Eutrophication of Soil and Water
Note the bright green colour caused by algae, stimulated by the experimental addition of phosphorus. The lake in the background is unfertilized.
Nutrients (mainly phosphorus and nitrogen) from sewage outfalls and fertilised farmland accelerate the growth of algae and other vegetation in lakes, watercourses and coastal waters. Nitrogen pollution of groundwater can amount to a health problem, and through the atmosphere forest land can also receive more nitrogen than is desirable.
Urban and industrial emissions of wastewater were one of the first environmental problems to become universally apparent. They were already causing problems at the beginning of the 20th century, in the form of stenches, filth and infection. Their abundant content of organic material was gradually broken down by micro-organisms in the water, but degradation of this kind consumes oxygen, producing knock-on effects in the form of oxygen deficiency and fish kill.
In addition, wastewater contained large quantities of nutrients. This nutrient increase caused heavy eutrophication, i.e. excess fertilisation, of the waters receiving the emissions. Vegetation densified along banks and shores, plankton algae became increasingly abundant, and dead plants and animals accumulated in ever-thicker benthic layers. In this way the polluted lakes and coastal inlets gradually silted up, and the open expanses of water grew smaller.
Lake overgrowth is partly a natural process which has been going
on ever since the lakes were formed after the glacial period. But the
anthropogenic eutrophication of recent years has drastically accelerated the
process in many places.
The effects of a moderate eutrophication of originally nutrient-poor water are not entirely negative. Increased growth of algae and other vegetation can be beneficial to the aquatic fauna, at least to begin with. Fish production rises, for example. If, on the other hand, eutrophication continues, plankton growth becomes so heavy as to cloud the water. The resultant darkness below the surface is harmful to the benthic vegetation. The process favours plankton-eating "coarse fish" like the roach and bream, while depleting the numbers of the predator fish species more sought after for human consumption.
In highly nutrient-rich waters, plankton production can be copious indeed. Certain plankton species appear intermittently in massive quantities, in what is termed algal bloom. Some such algae can give the water an unpleasant smell or taste, and some are even poisonous. Intensive algal production have the same consequences as if the water had received organic material from a pollution source: a large part of the oxygen content of the water is consumed when these masses of plankton die, sink to the bottom and are degraded. Oxygen deficiency can eliminate benthic life and sometimes fish as well. If the benthic water becomes completely de-oxygenated, hydrogen sulphide - toxic to all higher forms of life - is formed instead.
All in all, heavy eutrophication entails a distinct reduction in the number
of plant and animal species in the water. A few species benefit, but at the
expense of all the others.
The first measure taken to deal with the disruptive effects of wastewater emission was to trap the "rubbish" in wastewater with bar screens, and to allow solid particles to settle on the bottom of tanks. But primary treatment of this kind still left large quantities of nutrient and organic material dissolved in the water.
For this reason, starting in the 1950s, secondary or biological treatment was introduced at many municipal wastewater treatment plants. In biological treatment, micro-organisms are added to the water, consuming its content of organic material. In this way nearly 90 per cent of the oxygen-consuming degradation of organic substances is accomplished inside the treatment plant instead of in the wastewater recipient.
Industry, has caused even heavier emissions of organic material than urban communities, but those emissions have also been heavily reduced in recent decades. In the most heavily polluted lakes and coastal areas, the successful combating of emissions of organic material has led to a great improvement in oxygen conditions, with fauna returning to beds which had been completely lifeless.
Biological treatment, however, is only capable of separating a minor portion of the nutrients which wastewater contains. Consequently it could not prevent urban emissions of phosphorus breaking all records when phosphorus-containing detergents became widespread in the 1960s.
Phosphorus, normally, is in short supply in fresh water, and so every additional quantity of this substance augments the growth of algae and other vegetation. Phosphorus, in other words, is the growth-regulating nutrient in lakes and watercourses, and also in certain coastal areas where water exchange is limited. And so, in spite of biological treatment, overgrowth proceeded as rapidly as before. Algal production also perpetuated oxygen deficiency problems in many places.
