Underwater observations

It would simplify many problems in marine biology if the range of direct observation could be extended. The only marine populations which are easily accessible to close inspection are those of the seashore, and then only for a part of each tidal cycle. Our knowledge of the rest of marine life comes almost entirely from the incomplete samples obtained by nets, dredges, grabs and similar devices. Recently, new techniques for visual underwater exploration have been developed, and have already provided much new information on marine organisms.

3.3.1 Diving

Diving by means of air pumped down a tube from the surface to a man enclosed in a special helmet and diving suit was first introduced in 1819 by Auguste Siebe. This was the prototype of hard hat diving. Apparatus of this type is still used by commercial divers working in connection with underwater constructions or salvage operations, but has found little application in biological work. Apart from its unwieldiness and expense, it has the drawback that it does not permit free movement of the diver over a wide area because he is limited by the length of his breathing tube and the need to keep it free from snags.

This problem has disappeared with the development of the aqualung or SCUBA gear (Self-Contained Underwater Breathing Apparatus). This provides a diver with a means of moving freely underwater, unencumbered by an airpipe, his air supply being carried on his back in compressed air cylinders. In 1942, Jaques Cousteau and Emile Gagnan developed the first fully automatic regulator or 'demand valve' that provided air from the cylinder only 'on demand'. Modern equipment is based on their original design.

In recent years, the design and amount of diving equipment available have increased tremendously. Large-capacity diving cylinders and warm dry-suits have increased the efficiency and time under water of divers. Diving computers allow for easier decompression planning. Underwater tape recorders, communication systems and cameras enable more data to be collected. Towed and self-propelled sledges allow divers to survey large areas.

Diving carries with it certain inherent risks and proper training and certification are essential. In most instances, training with organizations such as the BSAC (British Sub Aqua Club) or PADI (Professional Association of Diving Instructors) is adequate. However, in Great Britain, since 1981, paid work involving diving has brought biologists and scientists under the HSE (Health and Safety Executive) regulations and additional training and certification may be required.

Physiological hazards

The physiology and problems associated with diving are described in detail in a wide variety of publications such as the British Sub-Aqua Club diving manual (BSAC, 1985; Bennett and Elliott, 1993; Moon and Bennett, 1995). Only a short summary of the major problems is given here.

The special problems of breathing underwater are due to the pressure of water surrounding the body. On the surface at sea level, the normal air pressure is 1 atmosphere (atm) or 1 bar absolute. The pressure increases approximately 1 atm for every 10 m of depth. For a diver to be able to expand his lungs against the water pressure, he must be supplied with air at a pressure equal to that of the water. Whereas at the surface we breathe air at atmospheric pressure (1 atm), at a depth of 10 m the diver must have air at double this pressure, that is at 2 atm; at 20 m, 3 atm and so on. The aqualung cylinder contains air at very high pressure (200-300 bars). Modern demand valves or regulators reduce this pressure in two stages to match the ambient pressure of the surrounding water. The first stage (the reducing valve) is attached to the valve of the diving cylinder and reduces the pressure to about 8-10 bars above ambient. The second stage (the demand valve) is held in the diver's mouth and reduces the pressure to ambient i.e. equal to the pressure of the surrounding water.

Diving using air is without serious physiological hazards down to about 9 m depth. Below this, precautions must be taken to avoid the dangerous condition of 'decompression sickness' during ascent. The 'bends', as it is often called, occurs when bubbles of gas (mainly nitrogen in air-breathing divers) are liberated into the tissues or the blood. As the diver ascends, the ambient pressure falls, and gas dissolved under pressure in the blood can come out of solution too rapidly - a process similar to the fizzing of soda-water when the bottle is unstoppered. Depending on how large the bubbles are and where they lodge, decompression sickness can produce a variety of severe symptoms including intense joint pains (bends), paralysis or, in extreme cases, death.

Decompression sickness is avoided by following a decompression table which tells the diver how long he can safely stay at a particular depth and still come straight to the surface. Beyond this time limit, it is necessary to make a gradual ascent involving a series of pauses, or 'decompression stops', to give ample time for the excess nitrogen dissolved in the blood and tissues to be eliminated in the expired breath. Nowadays, modern diving computers are often used in place of tables.

