Phytoplankton bloom in the Baltic Sea 3 July 2001. Photo: NASA/Wikimedia
The deposition of nitrogen compounds favours forest growth, but at the same time leads to the chemical disruption of a long list of ecosystems on land and in the sea, and results in the impoverishment of biodiversity.
In fresh water environments, eutrophication is almost always caused by phosphates, since phosphorus is the substance that usually limits biological growth in fresh water.
On land and in the sea, however, it is nitrogen that is the limiting factor in the majority of cases. The deposition of nitrogen - originating from emissions of nitrogen oxides and ammonia - therefore acts as a fertilizer in nature.
While this favours some species of plants that can easily make use of the extra nitrogen, it does so at the expense of others. It also affects the growth of mycorrhizal fungi.
The impoverishment of ecosystems that results from the deposition of nitrogen is a real and very serious problem in large parts of Europe. The increased growth rate that results from nitrogen deposition also increases biological acidification.
The atmospheric deposition of nitrogen compounds in Europe is due, in roughly equal parts, to emissions of nitrogen oxides and ammonia.
Nitrogen oxides are always produced during combustion. Emissions from land-based sources in Europe have fallen from 23 million tonnes a year in 1980 to 13 million tonnes a year in 2010, a reduction of 44 per cent. About half the emissions in Europe come from the transport sector, and most the rest from combustion plants. In the case of nitrogen oxides a large part of the emission reduction from land-based sources has been offset by rising emissions at sea. Ship emissions around Europe increased from 2.4 to 4.0 million tonnes between 1980 and 2010.
The main source of ammonia emissions is agriculture. The amount of ammonia that evaporates depends primarily on how the manure is handled during storage and spreading. According to statistics, emissions have fallen by 34 per cent between 1990 and 2010, from 7.6 to 5.0 million tonnes per year.
Due to long-range transport by winds, the problems are largely unrestricted by national borders, especially in the case of nitrogen oxides and their transformation products.
The atmospheric deposition of nitrogen compounds in Europe is greatest in the Netherlands, Belgium, France, southern England, northern Germany, and northern Italy.
The reason why the situation is worst in areas with intensive agriculture is that a relatively large proportion of nitrogen from ammonia, 90 per cent of which comes from livestock farming, is deposited relatively close to the source of emissions.
Note that it is not just airborne nitrogen that ends up in nature. In many environments nitrogen is also added in the form of fertilizer. Large amounts are spread on fields, and sometimes also on natural grazing land, which leads to impoverishment of the natural flora. Fertilizer is also spread on forest land to increase forestry yield. In addition to direct deposition, nitrogen also reaches the sea through leaching from the land and discharges from wastewater treatment plants and individual households.
Affected areas in Europe
In order to get a picture of the areas that are affected by eutrophication it is not sufficient to determine the quantities deposited - we also need information about the sensitivity of the ecosystems.
But nitrogen has a twofold effect; it causes both eutrophication and acidification. This fact, together with the complexity of the nitrogen cycle, makes it difficult to give unequivocal critical loads for different ecosystems.
One way to define the critical load for nitrogen is to calculate the level at which nitrogen starts to leak from the system into the groundwater. This is done with the aid of what are known as mass balances. These look at the way that nitrogen is converted in the ecosystem - its uptake by vegetation, fixing in the soil, conversion by micro-organisms in the soil (nitrification and denitrification), its removal if biomass is harvested, etc. These plus and minus entries are then weighed against each other to give a measure of how much nitrogen can be added without the loss from the system exceeding a certain limit.
Changes in ecosystems
Another way of determining the critical load for nitrogen is to study the deposition levels of nitrogen at which visible changes start to appear in ecosystems, e.g. changes in the composition of species. Knowledge in this area is however incomplete, since it is difficult to establish which changes are due to nitrogen deposition and which are caused by other changes, such as the way that land is used. Moreover, changes only appear in the flora after the critical limit has been exceeded, and in some cases only after it has been exceeded for an extended period of time.
Critical loads for nutrient nitrogen in Europe
The maps show areas where critical loads for eutrophication of freshwater and terrestrial habitats are exceeded (CSI 005) by nitrogen depositions caused by emissions between 1980 (top left) and 2030 (bottom right)
Percentage of total ecosystems receiving nitrogen deposition above the critical loads for eutrophication
Black colour = 100% exceedance, blue = less than 5%. For the emission levels in the year 2000 (left), and for two projected EU emission levels for 2020: Current legislation (CLE; centre) and Maximum Technically Feasible Reduction (MTFR; right). For details and country-by-country figures, see factsheet.
>> Further reading
CCE, Coordination Centre for Effects under LRTAP convention. More about mapping of critical loads and exceedances. (external link)