Most bioluminescence in the oceans is of a sheet-type display and is produced by one-celled organisms. This type is most commonly observed in coastal waters. The color is usually green or blue and many displays appear white when the organisms are present in great numbers.

Spark-type displays are created by a large number of crustaceans. Most of these displays occur in colder, disturbed waters and only rarely in tropical waters. The light emitted gives the ocean surface a "twinkling" appearance.

Glowing ball or globe-type displays are seen most frequently in the warmer waters of the world. The ocean may seem to be full of balls or discs of light, some flashing brightly as they are disturbed, and others dimming after the initial disruption has ceased. The light given off is usually blue or green; displays of white, yellow, orange, or red have occasionally been reported. Luminescent jellyfish also cause many glowing-ball displays. Large shining round or oval spots of light may appear in the water.

Exotic light formations like wheels, undulating waves of light, and bubbles of light appear to be separate and distinct from the displays previously discussed. The cause of such phenomena are still unknown.

ARABIAN SEA BIOLUMINESCENCE 

The Arabian Sea is one of the richest areas in the world for marine bioluminescence. It is known to appear with the onset of the southwest and northeast monsoons. Reports indicate that there is no correlation between this phenomena and meteorological conditions.

LEARNING OBJECTIVES: Recognize the six factors affecting underwater visibility.

Compare water transparency in various parts of the North Atlantic ocean.

Visibility in seawater is restricted in a manner somewhat similar to the restriction of visibility in the atmosphere. The restriction in seawater differs from that in the atmosphere primarily because of scattering (predominant in coastal waters) and absorption (predominant in deep, clear ocean waters).

FACTORS AFFECTING UNDERWATER VISIBILITY

Underwater visibility depends primarily upon the transparency of the water, reflectance and contrast, water color, sea state, incident illumination, and optical image.

The term transparency is often thought of as that property of water that permits light of different wavelengths to be transmitted; transparency is sometimes measured as the percent of radiation penetrating a path length of 1 meter. However, the most commonly used definition and measurement of transparency, as applied to underwater visibility, is the average depth below sea surface at which a Secchi disc (white disc) first disappears and then reappears at the surface to an observer who successively lowers and raises the disc.

The degree to which seawater becomes transparent is a function of the combined effects of scattering and absorption of light by the water surface, suspended, organic and inorganic particulate matter, dissolved substances, plankton, and the waters molecular structure.

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