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The effests of eutrophication are the occuring of biological changes.
A number of biological changes may occur as a result of eutrophication. Some of these are direct (e.g. stimulation of algal growth in water bodies), while others are indirect (e.g. changes in fish community composition due to reduced oxygen concentrations). This section summarizes some of the typical changes observed in aquatic, marine and terrestrial ecosystems following eutrophication.
The following are typical changes observed in lakes experiencing ‘artificial’ eutrophication.
• Turbidity increases, reducing the amount of light reaching submerged plants.
• Rate of sedimentation increases, shortening the life-span of open water bodies such as lakes.
• Primary productivity usually becomes much higher than in unpolluted water and may be manifest as extensive algal or bacterial blooms.
• Dissolved oxygen in water decreases, as organisms decomposing the increased biomass consume oxygen.
• Fish populations are adversely affected by reduced oxygen availability, and the fish community becomes dominated by surface-dwelling coarse fish, such as pike and perch.
• Drinking water quality may decline. Water may be difficult to treat for human consumption, for example due to blockage of filtering systems. Water may have unacceptable taste or odour due to the secretion of organic compounds by microbes.
• Water may cause human health problems, due to toxins secreted by the abundant microbes, causing symptoms that range from skin irritations to pneumonia.
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.
In rivers, the presence of plant species such as the yellow water-lily (Nuphar lutea) and the arrowhead (Sagittaria sagittifolia) are likely to indicate eutrophic conditions. In some rivers, the fennel-leaved pondweed (Potamogeton pectinatus) is tolerant of both sewage and industrial pollution.
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 and play an important part in balancing relative proportions of macrophytes and phytoplankton.
Submerged macrophytes also stabilize sediments and the banks of slow-flowing rivers or lakes. Bodies of water used for recreation (boating for example) become more vulnerable to bank destabilization and erosion in the absence of well-developed plant communities, making artificial bank stabilization necessary.
Submerged plants also have a role in the oxygenation of lower water layers and in the maintenance of aquatic pH.
The enrichment of water bodies by eutrophication may be followed by population explosions or ‘blooms’ of planktonic organisms. ‘Algal blooms’ are a well-publicized problem associated with increased nutrient levels in surface waters. The higher the concentration of nutrients, the greater the primary production that can be supported.
Opportunistic species like some algae are able to respond quickly, showing rapid increases in biomass. Decomposition of these algae by aerobic bacteria depletes oxygen levels, often very quickly. This can deprive fish and other aquatic organisms of their oxygen supply and cause high levels of mortality, resulting in systems with low diversity.
The odours associated with algal decay taint the water and may make drinking water unpalatable. Species of cyanobacteria that flourish in nutrient-rich waters can produce powerful toxins that are a health hazard to animals.
Such problems are well documented for a number of famous lakes. The Zurichsee in Switzerland has been subject to seasonal blooms of the cyanobacterium Oscillatoria rubescens due to increased sewage discharge from new building developments on its shores. For lakes in Wisconsin, USA, ‘nuisance’ blooms of algae or bacteria occur whenever concentrations of phosphate and nitrate rise.
Effects on consumers in freshwater ecosystems
In unpolluted water, mayfly larvae may be found. In polluted water, these species cannot survive due to reduced oxygen availability and are likely to be replaced by species, such as the bloodworm, which can tolerate lower oxygen concentrations.
Many species of coarse fish, such as roach, can also tolerate low oxygen concentrations in the water, sometimes gulping air. However these species are generally less desirable for commercial fishing than others such as salmon, which depend on cool, well-oxygenated surface water.
Populations of such species usually decline in waters that become eutrophic. They may be unable to live in a deoxygenated lake at all, resulting in fish kills. They may also be unable to migrate through deoxygenated waters to reach spawning grounds, resulting in longer-term population depressions.
Effects on terrestrial vegetation
Atmospheric deposition of nitrogen, together with the deposition of phosphorus-rich sediments by floods, can alter competitive relationships between plant species within a terrestrial community. This can cause significant changes in community composition, as species differ in their relative responses to elevated nutrient levels.
As is the case with aquatic vegetation, terrestrial species that are able to respond to extra nitrogen and phosphorus with elevated rates of photosynthesis will achieve higher rates of biomass production, and are likely to become increasingly dominant in the vegetation. Atmospheric deposition of nutrients can reduce, or even eliminate, populations of species that have become adapted to low nutrient conditions and are unable to respond to increased nutrient availability. Some vegetation communities of conservation interest are directly threatened by atmospheric pollution.
For example, coastal marshes and wetlands in many parts of the world have been affected by invasion of ‘weed’ or ‘alien’ species.
Eutrophication can accelerate invasion of aggressive, competitive species at the expense of slower growing native species.
In the USA, many coastal marshes have been invaded by the common reed (Phragmites australis). Phragmites is a fierce competitor and can out-compete and entirely displace native marsh plant communities, causing local extinction of plants and the insects and birds that feed on them.
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 areas, high nutrient levels appear to favour the growth of green macroalgae (‘seaweeds’) belonging to such genera as Enteromorpha and Ulva.
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.
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 high concentrations entering coastal water.
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