The Night Dive

The Night Dive; its a different style of diving to day diving, some divers either love it or they hate it. To those who have never been on a night dive, the obvious question is 'why bother?' The answer lies in the fact that many underwater animals, which hide during the day, come out to feed at night and can be easily spotted in a diver's torch beam. Many of the species of crustaceans and molluscs use the night hours in this way, and some of their predators, such as conger eels, follow suit. By day, many of these species can only be seen at the back of holes or under rocks, but by night they can be found in open spaces.
Although visibility under the water is obviously restricted by the darkness, this situation should not be confused with low-visibility situations caused by particles suspended in the water. At night, the clarity of the water is often very good, and this sensation is heightened owing to the fact that the diver's horizon is restricted to the penetration of his torch beam. This causes the diver to concentrate in the objects that his beam of light shines upon. Sites dived at night often seem to have a lot more life than when dived during the day. In reality this is not so, but during the day the diver's horizon is wider and he may not spot all that is there.
A trick worth remembering is that, because of the inability of low-angle sunlight to penetrate the water surface, the seabed becomes darkened (and nightlife becomes active) before it is fully dark at the surface. The clever night diver uses this fact to aid surface recovery by the cover party after his 'night' dive.
Even a complete night dive is seldom totally dark. If the diver switches off his torch, he will be suprised at the amount of light there is. Relected sunlight from the moon and starlight are major sources, but once the diver's eyes have become accustomed to the dark he may see the water apparently glowing, and when he moves in the water the disturbance causes a display of phosphorescence. This is caused by phosphorescent plankton giving off a glow when they are disturbed.
The use of a torch allows the diver to see easily under the water and, because the light source is close to the objects on which it is shone, the full spectrum of light is available to the objects on illuminate them. This means that instead of the usual blue-green look seen in daylight, the bright colours that actually exist are revealed to the diver. Plants and animals that look black by daylight are suddenly shown to be coloured in scarlets and purples. The same effect is, of course, available to photographers using artificial light and can lead to a rewarding pastime.
The night dive therefore provides an experience in which the senses are sharpened, the animals are more obvious and the natural colours are fully restored by the torchlight, which the diver carries.