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4.4.3 Buffer strips

The interface between aquatic ecosystems and the land is an ecotone that has a profound influence on the movement of water and water-borne contaminants. Vegetation adjacent to streams and water bodies can help to safeguard water quality, particularly in agricultural landscapes. Buffer strips are used to reduce the amounts of nutrients reaching water bodies from runoff or leaching. They usually take the form of vegetated strips of land alongside water bodies: grassland, woodland and wet
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4.4.1 Diversion of effluent

In some circumstances it may be possible to divert sewage effluent away from a water body in order to reduce nutrient loads. This was achieved at Lake Washington, near Seattle, USA, which is close to the sea. Lake Washington is surrounded by Seattle and its suburbs, and in 1955 a cyanobacterium, Oscitilloria rubescens, became dominant in the lake. The lake was receiving sewage effluent from about 70 000 people; this input represented about 56% of the total phosphorus load to the lake.
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4.2 Reducing eutrophication

In Britain, water supply companies have tended to regard eutrophication as a serious problem only when it becomes impossible to treat drinking water supplies in an economic way. Threshold concentrations at which action is taken to reduce nutrient loadings thus depend on economic factors, as well as wildlife conservation objectives.

There are two possible approaches to reducing eutrophication:

  1. Reduce the source of nutrients (e.g. by phosphate str
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3.3 Mechanisms of eutrophication

Direct effects of eutrophication occur when growth of organisms (usually the primary producers) is released from nutrient limitation. The resulting increased NPP becomes available for consumers, either as living biomass for herbivores or as detritus for detritivores. Associated indirect effects occur as eutrophication alters the food supply for other consumers. Changes in the amount, relative abundance, size or nutritional content of the food supply influence competitive relationships between
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3.2.2 Domestic detergents

Domestic detergents are a major source of phosphorus in sewage effluents. Phosphates are used as a ‘builder’ in washing powders to enhance the efficiency of surfactants by removing calcium and magnesium to make the water ‘softer’. In 1992, the UK used 845 600 tonnes of detergent of various types, all of which have different effects on the environment. Estimates of the relative contribution of domestic detergents to phosphorus build-up in Britain's watercourses vary from 20–60%. The
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3.2 Anthropogenic sources of nutrients

In addition to the natural sources of nutrients referred to above, nitrogen and phosphorus enter the environment from a number of anthropogenic sources. These are considered below.


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2.4.2 Saltmarshes

Marsh plant primary production is generally nitrogen limited, so saltmarsh vegetation responds readily to the artificial eutrophication that is now so common in nearshore waters. Eutrophication causes marked changes in plant communities in saltmarshes, just as it does in freshwater aquatic and terrestrial systems. Biomass production increases markedly as levels of eutrophication increase. Increases in the nitrogen content of plants cause dramatic changes in populations of marsh plant consumer
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2.4.1 Estuarine species

Nutrient runoff from the land is a major source of nutrients in estuarine habitats. Shallow-water estuaries are some of the most nutrient-rich ecosystems on Earth, due to coastal development and the effects of urbanization on nutrient runoff. Figure 2.19 shows some typical nitrogen pathways. Nitrogen loadings in rainfall are typically assimilated by plants or denitrified, but septic tanks tend to add nitrogen below the reach of plant roots, and if situated near the coast or rivers can lead to
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2.4 Effects on marine systems

In the marine environment, nutrient enrichment is suspected when surface phytoplankton blooms are seen to occur more frequently and for longer periods. Some species of phytoplankton release toxic compounds and can cause mass mortality of other marine life in the vicinity of the bloom. Changes in the relative abundance of phytoplankton species may also occur, with knock-on effects throughout the food web, as many zooplankton grazers have distinct feeding preferences. In sheltered estuarine are
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2.3 Effects on terrestrial vegetation

SAQ 13

Why do you think nitrogen is becoming increasingly available to terrestrial ecosystems in many parts of the world)

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2.2 Effects on consumers in freshwater ecosystems

Increased productivity tends to increase rates of deoxygenation in the surface layer of lakes. Although phytoplankton release oxygen to the water as a byproduct of photosynthesis during the day, water has a limited ability to store oxygen and much of it bubbles off as oxygen gas. At night, the phytoplankton themselves, the zooplankton and the decomposer organisms living on dead organic matter are all respiring and consuming oxygen. The store of dissolved oxygen thus becomes depleted and diffu
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2.1.2 Algal blooms

The enrichment of water bodies by eutrophication may be followed by population explosions or ‘blooms’ of planktonic organisms.

SAQ 10

Bursts of primary production in a
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2.1.1 Loss of submerged plant communities

One of the symptoms of extreme eutrophication in shallow waters is often a substantial or complete loss of submerged plant communities and their replacement by dense phytoplankton communities (algal blooms). This results not only in the loss of characteristic plant species (macrophytes) but also in reduced habitat structure within the water body. Submerged plants provide refuges for invertebrate species against predation by fish. Some of these invertebrate species are phytoplankton-grazers an
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2.1 Effects on primary producers in freshwater ecosystems

Plant species differ in their ability to compete as nutrient availability increases. Some floating and submerged macrophyte species are restricted to nutrient-poor waters, while others are typical of nutrient-rich sites (see Table 2.2). Figure 2.2 shows turbid water in a polluted drainage ditch associated with localized growth of algae. There are no aquatic plants present.

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1.4 Human-induced eutrophication

While eutrophication does occur independently of human activity, increasingly it is caused, or amplified, by human inputs. Human activities are causing pollution of water bodies and soils to occur to an unprecedented degree, resulting in an array of symptomatic changes in water quality and in species and communities of associated organisms. In 1848 W. Gardiner produced a flora of Forfarshire, in which he described the plants growing in Balgavies Loch. He talked of ‘potamogetons [pondweeds]
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1.3 Natural eutrophication

Eutrophication of habitat can occur without human interference. Nutrient enrichment may affect habitats of any initial trophic state, causing distinctive changes to plant and animal communities. The process of primary succession is normally associated with a gradual eutrophication of a site as nutrients are acquired and stored by vegetation both as living tissue and organic matter in the soil.

There is a long-standing theory that most water bodies go through a gradual process of nutrien
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1.2 Resource availability and species diversity

A wide range of ecosystems has been studied in terms of their species diversity and the availability of resources. Each produces an individual relationship between these two variables, but a common pattern emerges from most of them, especially when plant diversity is being considered. This pattern has been named the humped-back relationship and suggests diversity is greatest at intermediate levels of productivity in many systems (Figure 1.5).

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1.1 Origin of the term ‘eutrophication’

The levels of nutrients present determine the trophic state of a water body, where trophic means ‘feeding’.

SAQ 1

Give another example of the adjective trophic be
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Learning outcomes

By the end of this unit you should be able to:

  • Describe the principal differences between a eutrophic and an oligotrophic ecosystem.

  • Explain the mechanisms by which species diversity is reduced as a result of eutrophication. (Questions 2.1 and 2.2)

  • Contrast the anthropogenic sources that supply nitrogen and phosphorus to the wider environment, and describe how these sources can be controlled. (Question 3.1)

  • Describe h
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Introduction

Eutrophication describes the biological effects of an increase in the concentration of nutrients. The collective term ‘nutrients’ refers to those elements that are essential for primary production by plants or other photosynthetic organisms. Eutrophication is most often caused by increases in the availability of nitrogen and phosphorus, commonly present in soil and water in the form of nitrate and phosphate, respectively. However, altered concentrations of any plant nutrient may ha
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