10. LIGHT AND ANIMAL LIFE 



G. L. Clarke and E. J. Denton 



Light is important to marine organisms in relation to photosynthesis, vision 

 and other vital processes. Since the growth of green plants is the first step in 

 the ecological cycle of the sea, just as it is on land, the supply of energy for 

 photosynthesis by means of the penetration of sunlight into the water is of 

 critical importance, directly or indirectly, to all life throughout the entire 

 ocean. Due to the fact that a minimum light intensity of about 1% of noon 

 sunlight is required for sufficient photosynthesis for growth, green plants are 

 limited to the upper layers of the sea, the exact depth depending upon the 

 transparency (see Chapter 13). Many animals can use their visual powers at 

 light intensities as low as 10~io of sunlight, or even less, and such reactions as 

 photokinesis, phototaxis, phototropism and photoperiodism may also be 

 elicited by intensities far below that required by photosynthesis, and hence at 

 very much greater depths. Quantitative information on the condition of light 

 in all parts of the ocean is thus necessary for an understanding of the factors 

 controlling the occurrence and activities of the various types of marine life 

 (Clarke, 1954, chap. 6). 



The aspects of light having ecological importance in the sea are : the intensity 

 (or ambient light flux), length of day, quantity of light energy received per 24- 

 hour period, rate of change of light intensity in time and space, cyclic changes 

 (such as those of the day, month and year), spectral composition (including 

 special effects of ultra-violet radiation), angular distribution (including direc- 

 tion of maximum flux and degree of diffusion) and polarization. The control of 

 many of these aspects originates above the surface (such as the change in flux 

 due to the rising and setting of the sun), but for other aspects it originates 

 within the water (such as changes in diffusion due to a turbid stratum), or is 

 profoundly modified by the water (such as spectral changes due to selective 

 absorption). 



Direct measurements of ambient light flux have been made at a variety of 

 places and under various circumstances using watertight photometers lowered 

 into the sea. Values obtained indirectly by extrapolation from measurements 

 of water samples brought into the laboratory have been less reliable because of 

 the difficulty of, flrst, detecting minute differences between small samples of 

 clear sea-water, secondly, allowing for changes in the water sample after being 

 removed from the ocean, thirdly, allowing for differences caused by the walls 

 of the measuring tube and the use of an artificial light source, and, lastly, 

 determining the effects of scattering and absorption in overlying water strata 

 for samples coming from great depths (Clarke and James, 1939). 



No one instrument used at sea can provide information on all the optical 

 properties of the water of interest to the biologist, but measurements have 

 been obtained of the average extinction rates through the use of a type of 

 photometer containing a light-sensitive element behind a flat, horizontal 

 receiving window, usually covered with a transluscent diffusing disc and some- 

 times also by a colour filter (Angstrom et at., 1938). Observations are made of the 



[MS received June, 1960] 456 