During the 1970's, most municipal wastewater treatment plants installed chemical or advanced treatment, which can eliminate 90 per cent or more of the phosphorus content of unprocessed wastewater.
Chemical treatment has led to substantial improvement in lakes and other
waters which used to be highly eutrophic as a result of emissions from nearby
Some of the surplus nitrogen ends up in the groundwater. In this way, concentrations of nitrogen in nitrate form have gradually risen in many wells in agricultural areas, presenting a health hazard to people who depend on such wells for their drinking water. New-borns especially are sensitive to nitrate, which can also be converted into carcinogenic substances in the body.
The slow exchange of groundwater means that nitrate pollution may persist for
many years to come, even if nitrogen fertilising and nitrogen seepage were to be
Large quantities of surplus fertiliser nitrogen also leak out of the fields
into nearby watercourses which carry them seawards. In recent decades this has
helped to bring about a eutrophication, not only of coastal inlets and
archipelago areas but also of more open stretches of the coastline and even far
out to sea.
On the deeper bottoms a completely different situation prevails. At a depth of about 70 metres in the Baltic there is a salinity discontinuity, a halocline, between the surface water and the appreciably saltier bottom water. This halocline impedes the vertical water exchange, in this way preventing highly oxygenated surface water from penetrating downwards. Beneath the halocline, consequently, there is a permanent oxygen deficiency.This oxygen deficiency is partly natural, but it has been aggravated by eutrophication and the increase in plankton production. Today roughly one-third of the bottom of the Baltic is practically dead, and the deepest basins mostly contain hydrogen sulphide instead of oxygen.
The Kattegat has a distinct halocline at a depth of about 15 metres. Below
this, oxygen deficiency occurs every autumn concurrently with degradation of the
algae produced in spring and summer. Here as well, the oxygen deficiency was
aggravated during the 1980s, resulting in a heavy depletion of the benthic
fauna. In both the Baltic and the Kattegat, oxygen deficiency has also had
noticeable effects on fishing. The Norway lobster virtually disappeared from the
southern Kattegat, and stocks of cod have probably also suffered.
For several years now, the countries bordering on the Baltic and the North Sea have been engaged in a common endeavour to reduce inputs of nutrients into these sea areas. Several measures have been taken, for example, to restrict nitrogen seepage from arable land. Thus regulations have been implemented on the storage and distribution of manure and on the proportion of arable land which has to be under vegetation during the autumn and winter ("green" fields release far less nutrient substance than newly ploughed land with no vegetation).
A number of artificial wetlands are now being constructed along streams and rivers in European agricultural areas, to trap some of the nitrogen content of the water on its way to the sea. In addition, municipal wastewater treatment in the larger coastal towns and cities of Central and Southern Sweden have recently been made to include nitrogen removal, since ordinary chemical treatment removes only a minor portion of the nitrogen content of wastewater.
Even so, limiting inputs of nutrients into the sea has proved far more
difficult than expected.
The nitrogen load on the sea is also compounded by deposition of nitrogen compounds from the atmosphere. Precipitation today is appreciably richer not only in nitrate but also in ammonium than it was a few decades ago. Nitrate deposition derives mainly from emissions of nitrogen oxides, e.g. from vehicular traffic, while ammonium deposition derives primarily from the ammonia escaping into the air from manure and fertilised arable land. Altogether, air pollution accounts for roughly one-third of the quantities of nitrogen reaching the sea.
Nitrogen supply through the air and precipitation has also affected vegetation on land, above all in forest, meadow and grazing lands which, originally, were nutrient-poor to a greater or lesser extent. The fact of these lands now being more rich in nitrogen has benefited individual nitrophilous species such as cow parsley, stinging nettles and rose bay. Species of this kind have become more widespread, while many others have been displaced. Lichens and mosses are also very often adversely affected by atmospheric nitrogen supply. Tree trunks and other surfaces which used to be covered in lichen or moss are now liable to be covered instead by unicellular green algae, which flourish when nitrogen is abundant.
Several efforts have been made to restrict atmospheric emissions of nitrogen oxides and ammonia, but here again, progress has been slower than expected.
© 2006 Oasis Environmental Ltd