SKUT. 4] LUJHT AND ANIMAL LIFIO 457 



Hiix ])er unit area from sunlight plus skylight above the water surface, below 

 the surface, and at all desired depths in the water to the limit of the instrument. 

 The vertical extinction coefficient, k, is found from the equation ///o = e~^'^ 

 (where /o = intensity ^ at one depth, / = intensity^ at a second depth, L — 

 vertical distance between depths in metres, and e = 2.7). Many of the photo- 

 meters used for the measurement of light flux have broad spectral sensitivities. 

 The range of some covers the whole of the visible spectrum and this also in- 

 cludes the spectral regions of most significance to marine organisms (see below 

 for animals and Chapter 13 for plants). The sensitivity of other photometers 

 has been narrowed for the investigation of particular regions of the spectrum as 

 discussed elsewhere in this chapter (see also Tyler, 1960). Photometers employ- 

 ing photovoltaic cells, which can respond to intensities as low as 0.1% of 

 sunlight, or slightly lower, are useful for light measurements in the upper 

 water strata (to about 220 m in the clearest water) and particularly in relation 

 to photosynthesis. Photometers with photomultiplier tubes are sensitive to 

 intensities as low as 10~'^ (jiW/cm^ (about lO"^^ of sunlight), or slightly lower, 

 and hence may be used for light measurements at great depths and the study 

 of the photic reactions of animals (Clarke and Hubbard, 1959). 



When a photometer, sensitive to the visible spectrum and the near-ultra- 

 violet, is lowered into optically homogeneous water, the rate of reduction of 

 light is found to be higher near the surface. This is due to the rapid loss of 

 radiation near the limits of this portion of the spectrum in the upper water 

 layers. At greater depths the spectrum has been narrowed by the water to the 

 most penetrating wave-lengths. Since this effect becomes small below a depth of 

 about 30 m in the clearest water and at shallower depths in less transparent 

 water, photometers with a broad spectral sensitivity may be used for measuring 

 the extinction coefficient of the remaining portion of the spectrum at greater 

 depths. Photometers with a narrowly limited spectral sensitivity or field of 

 view, or with other special design, have provided additional information on 

 the optical conditions in the sea. Extensive studies of extinction rates, angular 

 distribution, and colour in a variety of water-masses around the world have been 

 reported by Jerlov (1951) and he has proposed a classification of sea- waters on 

 the basis of their optical properties. Recent investigations of the penetration of 

 ultra-violet radiation into the sea have been reported by Lenoble (1956). 



Coastal waters have generally been found to have their maximum trans- 

 parency in the yellow-green portion of the spectrum (500-600 mjj.) as is also 

 true of most fresh waters. Curves showing the transparencies of representative 

 natural waters based on the average extinction coefficient for this component 

 of daylight are given by Clarke (1954, fig. 6.8). Depths at which daylight is 

 reduced to 1% of its surface value usually range from 10 to 30 m (A; = 0.46 to 

 0.15) in coastal waters, as compared to about 100 m (A; = 0.046) for clear oceanic 



1 The word "intensity" refers to the flux collected by the diffusing disc described above. 

 If this disc is designed to collect according to the cosine law, then the "extinction coeffi- 

 cient" will be numerically the same as the "diffuse attenuation coefficient" described in 

 Chapter 8. 



