The word 'eutrophication' comes from the Greek eutrophos which means well nourished. It is applied to water that is enriched with nutrients, mainly phosphorus and nitrogen compounds, which encourage the growth of abnormally large number of algae and aquatic plants. The extent of nutrient enrichment of water (its 'trophic' state) is described by different prefixes, graded from ultra-oligotrophic water, which is very deficient in nutrients, through oligotrophic, mesotrophic and eutrophic, to hypertrophic, which has a great excess of nutrients.

The problem of eutrophication mainly applies to still water such as that in lakes, ponds and canals. This is because the static water allows sufficient time for the algae to grow and multiply, whereas in rivers they are constantly being moved and swept downstream. The excess nutrients in rivers, however, do encourage the growth of water weeds and attached algae, and the river can become choked with the mass of plants.

In modern times, the amount of eutrophication has increased rapidly as a result of human activities. Large quantities of nutrients are discharged into surface waters and the sea from industry and from sewage treatment works. They also originate from agriculture, especially from the application of fertilizers to crops and from the spreading of animal wastes onto fields.

Aquatic algae require four main components for their growth: carbon dioxide and sunlight for photosynthesis, warmth and nutrients. So in eutrophic waters in summer time, there is often a prolific growth of algae on the water surface. This causes problems in the lake or pond because:

• the water becomes turbid with tens of thousands of algal cells in each millilitre

• the sunlight cannot penetrate the deeper water because of the density of the algae

• the wind can blow the algae onto the shore line where they form unsightly and smelly deposits as they rot

• when the algae die off at the end of the summer, the decomposition process uses up dissolved oxygen from the water and kills off other aquatic life.

The main nutrients needed by the algae and other plants for growth are nitrogen, phosphorus, potassium and silicon. There are also many trace elements required, such as cobalt, iron, manganese, molybdenum, vanadium, etc. In fresh waters, the problem of excess algae (algal blooms) is caused mainly by too much phosphorus in the water, whilst in the sea it is often caused by excess nitrogen.

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