Cycles Part 4

Question 1

With respect to the global nitrogen cycle, what services do ecosystems provide to society?

Answer 1

Ecosystems provide the intermediate service of biological fixation, which in turn serves society with food production and soil formation services.

Question 2

What are ecosystem disservices?

Answer 2

ecosystem disservices (negative impacts on society) of climate change, food scarcity and soil erosion, among others.

Question 3

What can damage of the nitrogen scosystem structure lead to?

Answer 3

nitrogen cycle, damage to ecosystem structure can lead to leaching of nitrogen from the system. This, in turn, can cause food scarcity and soil erosion, and potentially climate change if changes to ecosystem structure lead to losses of N2O to the atmosphere.

Question 4

What are Provisioning services i.e.
products obtained
from ecosystems, give some examples?

Answer 4

Food e.g. crops, fruit, fish
• Fibre and fuel e.g. timber, wool
• Biochemicals, natural medicines and pharmaceuticals
• Genetic resources: genes and genetic information used for animal/plant breeding
and biotechnology
• Ornamental resources e.g. shells, flowers

Question 5

What are Regulating services i.e.
benefits obtained from the
regulation of
ecosystem processes, give some examples?

Answer 5

Air-quality maintenance: ecosystems contribute chemicals to and extract chemicals from
the atmosphere
• Climate regulation e.g. land cover can affect local temperature and precipitation; globally
ecosystems affect greenhouse gas sequestration and emissions
• Water regulation: ecosystems affect e.g. the timing and magnitude of runoff, flooding etc.
• Erosion control: vegetative cover plays an important role in soil retention/prevention of
land/asset erosion
• Water purification/detoxification: ecosystems can be a source of water impurities but can also
help to filter out/decompose organic waste
• Natural hazard protection e.g. storms, floods, landslides
• Bioremediation of waste i.e. removal of pollutants through storage, dilution, transformation
and burial

Question 6

What are Cultural services i.e. nonmaterial benefits that
people obtain through
spiritual enrichment,
cognitive development,
recreation etc, give some examples?

Answer 6

• Spiritual and religious value: many religions attach spiritual and religious values to ecosystems
• Inspiration for art, folklore, architecture etc
• Social relations: ecosystems affect the types of social relations that are established
e.g. fishing societies
• Aesthetic values: many people find beauty in various aspects of ecosystems
• Cultural heritage values: many societies place high value on the maintenance of important
landscapes or species
• Recreation and ecotourism

Question 7

What are Supporting services,
necessary for the
production of all other
ecosystem services, give some examples?

Answer 7

Soil formation and retention
• Nutrient cycling
• Primary production
• Water cycling
• Production of atmospheric oxygen
• Provision of habitat

Question 8

What is eutrophication caused by?

Answer 8

concentrations of nutrients in the water, The collective term ‘nutrients’ is used for the elements that are essential for primary production by plants or other photosynthetic organisms.

Question 9

name the nine macronutrients that are essential for plant growth?

Answer 9

Macronutrients are those required in relatively high amounts. Carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium and magnesium are all macronutrients.

Question 10

What elements most often causes eutrophication?

Answer 10

Eutrophication is most often caused by increases in the availability of nitrogen and phosphorus. These elements are common in soil and water in the form of nitrate and phosphate, respectively. However, alterations in the concentration of any plant nutrient may have a recognisable biological effect.

Question 11

What is another example of the adjective trophic being used in a scientific context?

Answer 11

Trophic levels, as applied to a food-chain.

Question 12

What are the series of stages to describe the trophic state?

Answer 12

A series of stages were used to describe the trophic state:

oligotrophic – mesotrophic – eutrophic – hypertrophic

where oligotrophic means ‘low in nutrients’, mesotrophic ‘with intermediate nutrient concentration’, eutrophic ‘high in nutrients’ and hypertrophic ‘very high in nutrients’.

Question 13

What is phytoplankton a collective term for?

