Which of the following would be a result of eutrophication of nitrogen?

A. Undergrowth of water plants.

B. Hypoxia, or low levels of oxygen.

C. Decreased decomposition.

D. Inactivity of fertilizers.

Photo of a very eutrophic pond

The correct answer is B. Hypoxia, or low levels of oxygen.

Eutrophication is the accumulation of excessive amounts of nutrients in the water leading to significant changes in the organisms that are present. In the past, this occurred naturally over a very long time and led to a succession in the environment. However, today humans have greatly accelerated the process leading to what is now called cultural eutrophication.

This process results from the use of fertilizers and also effluents from sewerage and industry entering into rivers and lakes. The nitrogen and phosphorous are the main two minerals that really cause many problems. The eutrophication problem is especially due to the use of fertilizers in agriculture.

The problem with adding so many extra minerals to water is that it promotes the growth of plants and of even more concern, the growth of blue-green algae. These algae are particularly harmful in large numbers since they tend to cause low levels of oxygen in the water and they stop light from moving down through the water column.

Hypoxia causes many animals in the water to die and this then influences the entire food web and ecosystem. Other problems are caused by eutrophication, including changes to carbon, pH of the water, and light availability. In addition, some blue-green algae are toxic and cause other animals to die.


Humans have created artificial ways to enrich the soil in order to increase plant growth and plant yields. They have done this by making fertilizers that contain large quantities of minerals such as phosphorous, potassium and nitrogen. These minerals are needed by plants which take them up via the root system.

The problem is that some of these minerals are washed into rivers and lakes. This runoff then accumulates in the aquatic ecosystem where it acts to promote the growth of plants and blue-green algae. These blue-green algae are classified as cyanobacteria and are actually prokaryotic organisms.

Other human activity can also add excess nutrients to the water; for instance, deforestation leading to soil erosion, and effluents containing human and industrial wastes. Eutrophication can also occur in estuaries and marine systems.

The North Sea, for instance, shows the impact of cultural eutrophication. It has decreasing levels of phytoplankton and increasingly high levels of blue-green algae. This is due to a drop in silica that is needed by diatoms, and instead, there is too much phosphorous and nitrogen.

Much of the problem is the rate and amount of nitrogen being added to freshwater and marine environments, with more nitrogen being added than can be used up by consumers.


Low oxygen levels can be caused when large numbers of the blue-green algae die and sink down to the bottom of the lake. Accumulations of organic matter may also lead to decreased water volume in the lake.

The problem with having depleted oxygen in water is that other animals are then adversely affected. Fish, for instance, are very sensitive to the oxygen concentration in the water, and these are often the first organisms to die under hypoxic conditions.

During the summer, some lakes can have very low levels of oxygen near the bottom. In such eutrophic systems, oxygen levels can be as low as 1mg/L. Oxygen can drop so low that anoxic conditions may occur which severely limits what organisms can be supported.

Carbon availability and pH

The overgrowth of cyanobacteria can also result in significant alterations in both the pH and the levels and amount of carbon available in the water.

The amounts of inorganic carbon can be decreased by the increased primary productivity. In addition, the pH may increase to unacceptable levels. This all has consequences for the organisms that live in the water.


The large blooms of cyanobacteria can, in fact, block sunlight and prevent it from reaching to lower levels of the water column. These organisms can form large scums on the surface because they have gas bubbles in the cells to allow them to float.

The reduced levels of light may cause the death of some species of submerged aquatic plants. These plants are often important in providing living spaces for small insects and crustaceans. Dragonfly and damselfly larvae use vegetation for perches to hide from predators and to hunt prey.

A lack of such vegetation would cause these animals to fall prey to other predators. This would then change the dynamics of the food web, causing a cascading effect.

Predatory fish may also be impacted as they rely heavily on vision to catch prey. The numbers of fish may then drop which again will have ramifications throughout the ecosystem. Species diversity, in general, is likely to drop under eutrophic conditions.


Blue-green algae include species that are actually toxic to wildlife. Species such as Microcystis, Anabaena and Cylindrospermopsis, have actually been discovered to be very toxic. In fact, livestock has been recorded to actually be killed by drinking water which has abundant levels of toxic cyanobacteria present.

The toxins of blue-green algae are called cyanotoxins, and they often are toxic to the liver; for instance in the case of Microcystis.

These microcystins are also particularly harmful because they are produced inside the cells of the blue-green algae. They thus rapidly accumulate as the biomass of the cells increases.

There are concerns in many countries, that people will be exposed to these toxins. It is due to this that monitoring programs are put in place to check levels of cyanobacterial biomass and toxins. In some cases. The World Health Organization has, in fact, developed a safety plan for dealing with blue-green algae.


  1. Editors of Encyclopedia Britannica (2018). Hydrosphere. Retrieved from Encyclopedia Britannica.
  2. Editors of Encyclopedia Britannica (2018). Eutrophication. Retrieved from Encyclopedia Britannica.
  3. MF Chislock, E Doster, RA Zitomer, AE Wilson (2013). Eutrophication: causes, consequences, and controls in aquatic ecosystems. Nature Education Knowledge.
  4. HW Paerl, MF Piehler (2008). Nitrogen and marine eutrophication. Nitrogen in the marine environment.
  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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