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The Effects of Nitrogen on Aquatic Animals

Nitrogen is one of the most abundant gases in the atmosphere and makes up 80 percent of the air we breathe. It is a key component in many biological processes like protein synthesis, and the nitrogen cycle is an essential ecological process. However, high levels of some compounds of nitrogen, namely ammonia, nitrates and nitrites cause havoc in aquatic environments. Aquatic animals and ecosystems are particularly sensitive to nitrites --- levels of just 2 parts per million are enough to kill some species of fish.

Nitrite Toxicity
- Nitrite toxicity, also known as brown blood disease, is a condition that is harmful to fish. It is caused by excessive levels of nitrites, a by-product in the breakdown of ammonia by bacteria. Nitrites enter the bodies of fish through the gills and cause their blood to turn dark brown. This happens because nitrites oxidize hemoglobin in the red blood cells, creating methemoglobin. Methemoglobin is a less efficient oxygen transporter than hemoglobin, so it causes the fish to slowly suffocate as their blood fails to take up enough oxygen.
Algal Blooms
- An algal bloom is a floating mass of algae or cyanobacteria on the surface of a body of water. Algae and cyanobacteria are very efficient at taking up nitrogen, so when the nitrogen level in the water rises they quickly proliferate. Algal blooms are often toxic and contaminate the water, poisioning the animals that live in it. By blocking sunlight, blooms reduce oxygen levels, which can lead to the suffocation of aquatic organisms. The die-off of an algal bloom can deplete the oxygen completely, causing mass fish deaths and creating a dead-zone.
Acidification
- When the nitrogen in acid rain is deposited into a body of water, it can cause the pH level to fall, resulting in acidification. Many fish eggs are unable to hatch at pH levels lower than 5, while bass do not survive if the pH level falls below 5.5. Some organisms are able to tolerate acidic water better than others. If the acidity kills off their food source, however, these animals are still unable to survive. As a result, some acidic lakes have no fish at all.
Eutrophication
- Ironically, eutrophication is also known as nutrient enrichment. Eutrophication happens when large amounts of nutrients, including nitrogen, are introduced to a previously nutrient-deficient body of water. This results in excessive growth of plants and algae, which, when they begin to decompose, cause oxygen depletion, which kills off many of the fish and other organisms. This can cause a lake to become a bog, which over time, due to continued deposits of sediment from decomposing plant matter, may convert to dry land.

Animals & Science