Answer 13

‘Phytoplankton’ is a collective term for the free-floating photosynthetic organisms within the water column. It encompasses both algae and photosynthetic bacteria. Therefore:

an oligotrophic lake has clear water with little phytoplankton
an eutrophic lake is more turbid and green from dense phytoplankton growth
a mesotrophic lake is intermediate between the tw

Question 14

What are dystrophic waters caused by?

Answer 14

A further term – dystrophic – describes ‘brown-water lakes’. These have heavily stained water caused by large amounts of organic matter, usually leached from peat soils.

Question 15

What will primary production be like in an oligotrophic and eutrophic lake?

Answer 15

Characteristic Oligotrophic Eutrophic

primary production low high

Question 16

What will the diversity of primary producers be like in an oligotrophic and eutrophic lake?

Answer 16

oligotrophic -high species diversity, low population densities
Eutrophic - low species diversity, high population densities

Question 17

What will the diversity of light penetration into water column be like in an oligotrophic and eutrophic lake?

Answer 17

oligotrophic - high

eutrophic - low

Question 18

What will toxic blooms be like in an oligotrophic and eutrophic lake?

Answer 18

oligotrophic - rare

eutrophic - frequent

Question 19

What will plant nutrient availability be like in an oligotrophic and eutrophic lake?

Answer 19

oligotrophic - low

eutrophic - high

Question 20

What will animal production be like in an oligotrophic and eutrophic lake?

Answer 20

oligotrophic - low

eutrophic - high

Question 21

What will oxygen status of surface water be like in an oligotrophic and eutrophic lake?

Answer 21

oligotrophic - high

eutrophic - low

Question 22

What will dominant fish be like in an oligotrophic and eutrophic lake?

Answer 22

oligotrophic - salmonid fish (e.g. trout, char)

eutrophic - coarse fish (e.g. perch, roach, carp)

Question 23

Why is light penetration poor in eutrophic lakes?

Answer 23

The high density of phytoplankton absorbs the light for photosynthesis and prevents it penetrating deeper into the water.

Question 24

Why are cyanobacteria so productive in eutrophic water bodies (Figure 5.5.3b) compared with oligotrophic ones?

Answer 24

The ready availability of nutrients allows rapid growth. In oligotrophic water, the rate of growth is limited by the nutrient supply. In eutrophic water, often only the availability of light regulates primary production.

Question 25

How does species diversity differ from species richness

Answer 25

Species diversity includes a measure of how evenly spread the biomass is between species (equitability), rather than just a simple count of the species present.

Question 26

Give an explanation for the classic ‘humped-back’ relationship with ecosystem productivity as inferred from the amount of biomass per unit area?

Answer 26

An explanation for this relationship is that, at very low resource availability, and hence ecosystem productivity, only a few species are suitably adapted to survive. As the limiting resource becomes more readily available, more species can grow. However, once resources are readily available, the more competitive species within a community can dominate it and exclude the less vigorous species.

Question 27

What type of trophic conditions do diatoms tolerate? and how can their presence be found in waters?

Answer 27

It is well established that some species of diatom can tolerate oligotrophic conditions whereas others flourish only in more eutrophic waters. When they die, their tiny (< 1 mm) silica-based skeletons, which can be identified to species level, sink to the bed and may be preserved for thousands of years. Therefore, a historical record of which species have lived within a water body can be constructed from the analysis of a core sample taken from its underlying sediment.

Question 28

How can rivers and lakes with high algae phytoplankton blooms be depleted of oxygen if they create oxygen?

Answer 28

the high productivity of the blooms means that, although oxygen is released by photosynthesis during the day, the effect of billions of cells respiring overnight can deplete the oxygen in the water. As a result, fish die through suffocation even if they can tolerate the toxins.
Although phytoplankton releases oxygen into the water as a by-product 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, the zooplankton and the decomposer organisms living on dead organic matter are all respiring and consuming oxygen. The store of dissolved oxygen therefore becomes depleted and the recovery of oxygen levels by the diffusion of atmospheric oxygen into the water is very slow if the water is not moving.

Question 29

Are fish most at risk of suffocation in warm or cool water?

Answer 29

In warm water because oxygen is less soluble at warmer temperatures. Therefore, oxygen is more rapidly depleted by respiring organisms, especially because respiration rate also increases with temperature.

