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Chapter 9
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IntroductionPlants, and in general autotrophs, the base in all ecosystems, grow and reproduce using sunshine, water, and air. Plants get their energy from sunlight, their hydrogen and oxygen from water, and their carbon dioxide from air. In addition, however, a series of other elements, referred to as nutrients, are needed to build plant cells. Among those are the macro-nutrients: nitrogen, phosphorus, calcium, potassium, and some others. The availability of these nutrients and their flow through natural cycles determine the conditions for growth and production in ecosystems. Nitrogen and phosphorus most often limit plant growth. In his efforts to master nature man learned in the early part of the 1900s to overcome these limitations. The addition of nitrogen and phosphorus from new sources greatly increased agricultural production. Industrial fixation of atmospheric nitrogen, the new source of nitrogenous salts, became as large as the biological nitrogen fixation. Likewise, phosphorus import from mines to biological systems greatly added to the natural fluxes from mineral weathering. This large scale manipulation of the fluxes of these two elements has resulted in drastic environmental consequences. The added N and P in an essentially linear flow ends in water bodies and drastically changes their character as nutrient conditions change. This "eutrophication" results in massive algal growth, oxygen deficiency, and thereby a changed species composition, and change of the coastal landscape. First only rivers and lakes were influenced. Not much later the Baltic Sea, an unusually sensitive water body, became increasingly eutrophic. Today, more than a billion people all over the world, who live along coasts and depend on marine food from coastal areas, destroy these through eutrophication. Land eutrophication also has drastic consequences on species composition. In fact what is happening is a large scale, all encompassing change of the chemistry of land and water. In this chapter the causes and consequences of this large scale change are discussed. Special attention is given to the Baltic Sea itself. Boosted nutrient flows are today part of many aspects of society. Agriculture leaks fertilizers, urban areas produce sewage, car traffic as well as power plants give rise to nitrogen emissions, as do several kinds of industries, all containing nutrients that find their way to land and water. Even if the processes described are longterm and include many functions of society they may be reasonably well controlled, at least on a local level. This is demonstrated by Lake Mälaren in central Sweden, with some two million people living in the drainage basin. This lake, which was once badly eutrophied, has today once again pleasant beaches and good fishing.
In the longterm perspective "end-of-pipe"
control is insufficient. Present nutrient flows are unsustainable, and
have to change from linear to cyclic. Nutrients from cities, which are
concentrated in sewage sludge, have to go back to farmland. Industrial
nitrogen fixation has to be limited, and phosphorus mines will at some
point be empty and no longer be able to provide fertilizer for
farmland. This will require drastic changes in our societies. Authors to this chapter are: |
