Unit 6.3 Eutrophication Flashcards
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.