Question 30

Refer back to Grime’s life strategies in Block 4 study session 4.8. Consider the differences in nutrient acquisition and use between his three principal categories: competitors, stress tolerators and ruderals.

In terrestrial systems, which life strategy is likely to be most successful in (a) oligotrophic and (b) eutrophic conditions?

Answer 30

a. In oligotrophic conditions, the stress-tolerant strategy is probably the most effective. Stress tolerators use nutrients very conservatively. They retain them within their tissues and have a slow turnover of tissues, to avoid releasing nutrients back to the soil.
b. In eutrophic conditions, the competitive strategy is probably the most successful. Competitors can increase their growth rate and productivity to use the extra nutrient availability. They are rather wasteful of the nutrients they acquire because of the high turnover rate of their tissues (both roots and leaves). However, they succeed through rapid upward growth, which allows them to shade, and hence exclude, their neighbours.

Question 31

In regards to eutrophication
Why is chlorophyll-a used as an indicator?

Is there any evidence of changes in the levels of eutrophication as measured by chlorophyll-a concentration?

Answer 31

Chlorophyll-a is used as an estimation of phytoplankton biomass. As eutrophication occurs an increase in phytoplankton biomass may occur due to increased frequency and duration of phytoplankton blooms and primary production. Measurements of water leaving radiance using satellite radiometers can be used to determine chlorophyll-a concentrations and hence eutrophication.

Data obtained from measuring stations in European seas, from 1985 to 2010, demonstrated that most of the stations (89%) did not show any change in summer chlorophyll-a concentrations.

Among stations which do show change:

in the Baltic Sea the values showed both an increase and decrease in chlorophyll-a concentrations
the greater North Sea values are generally declining
in the UK Celtic Seas the values are generally decreasing
in the Bay of Biscay the values are showing a decrease
in the western Mediterranean and Adriatic seas the values are showing a decrease.

Question 32

What happens to turbidity of water after artificial eutrophication?

Answer 32

Turbidity increases, reducing the amount of light reaching submerged plants.

Question 33

What happens to the rate of sediment of water after artificial eutrophication?

Answer 33

Rate of sedimentation increases, shortening the lifespan of open water bodies such as lakes.

Question 34

What happen to primary productivity in water after artificial eutrophication?

Answer 34

Primary productivity usually becomes much higher than in unpolluted water and may be manifested as extensive algal or bacterial blooms.

Question 35

What happens to dissolved oxygen in water after artificial eutrophication?

Answer 35

Dissolved oxygen in the water decreases, as the organisms decomposing the increased biomass consume oxygen.

Question 36

What happens to diversity of primary producers in water after artificial eutrophication?

Answer 36

The diversity of primary producers tends to decrease and the dominant species change. Initially, a temporary increase in the diversity of primary producers occurs, due to the number of green algae species; however, as eutrophication proceeds, cyanobacteria become dominant, displacing many algal species

Question 37

What happens to fish populations of water after artificial eutrophication?

Answer 37

Fish populations are adversely affected by reduced oxygen availability. The fish community becomes dominated by surface-dwelling coarse fish, such as pike (Esox lucius, Figure 5.5.13) and perch (Perca fluviatilis).

Question 38

What happens to zooplankton species in water after artificial eutrophication?

Answer 38

Zooplankton species (e.g. Daphnia spp.), which eat phytoplankton, are disadvantaged because of the loss of submerged macrophytes, which provide cover, exposing them to predation.

Question 39

What happen to the abundance of macrophytes in water after artificial eutrophication?

Answer 39

The increased abundance of competitive macrophytes (e.g. bulrushes) may impede water flow, increasing rates of silt deposition.

Question 40

What happens to drinking water quality in water after artificial eutrophication?

Answer 40

Drinking water quality may decline. Water may be difficult to treat for human consumption, for example because of blocked filtering systems. Water may have an unacceptable taste or odour caused by the secretion of organic compounds by microbes.

Question 41

What happens to microbes in water after artificial eutrophication?

Answer 41

Water may cause human health problems because of toxins secreted by the abundant microbes. Symptoms can range from skin irritations to pneumonia.

