Contrast the anthropogenic sources that supply nitrogen and phosphorus to the wider environment, and describe how these sources can be controlled.
- atmosphere: N from fossil fuel combustion (NOx); no atmospheric P reservoir.
- domestic detergents: P used in detergents as softener contributes 20-60% of P buildup in UK watercourses. Many compounds not removed, and degrade slowly.
- agricultural fertilisers: N is very soluble.
- land use: deforestation and mechanised agriculture can reduce soil stability, leasing to soil erosion and high sediment inputs into watercourses.
Describe the principle differences between a eutrophic and an oligotrophic ecosystem.
An oligotrophic ecosystem has low nutrient availability, and therefore low primary production, which is carried out by a diverse range of species at low population densities. In lakes, the water is clear, allowing light to penetrate, and is rich in oxygen.
In eutrophic systems, a plentiful nutrients allow high primary production, which is often carried out by a low diversity of species at high population densities. In lakes, turbidity prevents light penetration, and blooms may be frequent, leading to low oxygen availability. Coarse fish are often dominant.
Describe how living organisms can be used as monitors of the tropic status of ecosystems.
Characteristic assemblages of plants and associated animal species are found in water with different trophic states.
For example: bog pondweed and river water-crowfoot are associated with low P levels (oligotrophic) but spiked water-milfoil with high (eutrophic to hypertrophic).
A shift in dominance from one species to another can indicate changing conditions.
Compare the advantages and disadvantages of three different methods for combatting antropogenic eutrophication.
There are two main methods for reducing eutrophication: reduction at source, and reduction in-system.
- diversion of effluent: sewage discharge from Seattle, USA, constituted 56% of phosphorus input into a nearby lake until the sewerage system was diverted into the sea.
(-) can only be used where the diverted load is not a major water supply to the lake. - phosphate stripping: treatment at sewerage works can remove over 90% of phosphorous which comes from sewerage (approximately 45% of total P into freshwater). The sewerage is mixed with a precipitant, often Fe(II) ammonium sulfate, which combines with phosphorous to form a solid.
(+) extremely effective
(-) not universally installed - buffer strips and wetlands: reduce the amount of nutrients reaching water bodies from runoff or leaching. A riparian buffer strip 20-30m wide can remove up to 100% of incoming nitrate, which is either accumulated in soils or undergoes denitrification.
(+) cheap
(+) effective even at high loading rates
(-) requires land
(-) P removal highest in first few years, N removal highest afterwards - reducing in-system: e.g. nutrient-rich sediments sucked from lake and used as fertiliser, or mechanical removal of plant biomass.
(-) expensive
(-) high impact
(-) e.g. conservation grazing is very long term
List five trophic levels.
Oligotrophic: low in nutrients
Mesotrophic: with intermediate nutrient concentration
Eutrophic: high in nutrients
Hypertrophic: very high in nutrients
Dystrophic: ‘brown water lakes’, which have heavily stained waters due to large amounts of organic matter usually leached from peat soils. The presence of these organic compounds can reduce the availability of nutrients to organisms, making the water body even less productive than an oligotrophic one.
Explain the relationship between species diversity and nutrient availability.
The relationship is described as ‘humped-back’.
At very low resource availability, and hence ecosystem productivity, only a limited number of species are suitably adapt to survive. As the limiting resource becomes more readily available, then more species are able to grow. However, once resources are readily available, then the more competitive species within a community are able to dominate it and exclude less vigorous species.
Describe natural eutrophication of lakes.
Most water bodies go through a gradual process of nutrient enrichment as they age.
Lakes fill gradually with sediment and eventually become shallow enough for plants rooted in the bed sediment to dominate, at which point they develop into a closed swamp or fen and are eventually colonised by terrestrial vegetation.
Describe how past nutrient levels of lakes can be studied.
Diatoms are microscopic photosynthetic algae (of kingdom Protoctista), which live either free-floating in lakes or attached to the surface of rocks and aquatic vegetation.
Some species of diatom can tolerate oligotrophic conditions whereas others flourish only in more eutrophic waters.
When they die, their tiny (< 1mm) bony capsules, which can be identified to species level, sink to the bed and may be preserved for thousands of years. A historical record of which species have lived within a water body can therefore be constructed from an analysis of a core sample taken from its underlying sediment.
Give the typical changes observed in lakes experiencing artificial eutrophication.
- turbidity increases, reducing the amount of light reaching submerged plants
- rate of sedimentation increases, shortening the plies pan of open water bodies such as lakes
- primary production 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
- diversity of primary producers tends to decrease and the dominant species change
- fish populations are adversely affected by reduced oxygen availability, and the fish population becomes dominated by surface-dwelling coarse fish, such as pike
- zooplankton, which eat phytoplankton, are disadvantaged due to the loss of submerged macrophytes, which provide their cover, thereby exposing them to predation
- increased abundance of competitive macrophytes (e.g. bulrushes) may impede water flow, increasing rates of silt deposition
- 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
Give the impact of eutrophication on submerged plants.
Substantial or complete loss, and replacement by dense phytoplankton communities.
Macrophytes are thought to disappear because they lose their energy supply in the form of sunlight penetrating water. Following eutrophication, the sunlight is intercepted by the increased biomass of free-floating phytoplankton.
Give the impact of eutrophication on invertebrates.
Excessive populations of phytoplankton, zooplankton and decomposes are all respiring and using oxygen. At night, when this supply is not being replenished by photosynthesis, the store of dissolved oxygen becomes depleted.
