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Nitrogen is an abundant substance on earth; making up about 80% of the air we breathe as Ngas. Most plants cannot use it in this form, however, some algae and plants can convert N2  into NO3  (nitrate) which can then be used by plants. Plants use nitrates to build protein, and animals that eat plants also use organic nitrogen to build protein. When plants die or excrete waste, the nitrogen is released into the environment as ammonium, which is eventually oxidized by bacteria in to nitrite (NO2) , and then into nitrate. In this form, it is fairly common in freshwater ecosystems. Nitrate therefore enters freshwater from natural sources like decomposing plants and animal waste, as well as runoff from sewage and fertilizer. Excess nitrate increase plant growth and decay, promote bacterial decomposition, and therefore, decrease the amount of oxygen available in the water. 


Unlike temperature and dissolved oxygen, the presence of normal levels of nitrates usually does not have a direct effect on aquatic insects or fish.  However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.

Algae and other plants use nitrates as a source of food. If algae have an unlimited source of nitrates, their growth is unchecked.  So, Why is that a problem?

A bay or estuary that has the milky colour  of pea soup is showing the result of high concentrations of algae.  Large amounts of algae can cause extreme fluctuations in dissolved oxygen.  Photosynthesis by algae and other plants can generate oxygen during the day. However, at night, dissolved oxygen may decrease to very low levels as a result of large numbers of oxygen consuming bacteria feeding on dead or decaying algae and other plants. When levels of dissolved oxygen are unstable, this creates a stressful environment for aquatic organisms, such as fish. Unstable dissolved oxygen levels can affect the ability to reproduce in some creatures, and can ultimately result in die-offs. 

Excess plants and algae will also create conditions where organic matter accumulates.  High densities of algae will create a condition where sunlight cannot reach very far into the water.  Since plants and algae require some sunlight, plants and algae not receiving sunlight will die off.  These dead plant materials will settle to the bottom of the water and bacteria that feed on decaying organic material will greatly increase in numbers.  These bacteria will consume oxygen and, therefore, the level of dissolved oxygen in this water will fall to levels that are too low for many aquatic insects and fish to survive.  Also, this can cause extreme changes in habitat.  Fish that need gravel or sand for spawning may find nothing but mats of vegetation and muck so will be unable to produce offspring.