Question 42

Name three species of submerged macrophytes which can tolerate eutrophic water.

Answer 42

Spiked water-milfoil (Myriophyllum spicatum), fennel-leaved pondweed (Potamogeton pectinatus) and arrowhead (Sagittaria sagittifolia).

Question 43

Bursts of primary production in an aquatic ecosystem in response to an increased nutrient supply are commonly referred to as ‘algal blooms’. Which two groups of organisms contribute to this ‘bloom’?

Answer 43

The organisms responsible may be either algae or bacteria (including cyanobacteria), or a mixture of the two.

Question 44

What is the problem with eutrophication?

Answer 44

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, causing high levels of mortality, and resulting in systems with low diversity. The odours associated with algal decay taint the water and may make drinking water unpalatable. Species of cyanobacteria which flourish in nutrient-rich waters can produce powerful toxins that are a health hazard to animals. Such problems are well documented for several lakes. For example, the Zürichsee in Switzerland has had seasonal blooms of the cyanobacterium Oscillatoria rubescens because of increased sewage discharge from new building developments on its shores.

Question 45

What type of species can tolerate low levels of oxygen in water and why are these fish undesirable?

Answer 45

Many species of coarse fish can also tolerate low oxygen concentrations in the water, sometimes gulping air (e.g. roach, Rutilus rutilus, a cyprinid fish, Figure 5.5.18a).

Indeed, yields of fish may increase because of the high net primary production (NPP) of the system. However, these species are generally less desirable for commercial fishing than others which depend on cool, well-oxygenated surface water

Question 46

What term defines species that are found only there).

Answer 46

endemic

Question 47

In a South East Asian village where cyprinid fish from the local pond are an important source of protein, eutrophication of the water by domestic sewage is seen as advantageous. Why?

Answer 47

The cyprinid fish tolerate deoxygenation and the increased NPP boosts their food supply. Therefore, the yield of fish improves.

Question 48

Why is nitrogen becoming increasingly available to terrestrial ecosystems in many parts of the world?

Answer 48

The emission of nitrogen oxides from burning fossil fuels and of ammonia from intensive agriculture result in nitrogen compounds being transported and deposited by atmospheric processes.

Question 49

What is acid flush?

Answer 49

Snow is a very efficient scavenger of atmospheric pollution and melting snowbeds release their pollution load at high concentrations in episodes called ‘acid flushes’. The flush of nitrogen is received by the underlying vegetation when it has been exposed after snowmelt.

Question 50

What can happen with atmospheric nitrogen at high altitudes?

Answer 50

The deposition of atmospheric nitrogen can be enhanced at high-altitude sites as a consequence of the deposition of cloud droplets on hills. Sampling of upland plant species at sites in the north of the UK has shown marked increases in the nitrogen concentration in leaves with increasing nitrogen deposition. This, in turn, correlates with increasing altitude. The productivity of the species was also found to increase in line with the amount of nitrogen deposited. Therefore, plant species can respond directly to elevated levels of nitrogen. In the longer term, the relative dominance of species is likely to alter, depending on their ability to convert elevated levels of deposited nitrogen into biomass.

Question 51

What will be the impact on species diversity of increasing biomass?

Answer 51

As biomass increases beyond an optimal value, species diversity will decline.

Question 52

How do seagrass beds play an important role in coastal waters?

Answer 52

Seagrass beds play an important role in reducing the turbidity of coastal waters by reducing the quantity of sediment suspended in the water. They slow down currents near the bottom, which increases the deposition of small sediment particles and decreases their erosion and resuspension. Seagrass roots also play an important part in stabilising sediments and limiting disturbance caused by burrowing deposit-feeders. Many species depend on seagrass beds for food or nursery grounds. Seagrass increases the structural complexity of habitats near the sea floor, and provides a greater surface area for epiphytic organisms. The 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.
Predation on seagrass-associated prey such as the grass shrimp is much higher outside seagrass beds than within them, where the shrimps can hide and predator foraging is inhibited by grass cover. Without seagrasses, soft-substrate communities on the seabed are simpler, less heterogeneous and less diverse. By reducing the health of seagrasses, eutrophication contributes directly to biotic impoverishment

Question 53

Seagrass (Zostera marina) is a key species for maintaining the biodiversity of estuaries.

a. Why is its abundance reduced after the eutrophication of estuarine waters?
b. Suggest a mechanism by which the decline of seagrass promotes its further decline.

