5 Summary Eutrophication is a process in which an ecosystem accumulates mineral nutrients. It can occur naturally, but is usually associated with human activity that releases nutrients into the environment. Anthropogenic eutrophication has caused a widespread loss of biodiversity in many systems. Recent attempts to reverse the process are proving difficult and expensive. Symptoms of eutrophication are most readily seen in aquatic sys
4.4 Reducing nutrient availability Once nutrients are in an ecosystem, it is usually much harder and more expensive to remove them than 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); removal of nutrient-enriched sediments, for example by mud pumping; and removal of biomass (e.g. harvesting of common reed) and using it for thatchi
4.3.5 Domestic campaigns An important aspect of efforts to reduce nutrient inputs to water bodies is the modification of domestic behaviour. Public campaigns in Australia have encouraged people to: wash vehicles on porous surfaces away from drains or gutters reduce use of fertilizers on lawns and gardens compost garden and food waste use zero- or low-phosphorus detergents wash only full loads in washing machi
4.3.4 Wetlands Wetlands can be used in a similar way to buffer strips as a pollution control mechanism. They often present a relatively cost-effective and practical option for treatment, particularly in environmentally sensitive areas where large waste-water treatment plants are not acceptable. For example, Lake Manzala in Egypt has been suffering from severe pollution problems for several years. This lake is located on the northeastern edge of the Nile Delta, between Damietta and Port Said. Land reclamatio
4.3 Reducing the nutrient source Europe is the continent that has suffered most from eutrophication, and increasing efforts are being made to restore European water bodies damaged by nutrient enrichment. If the ultimate goal is to restore sites where nature conservation interest has been damaged by eutrophication, techniques are required for reducing external loadings of nutrients into ecosystems. Although algal production requires both nitrogen and phosphorus supplies, it is usually sufficient to reduce only one major
2.4.2 Saltmarshes Marsh plant primary production is generally nitrogen limited, so saltmarsh vegetation responds readily to the artificial eutrophication that is now so common in nearshore waters. Eutrophication causes marked changes in plant communities in saltmarshes, just as it does in freshwater aquatic and terrestrial systems. Biomass production increases markedly as levels of eutrophication increase. Increases in the nitrogen content of plants cause dramatic changes in populations of marsh plant consumer
1.2 Resource availability and species diversity A wide range of ecosystems has been studied in terms of their species diversity and the availability of resources. Each produces an individual relationship between these two variables, but a common pattern emerges from most of them, especially when plant diversity is being considered. This pattern has been named the humped-back relationship and suggests diversity is greatest at intermediate levels of productivity in many systems (Figure 1.5). Summary of Section 6 Sequence-specific protein–DNA interactions are achieved through the formation of non-covalent bonds between amino acid side-chains in the protein and bases in the major groove of the DNA. Non-sequence-specific protein–DNA interactions are achieved primarily through electrostatic interactions between positively charged amino acid side-chains and the negatively charged DNA backbone. Protein and DNA conformation can be alt Torsional energy can be taken up by alternative DNA conformations The energy introduced into DNA by twisting has great potential as a regulatory mechanism, since the free energy can be stored in a variety of different high-energy conformations along the chain. 1.1 Early observations Some of the earliest observations of macromolecules within living cells were of nucleic acids in the form of chromosomes. These long dark-staining objects, which became visible in the nucleus of cells at specific stages of cell division, were large enough to be detected using primitive light microscopes. Giant polytene chromosomes, found in certain cells such as the salivary gland cells of Drosophila (see Figure 1a), contain many thousands of copies of each chromosomal DNA align Acknowledgements Except for third party materials and otherwise stated (see terms and conditions). This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: 2.8.1 Adipose tissue In this unit, there have been several references to adipose tissue. You probably have a sense of a rather inert (and, perhaps, hard to shift!) tissue acting as a long-term energy store, but this is only a small part of the story. Adipose tissue consists of individual fat cells (adipocytes (Figure 12)) togeth 1.5 Obesity At the time of writing (2004) 20% of the adult population of the UK is classified as obese. The number of obese children has doubled since 1982, 10% of six year olds and 17% of fifteen year olds are now classified as obese. As shown in Table 4, obesity is recognized when the BMI exceeds 30 and occurs quite simply wh 7.5 Outstanding issues Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes. Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years. It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat 7.3 The central engine An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt. The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System. The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M ⊙ and Schwarzschild radius 7.2 Types of active galaxy All active galaxies have a compact, energetic nucleus – an AGN. Seyfert galaxies are spiral galaxies with bright, point-like nuclei which vary in brightness. They show excesses at far infrared and other wavelengths, and have strong, broad emission lines. Quasars resemble very distant Seyfert galaxies with very luminous nuclei. They are variable. About 10% are strong radio sources thought to be powered by jets of material Radio-quiet AGNs There has been a great deal of debate about whether there really are two different types of Seyfert or whether they can both be accounted for by the same model. For example, suppose that you look at the model AGN in Figure 36a from a direction not too far from the rotation axis of the torus. You will see Narrow-line region The model places the narrow-line region much further out from the central engine where orbital speeds are lower; 200–900 km s−1 is typical for the NLR. An important consequence of the NLR being outside the dust torus is that it is always in view, so narrow lines will be seen even if the broad-line emitting gas is obscured. 5.3 The broad- and narrow-line regions In our model, the engine is surrounded by gas clouds (Figure 32). You have already seen how common these are in our own and other galaxies, so it is reasonable to expect them to be present in at least the spiral galaxies that contain AGNs. If these gas clouds are illuminated by ultraviolet or X-rays from the e 4.7 Jets You have seen that two kinds of active galaxies – quasars and radio galaxies – are often seen to possess narrow features called jets projecting up to several hundred kiloparsecs from their nuclei. If these are indeed streams of energetic particles flowing from the central engine, how do they fit with the accretion disc model? How could the jets be produced? The answers to these questions are not fully resolved, but there are some aspects of the model of the central engine which prob
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