2018). Eutrophication. Retrieved from Encyclopedia Britannica.
  2. MF Chislock, E Doster, RA Zitomer, AE Wilson (2013). Eutrophication: causes, consequences, and controls in aquatic ecosystems. Nature Education Knowledge.
  3. HW Paerl, MF Piehler (2008). Nitrogen and marine eutrophication. Nitrogen in the marine environment.
  4. Editors of Encyclopedia Britannica (2018). Nitrogen cycle. Retrieved from Encyclopedia Britannica.
  5. BW Ibelings, LC Backer, WEA Kardinaal, I Chorus (2014).  Current approaches to cyanotoxin risk assessment and risk management around the globe. Harmful Algae.


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