Illumination

Compared to the depth of the ocean, light does not reach very far into the sea. Illumination of the surface layers varies with place, time and conditions depending upon the intensity of light penetrating the surface and upon the transparency of the water. The strength of the incident light varies diurnally, seasonally and with latitude, and is influenced by cloud conditions and atmospheric absorption. Much of the incident light is reflected from the surface, more light being reflected from a ruffled surface than a calm one, and reflection increases as the sun becomes lower in the sky. Depending on conditions some 3-50% of incident light is usually reflected. The light which penetrates the surface is quickly absorbed, partly by the water and dissolved substances but often largely by suspended matter including planktonic organisms, translucent water generally being indicative of a sparse plankton. Extinction coefficients (see Section 3.1.4) vary from about 1.0 to 0.1 between turbid inshore and clear offshore areas, but in exceptionally transparent ocean water may be as low as 0.02. Even in clear water about 80% of the total radiation entering the surface is absorbed within the uppermost 10 m, and in more turbid water the absorption is far more rapid than this. The heating effects of solar radiation are therefore confined to a very thin surface layer.

Different wavelengths of light do not penetrate equally. Infrared radiation penetrates least, being almost entirely absorbed within the top 2 m, and ultraviolet light is also rapidly absorbed. Within the visible spectrum, red light is absorbed first, much of it within the first 5 m. This is why underwater photographs taken without a flash have an overall bluish-green colour. In clear water the greatest penetration is by the blue-green region of the spectrum, while in more turbid conditions the penetration of blue rays is often reduced to a greater extent than that of the red-yellow wavelengths. This differential absorption of the solar spectrum partly accounts for the colour of the sea's surface by its effect on the spectral composition of reflected light. In bright sunlight, clear ocean water may appear very blue because the yellow and red rays are largely absorbed, and blue rays predominate in light reflected from below the surface. In more turbid coastal waters, their greener appearance may result from the relatively greater absorption of blue light. Sometimes the colour of the water is due to the pigmentation of minute organisms.

Light is of supreme biological importance as the source of energy for photosynthesis (see Section 5.3.1). Primary food production in the marine environment is virtually confined to the illuminated surface layers of the sea where there is sufficient light to support plant life. The depth of this photosynthetic or euphotic zone varies with conditions, extending to some 40-50 m from the surface in middle latitudes during the summer months and to 100 m or more in low latitudes if the water is fairly clear. Below the euphotic zone, down to about 200 m, is the dimly illuminated dysphotic zone where light is insufficient for the survival of plants. The water below 200 m is termed the aphotic zone because there is little or no light, but in clear tropical waters a small amount of blue radiation penetrates to at least 1000 m.

Light is certainly one of the major factors controlling the distribution of marine organisms, but in many cases its effects are not easy to understand. Plants are restricted to the euphotic zone by their dependence upon light for energy; and animals are most numerous in or near the surface layers because they derive their food, directly or indirectly, from plants. Below the productive surface zone, animals are almost entirely dependent upon food sinking to them from above, and the deeper they are, the less food is likely to reach them because much of the assimilable material is decomposed on the way down. Broadly, the deeper the level, the less the food supply and the fewer the population. But numbers do not fall off evenly with depth. Although we do not have much knowledge about the distribution of mesopelagic and bathypelagic forms, it appears that populations tend to congregate at certain levels. There is commonly a concentration of animals somewhere between 300 and 1000 m, sometimes coinciding with the oxygen-minimum zone, and lesser concentrations below this. Many organisms, however, do not remain consistently at one level but perform vertical movements, often over a considerable distance, which are related to changes of illumination (Figure 4.10).

Figure 4.10 Day and night distributions of total migrant and non-migrant mesopelagic euphausids off the Canary Islands taken during a cruise by RRS Discovery, l965. A - Total numbers per haul; B - Total weight per haul. Pecked line in upper 100 m of night distribution indicates catches adjusted by addition of species assumed to be missed at night by migrating above 50 m. Pecked line at 250 m in night distribution indicates catch when a large sample of Nematoscelis megalops is omitted.

(From Baker, A. de C., J. Mar. Biol. Ass. UK, 50, 301-42 (1970) published by Cambridge University Press.)

Figure 4.10 Day and night distributions of total migrant and non-migrant mesopelagic euphausids off the Canary Islands taken during a cruise by RRS Discovery, l965. A - Total numbers per haul; B - Total weight per haul. Pecked line in upper 100 m of night distribution indicates catches adjusted by addition of species assumed to be missed at night by migrating above 50 m. Pecked line at 250 m in night distribution indicates catch when a large sample of Nematoscelis megalops is omitted.

(From Baker, A. de C., J. Mar. Biol. Ass. UK, 50, 301-42 (1970) published by Cambridge University Press.)

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