Total Nitrogen: Environmental effects
Description
High concentration of total nitrogen are, in conjunction with other factors, often associated with algal blooms (including toxic blue-green algal blooms), as well as dense aquatic plant growth. This process of high nutrient input and algal growth is known as eutrophication, which can lead water which does not support aquatic life.
Entering the environment
Most total nitrogen is transported by fluvial processes such as runoff and streamflow, although aeolian processes (wind) may at times also transport components of total nitrogen around the landscape. See also Sources (above) for human mediated transport mechanisms of total nitrogen.
Where it ends up
In rivers and lakes the inorganic components of total nitrogen (ammonia, nitrate and nitrite) will become available for algal growth. High total nitrogen levels together with high phosphorus levels and in conjunction with favourable physical characteristics of aquatic environments this may result in algal blooms. After assimilation in algal (plant) growth, microbial breakdown and other processes such as mineralisation and nitrification may transform organic nitrogen through various steps into inorganic forms of nitrogen such as ammonia, nitrite and nitrate. As a generalisation, however, natural surface waters may contain significant amounts of organic-nitrogen and nitrate, but generally little ammonia or nitrite. Two other important microbiologically mediated processes influence total nitrogen levels in water - denitrification and nitrification. Denitrification results in the breakdown of nitrate-nitrogen by bacteria into nitrogen gas, which may remain dissolved in the water or be lost to the atmosphere. In either case the molecular nitrogen is no longer available for biological activity. Conversely some blue-green algae (cyanobacteria) can, under certain conditions, convert gaseous nitrogen dissolved in water into nitrate. This process is called nitrogen fixation or nitrification. These same types of cyanobacteria assist legumes to 'fix' atmospheric nitrogen in the case of terrestrial plants.
Environmental guidelines
Because various factors, such as flow, light, turbidity, temperature, phosphorus levels, zooplankton grazing etc., can limit algal growth, it is not possible to recommend absolute total nitrogen concentration for aquatic environments that will prevent algal blooms. Nevertheless the following environmental guidelines have been set for total nitrogen (ANZECC, 1992).
Rivers and steams: 100-750 micrograms/L (0.0001 to 0.00075 g/L) (as total-nitrogen)
Lakes and reservoirs: 100-500 micrograms /L (0.0001 to 0.0005 g/L) (as total-nitrogen)
Estuaries and embayments: 10-100 micrograms /L (0.00001 to 0.0001 g/L) (as nitrate-nitrogen)
Coastal waters: 10-60 micrograms /L (0.00001 to 0.00006 g/L) (as nitrate-nitrogen)
In recognition that other factors can have a major influence on the effects of nutrient levels, the future ANZECC guidelines will adopt a risk-based approach. This will involve the use of a decision-tree with guideline levels for nitrogen (and phosphorus) largely determined by these other influencing factors.
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