Answer 53

a. Seagrass needs sufficient light to photosynthesise effectively. An increase in nutrient levels leads to a greater abundance of phytoplankton in the water column. This increases the turbidity of the water and blocks out the light.
b. As the seagrass beds recede, the exposed sediment may be resuspended, further increasing the turbidity of the water. This exacerbates the problem in a classic positive feedback response. Another possible positive feedback loop is the loss of habitat for filter-feeding animals. These live in the shelter of seagrass beds and help to keep the water clear by consuming microbes.

Question 54

At high nutrient levels, what is the major limiting factor?

Answer 54

Light. As nitrogen availability increases, competition for light becomes relatively more important.

Question 55

What are the life-history strategies of (a) heather and (b) birch?

Answer 55

Heather is a stress tolerator. It is adapted to survive in a difficult environment, where nutrients are very scarce.
b.Birch is a more competitive species. It can use additional nutrients, when available, to increase its growth rate.

Question 56

How does eutrophication play an indirect loss of wetland habitats?

Answer 56

Phragmites can spread by underground rhizomes and rapidly colonise large areas. However, it is the target of conservation effort in some areas, including the UK, because the reedbeds it produces provide an ideal habitat for rare bird species such as bitterns (Botaurus stellarus). But the spread of common reed is not always beneficial for nature conservation because it often dries out marsh soils, making them less suitable for typical wetland species and more suitable for terrestrial species. This is because Phragmites is very productive and can cause ground levels to rise by depositing litter and entrapping sediment. Thus eutrophication can also play an indirect part in the loss of wetland habitats.

Question 57

What are the 4 most important factors that determine net primary production?

Answer 57

The four most important factors determining net primary production (NPP) are:

light availability
water availability
temperature
the supply of plant nutrients.

Question 58

Which two elements most often limit NPP?

Answer 58

Phosphorus and nitrogen are the main limiting nutrients.

Question 59

What is a more limiting factor for plant growth, phosphorus or nitrogen?

Answer 59

As a result, phosphorus is generally unavailable for plant growth. In natural systems, phosphorus is more likely to be the growth-limiting nutrient than nitrogen

Question 60

what is the form of phosphorus that can be taken up by organisms?

Answer 60

Phosphorus can be assimilated by organisms as soluble phosphate (PO43−). On land, phosphate is produced mostly by the weathering of phosphorous minerals

Question 61

How do the mechanisms of eutrophication caused by phosphorus vary for terrestrial and aquatic systems?

Answer 61

The mechanisms of eutrophication caused by phosphorus vary for terrestrial and aquatic systems.

In soils, some phosphorus comes out of solution to form insoluble iron and aluminium compounds. These are then immobilised until the soil itself is moved by erosion.
Eroded soil entering watercourses may release its phosphorus, especially under anoxic conditions.

Question 62

What changes occur to iron(III) compounds (Fe3+) as a result of bacterial respiration in anoxic environments, and how is their solubility affected?

Answer 62

Bacterial respiration can reduce Fe3+ to Fe2+, increasing the solubility of iron salts, including phosphates of iron.

Question 63

Nearly 80% of the atmosphere is nitrogen. Despite the huge supply potentially available, nitrogen gas is directly available as a nutrient to only a few organisms.

Why are the majority of organisms unable to use gaseous nitrogen?

Answer 63

Nearly 80% of the atmosphere is nitrogen. Despite the huge supply potentially available, nitrogen gas is directly available as a nutrient to only a few organisms.

Why are the majority of organisms unable to use gaseous nitrogen?

Question 64

What is the main growth limiting nutrient in aquatic and terrestrial system?

