At one time the depths of the ocean were thought to be totally lifeless. But now scientists know that the whole of the ocean environment, down to the very greatest depths at more than 11 kilometres, is populated by living organisms. The oceans provide about 170 times as much living space as all of the Earth's other environments--soil, air and fresh water--put together. The floor of the deep sea may harbour many times more species than these other environments. In this session we will examine the general characteristics of the oceans--geography, sea water, temperature and currents.

NASA

The Earth from space. The dominance of the oceans, and why the Earth is called the 'Blue Planet,' becomes obvious from this viewpoint. The oceans cover two-thirds of the Earth's surface to an average depth of almost 4 km and to an extreme depth of more than 11 km. Life on Earth almost certainly evolved in the ocean and the whole ocean environment, down to the very greatest depths, is populated by living organisms.

The shape of the ocean basins

	The Problem of Pressure
	The weight of air in the earth's atmosphere squeezes everything in it equally in all directions. This atmospheric pressure is at its maximum at sea level where it is almost one kilogram per square centimetre (by convention called 1 atmosphere). The weight of water in the sea also creates pressure, and because water is so heavy, the pressure changes rapidly by about one atmosphere for every 10m of depth. So the pressure at 100m depth is about 10 kilos per square centimetre, at 1,000m it is about 100 kilos and so on. It seems inconceivable that any deep-sea animal could withstand these crushing pressures, almost half a tonne on every square centimetre at the ocean's average depth of 4,000m. But the bodies of most marine animals are made up mainly of water and, because liquids are almost incompressible, the animals suffer very little from the effects of quite large pressure changes. However, many fish, including deep-sea species, have gas-filled bladders that allow them to stay neutrally buoyant. To do so the bladder volume must remain more or less constant. But unlike liquids, gases are very compressible. So when the pressure doubles, the volume of the bladder is halved, whereas if pressure is halved, the volume is doubled. Since the relative change in pressure for each unit change in depth is greatest near the sea surface, fish with gas bladders living in this region have to be able to add or remove gas from their bladders as they move up and down, otherwise the volume of the bladder would change and upset the buoyancy. The volume of the gas bladder of a herring, for example, moving from a depth of 30m to the surface, would increase fourfold. To experience a similar fourfold decrease in pressure and therefore a fourfold increase in the volume of its gas bladder, a fish living at a depth of 10,000m would have to swim up to a depth of 2,500m. If, as in the example of a herring, it swam upwards for only 30m, that is to 9,970m, the pressure would change only from 1,000 to 997 atmospheres and the volume of its bladder would similarly change by only three one-thousandths, almost nothing. So deep-sea animals, including fish, can wander quite freely over large depth ranges without being concerned by the pressure changes involved.

All the Earth's major land masses are surrounded by relatively shallow seas with fairly flat bottoms no more than about 200 metres deep. These areas, the continental shelves, on average extend to roughly 60 kilometres from the shore line. They represent only about one-twentieth of the total area of the oceans, but they are by far the richest parts biologically and provide us with most of the marine fish we eat.

At the outer edges of the continental shelves the sea bed gradually falls away, down a shallow gradient, into the deep sea proper. These continental slopes and rises represent about one-quarter of the total surface area of the oceans. The slopes vary enormously in steepness, in some places being dissected by dramatic canyons with near-vertical walls.

At a depth of around 3,500-4,000 metres the bottom flattens out to an almost imperceptible slope. This is the abyssal plain, the largest single environment on earth, which underlies over half the ocean surface with depths down to 6,000m. Finally, in some small areas of the oceans, particularly in the western Pacific, the sea floor drops away even further into elongated gashes, the trenches, with water depths of 10-11 kilometres.

Interrupting the abyssal plain is a vast interconnected mountain chain, the mid-oceanic ridge system, extending over some 45,000 km and crossing all the major oceans except the North Pacific. In some places the ridges actually rise above the sea surface as oceanic islands. The ridges are the sites of production of new sea floor which rises as molten rock from deep within the Earth and then cools and moves away from the ridges at a rate of a few centimetres a year, like a great conveyor belt. On its slow journey away from the ridges, the sea floor becomes carpeted with muddy sediment. This is made up of the remains of billions of tiny animals and plants that lived in the overlying waters and whose skeletons sank to the bottom after they died, along with volcanic ash and land-based material carried into the sea by rivers and winds. The sediment has accumulated over millions of years, so most of the irregularities on the sea floor are smothered by a layer of mud, hundreds or even thousands of metres deep. The upper layers of this sediment provide the living space for the animals of the deep-sea floor.

