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ch7

Terms in this set (10)

Sea water is the water in the sea or an ocean and on average it has a salinity of about 35 parts per thousand (35 ‰). The salinity varies and may be significantly lower than this, for example where sea water is diluted by freshwater from a river, or as a result of melting glaciers.
The chemical composition of sea water remains fairly constant and the salinity is mainly due to the presence of sodium ions and chloride ions. Other ions, including sulphate, magnesium, hydrogencarbonate and potassium, are also present. Table 4.1 shows the concentrations of major ions present in typical sea water.
Although the composition of sea water remains fairly constant, and probably has done for many millions of years, local changes can occur as a result of volcanic activity, runoff and atmospheric dissolution. Gases emitted from volcanoes include carbon dioxide, sulphur dioxide, hydrogen sulphide and hydrogen chloride. These gases dissolve in atmospheric water and enter sea water in precipitation, as part of the hydrological cycle. Submerged volcanoes at tectonic plate boundaries emit gases, including chlorine, and gases from volcanoes are the major source of the chloride ions present in sea water.
Runoff refers to the flow of water from land, arising from rain or from melting snow and ice. As part of the hydrological cycle, much of this water may eventually drain into oceans, either directly, or from rivers. Water passing through soil, or water from urban runoff flowing into drains, may pick up a variety of pollutants, including pesticides, fertilizers and oil-derived substances. Some pollutants have initially have a very low concentration, but if taken up by living organisms, the pollutants can pass through food chains and food webs, increasing in concentration at each trophic level. An example of this is the release of mercury compounds in industrial wastewater entering Minamata Bay from 1932 until 1968. These mercury compounds accumulated in shellfish and other marine organisms. When shellfish were eaten by humans, the mercury compounds caused neurological disorders, paralysis and death.
Gases dissolved in sea water are in equilibrium with the atmosphere. However, the actual concentration of gases in sea water depends on their relative solubility, and the temperature and salinity of the water. The gases dissolved in sea water consist mainly of nitrogen, oxygen and carbon dioxide. Nitrogen may be fixed by nitrogen-fixing microorganisms into products which are usable by other organisms. Oxygen is essential for respiration and carbon dioxide is used in the process of photosynthesis
The density of water depends on temperature and salinity. As the temperature increases, the density of water decreases. There is a tendency, therefore, for warm water to form a layer on top of colder, denser water. The result of this is the formation of a temperature gradient and the temperature generally decreases as the depth increases. There is a relatively shallow layer of warn water floating on a deep layer of colder water. The interface between the two layers, where the temperature decreases abruptly as the depth increases is referred to as the thermocline. The surface layer of water in an ocean may reach a temperature of 25 °C or higher, the temperature decreases to about 1 °C at depths of over 2000 m.
There is a similar gradient of salinity in the oceans. As the salinity of water increases, the density of water also increases. There is a tendency therefore, for water with as lower salinity, and therefore lower density, to float on top of water with a higher salinity. This effect results in a general increase in salinity as the depth increases, with a region referred to as the halocline where there is a significant change in salinity as the depth increases.
Wind blowing across the surface of the sea can set up turbulence and currents, resulting in mixing of the surface layers, down to a depth of about 200 m. Mixing can also occur as a result of temperature changes; if the surface layer cools, for example, the density will increase and the water will tend to sink.
(d) demonstrate an understanding of the physical and biological reasons for the variability of the concentration of dissolved oxygen
The concentration of dissolved oxygen in water is very variable and depends upon a number of factors. As a general rule, as the temperature of water increases, the solubility of oxygen in water decreases
The regular rise and fall of sea level is referred to as the tide and is due to the gravitational effects of the Sun, Moon, Earth and the rotation of the Earth. Tides have a cycle of approximately 12.5 hours; so most coastal areas experience two high tides and two low tides every day.
The tidal amplitude varies. When the Earth, Moon and Sun are aligned, the amplitude is greatest, resulting in what are known as spring tides (see Figure 7.1. At other phases of the Moon, such as first quarter, the tidal range is smaller; these are referred to as neap tides. 29
oxygen as a by-product. Dissolved oxygen is removed from water by respiration of marine organisms.
The concentration of dissolved oxygen changes as the depth of water changes. It is generally high in the surface layers, but decreases to a minimum as the depth increases, before increasing again as the depth continues to increase. The depth at which the concentration of dissolved oxygen is lowest is referred to as the oxygen minimum layer. This is usually between depths of 100 m and 1000 m.
(e) describe how tides are produced, and how the alignment of the Moon and Sun, coastal geomorphology, wind, air pressure and size of water body affect the tidal range
The regular rise and fall of sea level is referred to as the tide and is due to the gravitational effects of the Sun, Moon, Earth and the rotation of the Earth. Tides have a cycle of approximately 12.5 hours; so most coastal areas experience two high tides and two low tides every day.
The tidal amplitude varies. When the Earth, Moon and Sun are aligned, the amplitude is greatest, resulting in what are known as spring tides (see Figure 7.1. At other phases of the Moon, such as first quarter, the tidal range is smaller; these are referred to as neap tides.
Figure 7.1: Spring tides occur when the Earth, Moon and Sun are aligned during a new moon (as illustrated here) or during a full moon.
The tidal range (tidal amplitude) is the difference in height between low water and high water and varies considerably in different parts of the World, from over 12 m to practically nothing. The nature of the coast, including slope, size of the body of water, and weather conditions can all affect the tidal range.
The shape of the coastline can influence the tidal range. For example, where the tide enters a tapering river mouth, the height of the tide is increased by the opposite sides of the channel. A similar effect occurs in a bay. Changes in wind and air pressure can have a significant effect on the tidal range. For example, a strong on-shore wind and low atmospheric pressure can produce a 'tidal surge' resulting in an exceptionally high tide.
In the oceans, the tidal amplitude is about 0.6 m; this is increased as the oceanic tide enters the shallow continental margins. The lowest tidal amplitudes are found in relatively small bodies of water, including the Mediterranean Sea, Red Sea and the Baltic Sea. Small tidal ranges occur in large lakes, but this effect is often masked by the effect of wind.
El Niño (also referred to as the El Niño southern oscillation) is a sequence of events that occurs in the southern Pacific Ocean. In normal conditions, cold water, rich in nutrients, flows in a northerly direction along the west coast of South America. This is accompanied by an upwelling of nutrients, caused by winds blowing form the south. This results in the water having a high productivity, with very large numbers of anchovies and sardines feeding in the plankton-rich water. This high productivity supports a substantial fisheries industry and many species of sea birds and other organisms.
Approximately every 7-10 years, the prevailing winds stop blowing in their normal pattern from the east or south-east. Warm equatorial water is blown by abnormal winds from the west. As a result, pressure gradients in the west and east Pacific Ocean are reversed, causing a reversal of wind direction and equatorial currents. This creates a large area of warm water; upwelling stops and so the supply of nutrients to the surface water is reduced. The increase in temperature results in the death of many cold-water species and, coupled with the lack of nutrients, this causes the primary production to decrease dramatically. This affects higher trophic levels in food chains and food webs with the consequent collapse of commercial fish stocks.
A major El Niño event occurred in 1982-1983. Surface temperatures increased by 5 °C, accompanied by heavy rain in the normally dry eastern Pacific. The exact cause of El Niño is not known, but it has been suggested that it could be a consequence of global warming.