Answer 64

Nitrogen is only likely to become the main growth-limiting nutrient in aquatic systems where rocks are particularly phosphate-rich or where artificial phosphate enrichment has occurred. However, nitrogen is more likely to be the limiting nutrient in terrestrial ecosystems, where soils can typically retain phosphorus while nitrogen is leached away.

Question 65

Why did the RCEP recommend reducing phosphate use, particularly in ‘soft’ water areas?

Answer 65

The reason for including phosphates in detergents is to soften the water. So in areas with naturally soft water, they provide no benefit yet still cause pollution.

Question 66

How can diatoms cause eutrophication?

Answer 66

Other compounds added to detergents may also contribute to eutrophication. For example, silicates can lead to the increased growth of diatoms, particularly if they are used as a partial replacement for phosphates in detergents. Diatoms require silicates to build their ‘skeleton’ and their growth can be limited by silicate availability. When silicates are readily available, diatoms characteristically have ‘spring blooms’ of rapid growth. They can smother the surfaces of submerged macrophytes, depriving them of light. The loss of submerged macrophytes is a problem because it results in the loss of habitat for organisms feeding on phytoplankton, enhancing the risk of blooms by other species.

Question 67

How could inorganic fertilisers infiltrate the wider environment?

Answer 67

drainage water percolating through the soil, leaching soluble plant nutrients
animal excreta applied to the land as fertiliser being washed into watercourses
the erosion of surface soils or the movement of fine soil particles into subsoil drainage systems.

Question 68

What are the main sources of phosphorus and nitrogen that enrich rivers in a developed country such as the UK?

Answer 68

Phosphorus comes primarily from domestic wastewater, while nitrogen comes primarily from intensive agriculture.

Question 69

Why is a lake in a catchment dominated by arable agriculture much more prone to eutrophication than one in a forested catchment?

Answer 69

First, the arable catchment probably receives much more nutrient input in the form of fertilisers. Second, and equally important, the soil structure is much less stable under arable systems. Therefore, it is more likely to erode and carry nutrients to the lake as suspended sediment.

Question 70

What major problems does increased phosphorus from human activities cause?

Answer 70

Worldwide, human activities have intensified the release of phosphorus considerably. The major causes of this increase are:

increased soil erosion
agricultural runoff
recycling of crop residues and manures
discharges of domestic and industrial wastes
most importantly, applications of inorganic fertilisers.

Question 71

What is the point source and diffuse source?

Answer 71

Studies of nutrient runoff have shown a mixture of inputs into most river and lake catchments: both point source (such as sewage treatment works) and diffuse source (such as agriculture). Point sources are usually most important in the supply of phosphorus, whereas nitrogen is more likely to be derived from diffuse sources.

Question 72

What are the main sources of anthropogenic phosphorus?

Answer 72

The sources of anthropogenic phosphorus entering the environment include sewage discharges, intensive livestock farms and spreading artificial fertilisers and animal manures on agricultural land.

Question 73

What are the main sources of anthropogenic nitrogen?

Answer 73

The sources of anthropogenic nitrogen entering the environment include gaseous emissions from vehicle exhausts and power stations and artificial fertilisers applied to agricultural land.

Question 74

Define the term The interface between aquatic ecosystems and the land?

Answer 74

ecotone

Question 75

How does plants pay bacteria for nitrogen?

Answer 75

The plants take up nitrogen directly, provide a source of carbon for denitrifying bacteria, and create oxidised rhizospheres in which denitrification can occur.

Question 76

What mainly controls the retention of nitrogen?

Answer 76

Nitrogen is more likely to be controlled by biological processes (e.g. organic matter accumulation, denitrification).

Question 77

What is the surface retention of sediment by vegetated buffer strips a function of?

Answer 77

The surface retention of sediment by vegetated buffer strips is a function of slope length and gradient, vegetation density and flow rates. Therefore, the construction of effective buffer strips requires detailed knowledge of an area’s hydrology and ecology. Overall, the restoration of riparian zones in order to improve water quality may have greater economic benefits than allocating the same land to cultivating crops.

Question 78

What is a wetland treatment process?

