SECT. 4] GEOGRAPHIC VARIATIONS IN PRODUCTIVITY 349 



of the water. It is a common misconception that the compensation depth is 

 constant for a given transparency. Obviously it changes throughout the day 

 and has no meaning at night. Hence it is convenient to define the compensation 

 depth more liberally as the maximum depth at which plant growth may occur 

 under clear skies and with the sun overhead. Thus qualified, this parameter is 

 more or less constant for a given body of water and hence is a useful concept 

 for comparative purposes. Since the maximum intensity of visible sunlight at 

 noon is of the order of 10,000 ft-candles and the compensation intensity about 

 100 ft-candles, it is adequate for most purposes to consider the compensation 

 depth as that depth to which 1% of the incident visible radiation penetrates. 



Sunlight in the ocean is attenuated with depth at an exponential rate which 

 may be expressed as the attenuation or extinction coefficient (k), the natural 

 logarithm of the fraction of incident light penetrating to a given depth. Con- 

 ventionally, k is expressed as light extinction per meter of depth. There have 

 been many local studies of light penetration, at various times and places, and 

 extensive surveys of large parts of the earth's oceans by Jerlov (1951) and 

 Steemann Nielsen and Jensen (1957). 



If we disregard the selective absorption of light of different wavelengths and 

 consider the total visible spectrum, we may say that for clear ocean water k 

 equals 0.04-0.05, representing a compensation depth of 100-120 m. These 

 values are typical of tropical and semi-tropical seas (the Sargasso, western 

 Pacific, Mediterranean and Caribbean Seas) and appear to be fairly constant 

 throughout the year. 



The temperate and northern parts of the oceans tend to be more turbid and 

 more seasonally variable, with values for k typically ranging between 0.10 and 

 0.20 and corresponding compensation depths of 50-25 m. This is largely due to 

 the greater abundance of phytoplankton, other organisms and their decomposi- 

 tion products in these waters. Although the annual rates of primary production 

 in temperate and tropical seas may not differ appreciably, as will be discussed 

 below, much larger populations of plants and animals develop in the former 

 regions. These populations and the detritus associated with them are an 

 important factor in reducing the transparency of the water. 



In coastal and inshore regions, transparency is too variable to be assigned 

 an average value, but it is generally more turbid than offshore waters, with 

 extinction coefficients as high as 1.0 (compensation depth 5 m) not uncommon. 

 This is again due partially to higher densities of plankton and organic detritus, 

 but may also result from suspended bottom sediments, particularly in turbulent 

 shallow waters. Furthermore, there is associated with coastal waters a dissolved 

 organic "yellow substance", described and studied by Kalle (1938), which may 

 contribute significantly to the absorption of daylight. 



The highest possible rates of organic production occur in situations approach- 

 ing a hypothetical optimum where all incident radiation is absorbed by plants. 

 This possibility is precluded in the clear blue open sea where most of the 

 radiation is absorbed by the water itself. Since the extinction coefficient of 

 visible light in pure sea-water is about 0.035, a coefficient of 0.040 implies that 