The normal safe depth limit for compressed air diving is considered to be 50 m and, in Britain, divers who are covered by the HSE safety regulations are limited to this depth. Below about 30 m, air-breathing divers face another problem known as nitrogen narcosis. Large quantities of nitrogen dissolved under pressure in the blood have an effect on the brain producing a condition of rapturous inebriation in which the diver loses control of his actions, with possibly fatal results. Commercial divers overcome this problem by using 'heliox' - a mixture of oxygen and helium. This can be used down to about 500 m and is much safer than compressed air but has the drawback of causing distortion of the voice, making speech communication difficult. Experiments are now underway in which oxygen is mixed with hydrogen to produce 'hydrox' which can be used at even greater depths. Use of such equipment is expensive, requires special training and is not much used in scientific work. However, recent developments in the field of 'nitrox' diving are now allowing safer, deeper diving with an acceptable level of extra cost and training. Nitrox gas consists of an oxygen/nitrogen mix in different proportions to atmospheric air. Closed-circuit, re-breathing technology also looks set to extend diving limits for scientific divers in the future.

Saturation diving from underwater chambers in which divers can live at pressure has obvious advantages in time and costs and is much used by the offshore oil industry. At any particular pressure the body can absorb only a certain amount of gas before becoming saturated. Once the saturation point is reached, no more gas will be absorbed and the time required for decompression will then be the same, however long the duration of the dive. Divers return to their submerged 'house' to feed or sleep and replenish their gas supplies, allowing them to have relatively long working periods at depth without time wasted on frequent decompression and resurfacing. A diver who spends a week at 150 m requires only the same decompression period as one who descends to that depth for only one hour.

Attempts to dive much deeper than 200 m encounter additional dangers. Modern medicine is suggesting that the breathing of oxygen and inert gases such as helium at high pressure causes causes long-term changes in the diver's nervous system and physiology. Another serious hazard of deep saturation diving is bone necrosis, the death of areas of bone apparently caused by blockage of blood vessels, sometimes leading to severe arthritis. Recent predictions put the lowest limit to which divers exposed to pressure may be able to work at between 500 and 1000 m. At the time of writing, the record for successful descent and return using aqualung equipment appears to be held by six French divers operating from a diving bell and breathing an oxygen-helium mixture. At a depth of 460 m they each worked for periods up to 2 hours 20 minutes over four days. Two of them then descended for ten minutes to 501 m, after which about ten days was spent on decompression.

The aqualung in ecological research

The aqualung is a tool that has many applications in marine biological investigations in shallow water. It makes possible many quantitative studies on distribution and growth of marine organisms by direct observation with minimal disturbance of their natural environment. The behaviour of marine animals can be recorded in their normal surroundings. Photographs can be taken of precisely selected areas and events and changes can be monitored. Divers can operate many types of underwater equipment which would otherwise have to be remotely controlled from the surface or might not be usable at all in particular localities (Kritzler and Eidemuller, 1972; Potts, 1976).

Plankton receiver

Propulsion unit

Plankton receiver

Throttle cord Removable skid

10cm

Figure 3.28 A diver-controlled plankton net.

(From Potts (1976) by courtesy of Cambridge University Press.)

Propulsion unit

Propeller cowl

Steering handle

Steering handle

Propeller cowl

Throttle cord Removable skid

10cm

Figure 3.28 A diver-controlled plankton net.

(From Potts (1976) by courtesy of Cambridge University Press.)

Diver-controlled nets (Figure 3.28) can be used in rock gullies and around submerged reefs where it would not be possible to tow conventional nets from the surface. Diver-controlled dredges can be opened and closed so as to sample only selected parts of the sea floor, and can be raised or steered to avoid snags and obstacles. Grabs and corers can be exactly positioned to take only the material needed (Figure 3.27). In sediment areas, divers have been used to study crab and fish burrows by taking resin casts - rather like animal track casts on land.

Diving is particularly useful when studying the shallow, rocky areas just below the shore - the sublittoral. Remote sampling is extremely difficult in such areas and it is only in the past 15 to 20 years that an accurate picture of such areas has been obtained. A similar comment applies to the study of coral reefs. Baseline ecological surveys of marine habitats and species are vitally important in terms of marine conservation. It is essential to know what is there before sensible decisions can be taken regarding development (e.g. oil exploration, marina development, etc.) and response to pollution incidents. With modern equipment, divers are able to carry out research even in such adverse places as under the Antarctic ice.

In Britain, divers are playing an important part in the Marine Nature Conservation Review, a project started in 1987 to survey and assess coastal marine habitats (see page 231). Standard recording methods for the survey of shallow sublittoral areas using divers have been developed. The massive increase in sport diving in recent years has also led to the development of a breed of 'underwater naturalists'. There is now a wide range of marine projects, expeditions and surveys in which amateur divers can participate and the results are providing many useful data.

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Renewable Energy 101

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