Effects of excess nitrogen on waterways

Nitrogen A chemical element that is the most abundant uncombined element in Earth’s atmosphere and occurs in all organisms. To be used by plants and animals, nitrogen must be fixed from the atmosphere and converted to ammonium (NH4) or nitrites (NO2-) and nitrates (NO3-). 

Nitrate-nitrogen (NO3-N) is an important plant fertiliser but it is highly water soluble so it leaches through soils very easily and reaches waterways. Sources of NO3-N include inorganic fertiliser, septic tanks and leaking sewerage systems.

Nitrate can also enter waterways from the nitrification of the ammonia in animal waste. 

The main ways that nutrients end up in water from diffuse pollution is through leaching and runoff. Nitrogen makes its way to water mainly through leaching which occurs when the plant cover cannot take up all the nitrogen so it moves down past the shallow root zone and through the soil. The nitrogen eventually moves into water, either groundwater storage areas or laterally into streams and then lakes and estuaries. The main form of nitrogen leaching is from cow urine because it occurs in small concentrated patches that the pasture cannot take up.

Nitrogen itself is only harmful to humans at very high concentrations (11.3mg/l)

Nitrate has ecosystem consequences at much lower levels than when it becomes unsafe to drink.  Ecosystem effects can start to occur at levels below 1 mg/l in water.

Nitrogen is toxic to fish at far lower levels than it is toxic to humans.

Too many nutrients (mainly nitrogen and phosphorus) in rivers and lakes cause unwanted plants (weeds and algae) to grow in excessive amounts which form into mats of slimy growth and soupy, green, smelly water.

 Excess nutrients cause growths of algal mats like this from the Oroua River.

So an overabundance of weed growth in water can cause oxygen levels in the water to fluctuate between very high and very low levels from day to night (diurnal fluctuations). 

In contrast, a healthy stream has constant oxygen levels. As streams become more enriched with nutrients, and the algal/plant life blooms, the more the oxygen fluctuates. High fluctuations in oxygen levels can cause high rates of gross primary productivity (GPP), which is bad for biodiversity.

These changes are harmful and eventually lethal for river ecology, making it impossible for fish and insects to live. 

Water Quality