Sea water
The most characteristic feature of sea water is that it is salty. It is a complex solution of many different chemicals--there are about 35 grammes of salt in every litre of sea water. Almost nine-tenths of this is common salt or sodium chloride. The remainder probably includes all the chemicals that occur naturally on Earth--along with a few that have been introduced by humans. Many of the chemicals in sea water occur in very low concentrations and vary over time and space, as a result of biological processes going on within the oceans. But several of the major constituents, including sodium and chlorine, maintain very similar concentrations throughout the seas and have done so over many millions of years. This constancy, relative to fresh water and dry land, makes sea water an easy environment for animals and plants to cope with physiologically. It is also the reason why the very first life on Earth almost certainly evolved in the ancient seas.

Restless waters
Although sea water is fairly constant in its chemical composition, it is certainly not the case where movement is concerned. The most obvious water movements are waves and tides, but although tidal currents can be detected even at the bottom of the deepest oceans, their main effects, and certainly those of waves, are restricted to very shallow layers. In the ocean depths the more important movements are those of the global system of oceanic currents, which are crucial in controlling the Earth's climate.

Surface Currents
Driven mainly by the winds, surface currents form a complicated system of gyres--large circular currents that move clockwise in the northern parts of the ocean and anticlockwise in the south. The best he North Atlantic, includes the Gulf Stream, which transports warm water from the Caribbean along the eastern seaboard of North America and then across the ocean to Europe. This gyre is completed by the Canaries Current in the Eastern Atlantic which transports relatively cold water back towards the Caribbean. The Gulf Stream is not a simple current but a vast, moving mass of water, which gives rise to complex swirls and eddies on either side. The enormous amount of heat transported by it helps to make the climate in north-western Europe much warmer than that at similar latitudes on the other side of the ocean. One of the major worries about global warming is that this system could be stopped, paradoxically making Europe much colder than it is today.

	Ready Reference
	Abyss Greek word meaning 'bottomless'. The deep part of the oceans, between about 3,000m and 6,000m deep. Abyssal plain The vast flat sea floor beneath the abyss. Almost always covered in fine mud. Continental rise The bottom part of the continental slope, where the deep-sea floor begins to flatten out on to the abyssal plain. Continental slope Fairly steeply sloping part of the sea floor between the edge of the continental shelf, at about 200 metres, and the continental rise at 3,000-4,000m. Global warming The processes resulting in a rise in the average temperature of the atmosphere and the oceans. Gyres Circular movements of the oceans, particularly the enormous ones stirring the surface layers of the Atlantic and Pacific in clockwise directions in the northern hemisphere and anticlockwise in the southern Hemisphere. Hydrothermal vents Springs of very hot, chemically-laden water gushing through the sea floor along the mid-ocean ridge system. Mid-ocean ridge system A more-or-less continuous, 45,000 km long, submarine mountain range running through the world's oceans. The ridge is the site of production of new sea floor and of hydrothermal vents. Trenches The deepest parts of the ocean at depths greater than 6,000m.

A similar gyre occurs in the Pacific. This includes the infamous El Niņo phenomenon. The eastern part of this gyre, the northward-flowing Peru Current, is associated with rich plant and animal growth in the surface waters and very important fisheries. Every few years the system fails and the 'rich' Peru Current is displaced by a very 'poor' southward-flowing current; this is El Niņo. The major changes in water flow associated with El Niņo not only cause dramatic changes in the marine conditions off the coast of South America, but may have important effects on the weather in many other parts of the world.

Deep circulation
Deep beneath the surface another current system gently stirs the waters of the ocean and is much more important for the animals of the deep sea. This thermo-haline (heat and salt) system is driven by the sinking and rising of waters of different heaviness or density, caused by differences in salt content and particularly temperature, rather than by the wind. Heavy (cold and salty) water sinks at high latitudes, especially in the North Atlantic and Pacific oceans, and around Antarctica, to be replaced by warmer, less salty water from lower latitudes. The result is a complex 'layer cake' of interleaved water masses flowing slowly under and over one another in the depths of the ocean. This deep circulation is important--it mixes the water, keeps chemistry more or less uniform and carries oxygen from the atmosphere into the deeper layers, making life there possible.

Temperature
The surface temperatures of the oceans range from 40°Celsius or so in shallow tropical lagoons to -1.9°C, the typical freezing point for sea water, in polar regions. Most of the water in the deeper layers is very cold. On the whole, any warm water in the open ocean is restricted to a shallow, near-surface band. No matter how warm the surface layers are, between 300 and 1,000m beneath the surface the temperature falls to about 5°C and then continues to fall slowly with increasing depth. As a result, even beneath the hottest tropical regions the water at a depth of 2,000-3,000m almost never rises above 4°C--with one dramatic exception. In some places along the oceanic ridge systems, extremely hot sea water gushes out of fissures in the underlying rocks. The water from these hydrothermal vents emerges at incredibly high temperatures--up to 300-400°C but, because of the vast mass of surrounding cold water, the temperature drops to the normal 3-4°C within a metre or so of the vent opening.