Some species, such as mayfly larvae, cannot survive such low oxygen levels; others, such as the bloodworm, can tolerate low oxygen concentrations.
Give the impact of eutrophication on fish.
Many species of coarse fish, such as roach and pike, can tolerate low oxygen concentrations in the water, as they sometimes gulp air. Yields of these fish may increase due to the high net primary production (NNP).
In oligotrophic conditions, salmonids will often dominate; in eutrophic conditions coarse fish.
In contrast, salmonid fish 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.
Give the impact of eutrophication on mountain ecosystems.
In Britain, rare bryophytes are found associated with snowbeds. Most of these are found in the Central Highlands of Scotland, which are also areas of very high deposition of nitrogenous air pollutants.
Snow is a very efficient scavenger of atmospheric pollution and melting snowbeds release their pollution load at higher concentrations in episodes known as ‘acid flushes’. The flush of nitrogen is received by the underlying vegetation when it has been exposed following snowmelt.
Concentrations of nutrients in the meltwater of Scottish have already been shown to damage underlying bryophytes. Recovery from damage is slow. Given the very short growing season, this persistent damage can greatly reduce the viability and survival of the plants.
Give the impact of eutrophication on lowland heath ecosystems.
Lowland heath ecosystems typically have low soil nutrient levels and a vegetation characterised by heather. Under elevated atmospheric deposition of nitrogen, they tend to be invaded by taller species, including birch, bracken and rhododendron.
Give the impact of eutrophication on ‘alien’ species.
Eutrophication can accelerate the invasion of aggressive, competitive species at the expense of slower growing native species.
Give the impact of eutrophication on estuarine ecosystems.
Eutrophication can significantly reduce populations of seagrass and oysters. Oysters live in bottom water which is vulnerable to oxygen depletion in eutrophic conditions. As waters become more turbid, seagrass is unable to photosynthesise.
Many species depend on seagrass beds for food or nursery grounds. Seagrass increases the structural complexity of habitat near the sea floor, and provide greater surface area for epiphytes organisms. Seagrass leaves support rich communities of organisms on their surface, including microalgae, stationary invertebrates (such as sponges and barnacles) and grazers (such as limpets and whelks). The plants also provide refuges from predatory fishes and crabs. Without seagrasses, soft-substrate communities on the sea-bed are simpler, less heterogenous and less diverse.
Oysters filter out water during feeding, while seagrass slows down currents leading to sediment deposition. Loss of either species can therefore bring about positive feedback.
Explain why phosphorous is often a limiting factor.
Phosphorous has a number of indispensable biochemical roles and is an essential element of growth for all organisms, being a component of nucleus acids and DNA.
However, phosphorous is a scarce element in the Earth’s crust and natural mobilisation of phosphorous from rocks is slow. Its compounds are relatively insoluble, there is no reservoir of gaseous phosphorous compounds available in the atmosphere, and phosphorous is also readily and rapidly transformed into insoluble forms that are unavailable to plants. This tends to make phosphorous generally unavailable for plant growth.
In natural systems, phosphorous is more likely to be the growth-limiting nutrient than is nitrogen, which has a relatively rapid global cycle and whose compounds tend to be highly soluble.
Explain why nitrogen is more likely to be a limiting factor in terrestrial ecosystem, and phosphorous in aquatic.
Soils can retain phosphorous while nitrogen is leached away.
Give the ways in which eutrophication can directly and indirectly alter an ecosystem.
Direct effects:
- increased NNP available for consumers
Indirect effects:
- changes in the amount, relative abundance, size or nutritional content of food supply influences competitive relationships between consumers, and hence the relative success and survival of different species
- changes in environment conditions, e.g. reduced oxygen concentrations caused by decay of biomass
Explain how pesticides can influence the impact of eutrophication.
Pesticide residues in sediments can reduce herbivore populations for long enough to reduce algal consumption, allowing algae to out-compete macrophytes.
Describe the legislation which deals with eutrophication, and explain weaknesses in these laws.
No single piece of UK legislation dealing comprehensively with the problem of eutrophication in the UK.
European Community Urban Wastewater Treatment Directive (EC UWWTD):
- 62 rivers and canals, 13 lakes designated as sensitive areas
- sewage treatment works in sensitive areas and which serve populations of 10,000 or more must have P-stripping equipment
- most sensitive sites are rural, with low populations and therefore equipment is not yet required
EC Nitrates Directive
- monitor nitrate levels
- establish ‘nitrate vulnerable zones’ (NVZs)
- produce and promote a code of good agricultural practice
- focused on drinking water not conservation
- defines eutrophication only in terms of nitrogen compounds
Explain the mean trophic rank (MTR) method for assessing eutrophication.
- uses a scoring system based on species and their recorded abundances at river sites
- each species allocated a score (Species Trophic Rank: STR) dependant on its tolerance of eutrophication
- for a given site, the mean score for all species present is calculated
- tolerant species have a low score, so a low-MTR tends to indicate a nutrient-rich river
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Unit 2.1 Air74
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Unit 2.2 Earth79
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Unit 3.1 Water38
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Unit 3.2 Life*46
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Unit 4.1 Landforms47
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Unit 4.2 Cycles*30
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Unit 5.3 Wetlands and the Carbon Cycle [#]30
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Unit 5.4 Cryosphere23
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Unit 6.1 Oceans and Climate30
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Unit 5.1 Extreme Weather [#] *0
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Unit 6.2 Water Quality [#] *0
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Unit 6.3 Eutrophication22
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Unit 6.4 Acid Rain*17
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Unit 7.1 Grasslands*16
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Unit 7.3 - Tropical Forests*12
