An Introduction to Eutrophication in the UK

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Aquatic eutrophication (from the Greek, meaning 'well nourish') is the enrichment of natural waters with plant nutrients, which results in the stimulation of an array of symptomatic changes. These include the increased production of algae and other aquatic plants, affecting the quality of the water and the balance of organisms present within it. Such changes may be undesirable and interfere with water uses.

The nutrient status of any natural water is determined by the supply of nutrients from its catchment, which in turn is influenced by the geology. Thus waters will vary in their natural nutrient status. Some waters are naturally eutrophic. However enrichment and consequential impacts can occur if there are changes to the nature of the catchment. This becomes a concern when human activities accelerate the enrichment process, and it is this artificial eutrophication, which warrants attention.

Causes of eutrophication
The growth of plants in natural waters is influenced by the supply of nutrients, light, temperature, flow regime, turbidity, zooplankton grazing and toxic substances. The properties of the catchment and the water body and the impact of human activities affect these factors. While it is important to understand the role of all these factors, it is widely accepted that the most important is the supply of nutrients.

In temperate freshwater systems, phosphorus is generally the key limiting nutrient. However, silicon may limit blooms of diatoms in spring and, in areas where phosphate levels are naturally high because of the underlying geology, waterbodies may be nitrate-limited, e.g. the meres of Shropshire and Cheshire. Estuarine ecosystems tend to show phosphorus-limitation at their freshwater extreme, grading through to nitrogen-limitation at their seaward end. However, the common occurrence of suspended sediments in estuaries with resulting turbidity often means that light limits algal growth. Where nutrients are the limiting factor in coastal waters, nitrogen is generally believed to be the key nutrient.

Sources of nutrients
Nutrient sources can be broadly segregated into two categories: readily identifiable point sources (such as sewage effluents), and diffuse sources (such as the run-off from agricultural land), with the relative contribution of each varying between catchments. The contributions (in percentage terms) of the main sources of phosphorus entering surface waters in the UK have been estimated as: -

Agricultural 43%
Human & household waste including detergents 43%
Industry 8%
Background source 6%

The source of these figures is Morse et al., The economic & environmental impact of phosphorus removal from wastewater in the European Community, Selper Publications, London 1993. The actual contributions in any given catchment will depend on the nature of the catchment and the human activities within it.

For nitrogen, inputs to fresh waters in Europe come principally from diffuse sources, particularly agriculture, although point sources (usually urban wastewater) also contribute significantly in many regions. In England and Wales 70% of the total input of nitrogen to inland surface waters is estimated to come from diffuse sources (agriculture, precipitation and urban run-off, in order of decreasing importance). The remaining 30% comes from sewage effluent and industrial discharges. As for P, the actual contributions in any given catchment will depend on the nature of the catchment and the human activities within it.

Impacts of eutrophication

Eutrophication can have both temporary and long-term effects on aquatic ecosystems. Large fluctuations in dissolved oxygen concentrations can occur between day and night. Low oxygen levels, the result of plant respiration, may lead to the death of invertebrates and fish. This process can be compounded when algal blooms, through their decay, further reduce the oxygen content of water. The growth or decay of benthic (bottom-dwelling) mats of macro-algae can also lead to the deoxygenation of sediments.

Certain algal species, particularly freshwater blue-green algae, and marine dinoflagellates can produce toxins, which may seriously affect the health of mammals (including humans), fish and birds. This occurs either through the food chain, or through contact with, or ingestion of, the algae. Algal species also cause fish deaths, for example by physically clogging or damaging gills, causing asphyxiation. Eutrophication ultimately detracts from biodiversity, through the proliferation and dominance of nutrient-tolerant plants and algal species. These tend to displace more sensitive species of higher conservation value, changing the structure of ecological communities.

Eutrophication can also adversely affect a wide variety of water uses such as water supply (e.g. algae clogging filters in treatment works), livestock watering, irrigation, fisheries, navigation, water sports, angling and nature conservation. It can give rise to undesirable aesthetic impacts in the form of increased turbidity, discolouration, unpleasant odours, slimes and foam formation.

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