Solutions

SAQ 1

Answer

Trophic levels, as applied to a food chain.

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SAQ 2

Answer

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

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SAQ 3

Answer

Leaching.

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SAQ 4

Answer

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

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SAQ 5

Answer

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

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SAQ 6

Answer

The phosphorus concentration of the water may have increased.

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SAQ 7

Answer

In 1800 the water was at the upper end of the oligotrophic range; it had moved through the mesotrophic range to become eutrophic by 1900, and by 1940 would be classed as hypertrophic. Between 1940 and 1975 there was a further threefold increase in the concentration of total phosphorus.

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SAQ 8

Answer

Warm water, because oxygen is less soluble at warmer temperatures and is therefore more rapidly depleted by respiring organisms, especially as respiration rate also increases with temperature.

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Question 2.1

Answer

River A has a soluble reactive phosphorus concentration in the range 0.1-0.2 mg 1−1, which corresponds to the ‘degraded’ category in Table 2.3. This suggests that the diversity of macrophytes would be less than in the pristine natural state, due to a limited eutrophication effect. Neither form of nitrogen is present at concentrations above the natural range, so primary productivity may become limited by nitrogen rather than phosphorus, limiting the impact of the elevated phosphorus concentration.

River B has a similar concentration of SRP to river A, which would again place it in the ‘degraded’ category, but a much higher concentration of nitrogen in both its forms, especially nitrate at 12.1 mg 1−1, taking it into the ‘severe loss of species’ category. The elevated availability of both P and N would boost primary production in the watercourse, favouring algal communities and leading to a decline in macrophyte populations and diversity. The more competitive macrophytes may benefit from the increased nutrient availability, but their increased growth would further exclude less competitive species, resulting in lower diversity.

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SAQ 9

Answer

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

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SAQ 10

Answer

The organisms responsible may be either algae or bacteria, or a mixture of the two. It is incorrect to refer to bacteria as algae as they belong to a completely different taxonomic kingdom.

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SAQ 11

Answer

Oxygen diffuses through water at approximately one ten-thousandth of its rate through air.

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SAQ 12

Answer

The cyprinid fish are tolerant of deoxygenation, and the increased NPP boosts their food supply; therefore the yield of fish improves.

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SAQ 13

Answer

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

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SAQ 14

Answer

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

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Question 2.2

Answer

Seagrass needs sufficient light to photosynthesize effectively. An increase in nutrient levels leads to a greater abundance of phytoplankton in the water column, which increases the turbidity of the water and blocks out the light. As the seagrass beds recede, the exposed sediment may be re-suspended and further increase the turbidity of the water, thereby exacerbating the problem in a classic positive feedback response. Another possible positive feedback loop is the loss of habitat for filter-feeding animals, which lived in the shelter of seagrass beds and helped keep the water clear by consuming microbes.

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SAQ 15a

Answer

Phosphorus and nitrogen are the main limiting nutrients.

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SAQ 16

Answer

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

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SAQ 17

Answer

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.

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SAQ 18

Answer

Phosphorus comes primarily from domestic waste water, whilst nitrogen comes primarily from intensive agriculture.

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SAQ 19

Answer

First the arable catchment is likely to be receiving much more nutrient input in the form of fertilizers. Second and equally importantly, the soil structure is much less stable under arable systems and therefore more likely to erode and carry nutrients to the lake as suspended sediment.

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Question 3.1

Answer

Figure 1.13 shows that total phosphorus concentrations had fallen to 0.2 mg 1−1 in 2000, compared with their peak concentration of 0.36 mg 1−1 in 1975. In terms of SRP (the form of phosphorus that affects ecosystems most directly), we assume levels have fallen from 0.29 mg 1−1 to 0.16 mg 1−1. Comparing these figures with those in Table 2.3, we see that the SRP concentration put the system in the ‘severe loss of species’ category in 1975, but only the ‘degraded’ category in 2000. This suggests that some recovery of macrophyte species would be possible. Actual re-colonization may be a slow process, however. Ecosystems can take many years to come back to equilibrium after a perturbation, and if an algal-dominated state has established, it will inhibit macrophyte recovery.

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Question 4.1

Answer

The River Great Ouse has a MTR of 3, suggesting it is enriched with nutrients and therefore eutrophic, but it is on the mildest edge of this category so the eutrophication is not severe.

The River Eden has an MTR of 6, indicating that its plant community is composed of species that are moderately sensitive to enrichment, so it can be assumed that this stretch has not undergone substantial eutrophication. The most sensitive species are absent, suggesting that the waters may naturally carry a moderate concentration of nutrients or that some very mild enrichment has occurred.

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Question 4.2

Answer

Prevention of eutrophication at source compared with treating its effects (or reversing the process) has the following advantages.

  • Technical feasibility. In some situations prevention at source may be simply engineered by diverting a polluted watercourse away from the sensitive ecosystem, while removal of nutrients from a system by techniques such as mud-pumping is more of a technical challenge.

  • Cost. Nutrient stripping at source using a precipitant is relatively cheap and simple to implement. Biomass stripping of affected water is labour-intensive and therefore expensive.

  • Habitat availability. Buffer strips and wetlands can provide a stable wildlife habitat whilst performing a nutrient trapping role on throughflow water. Habitats, whether aquatic or terrestrial, can be compromised in terms of their wildlife value, due to the degree of disturbance involved in biomass stripping.

  • Products. Constructed wetlands may be managed to provide economic products such as fuel, compost or thatching material more easily than trying to use the biomass stripped from a less managed system.

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