Answer 78

The treatment process involves passing the drain water through basins and ponds, designed to have specific retention times. The pumped water first passes through sedimentation basins, to allow suspended solids to settle out (primary treatment). This is followed by several wetland ponds (secondary treatment). The ponds are cultivated with different types of aquatic plant. These include emergent macrophytes (e.g. Phragmites) with well-developed aerenchyma systems to oxygenate the rhizosphere, allowing the oxidation of ammonium ions to nitrate. Subsequent denitrification removes the nitrogen to the atmosphere.
The wastewater treatment mechanism depends on a wide diversity of highly productive organisms, which produce the biological activity required for treatment. These include decomposers (bacteria and fungi), which break down particulate and dissolved organic material into carbon dioxide and water, and aquatic plants. Some of the latter can convey atmospheric oxygen to submerged roots and stems, and some of this oxygen is available to microbial decomposers.

Question 79

What type of species are used in wetland treatment process?

Answer 79

Aquatic plants also sequester nitrogen and phosphorus. Species such as common reed (Phragmites australis, Figure 5.5.46) yield a large quantity of biomass, which has a range of commercial uses in the region. Another highly productive species is the water hyacinth (Eichhornia crassipes, Figure 5.5.47). This species is regarded as a serious weed on lakes and is regularly harvested to reduce eutrophication. However, it has a potential role in water treatment because of its high productivity and rapid rates of growth. The resulting biomass could be harvested and used for producing nutrient-rich animal feed, or for composting and producing fertiliser. Further research is required to develop practical options.

Question 80

How do plants help to improve water quality?

Answer 80

The passage of water through emergent plants reduces turbidity because the large surface area of stems and leaves acts as a filter for particulate matter. The transmission of light through the water column is improved, enhancing photosynthesis in the attached algae. These contribute further to nutrient reduction in through-flowing water. The mixture of floating plants and emergent macrophytes contributes to the removal of suspended solids, improved light penetration, increased photosynthesis, and the removal of toxic chemicals and heavy metals.

Question 81

Domestically how can you reduce nutrient inputs to water bodies?

Answer 81

An important aspect of efforts to reduce nutrient inputs to water bodies is modifying domestic behaviour. For example, public campaigns in Australia have encouraged people to:

wash their vehicles on porous surfaces, away from drains or gutters
reduce the use of fertilisers on lawns and gardens
compost garden and food waste
use zero- or low-phosphorus detergents
wash only full loads in washing machines
collect and bury pets’ faeces.
These campaigns have combined local lobbying with national strategies to tackle pollution from other sources.

Question 82

What are the main methods to tackle eutrophication at source?

Answer 82

Once nutrients are in an ecosystem, it is usually much harder and more expensive to remove them than to tackle the eutrophication at source.

The main methods available are:

precipitation (e.g. treatment with a solution of aluminium or ferrous salt to precipitate phosphates)
remove nutrient-enriched sediments (e.g. by mud pumping)
remove biomass (e.g. harvest common reed) and use it for thatching or fuel.

Question 83

How do submerged and floating macrophytes differ in treatment process?

Answer 83

Alternatively, plants may be introduced deliberately, to ‘mop up’ excess nutrients. Although water hyacinth can be used in water treatment, the water that results from treatment solely with floating macrophytes tends to have low dissolved oxygen. The addition of submerged macrophytes, together with floating or emergent macrophytes, usually gives better results.

Submerged plants are not always as efficient as floating ones at assimilating nitrogen and phosphorus because of their slower growth. This is a result of poor light transmission through water (particularly if it is turbid) and slow rates of carbon dioxide diffusion down through the water column. However, many submerged macrophytes have a high capacity to elevate pH and dissolved oxygen, which improves conditions for other mechanisms of nutrient removal. For example, at higher pH, soluble phosphates can precipitate with calcium, forming insoluble calcium phosphates, so removing soluble phosphates from the water.

Various species have been used in this way. One submerged macrophyte, a pondweed (Elodea densa), removes nitrogen and phosphorus from nutrient-enriched water, its efficiency varying according to the loading rate. Nitrogen removal rates reach 400 mg N m−2 per day during summer, while phosphorus is removed at over 200 mg P m−2 per day.