28 



PROBLEMS OF LAKE BIOLOGY 



PERCENTAGE OF INCIDENT LIGHT 



I 5 10 50 100 



M. 



10 



-20 



O 





30 O 



— 40 H 



m 



— 50 



33 

 > 

 < 



m 



m 

 o 



70 



Fig. 1. Transmission of light by distilled water 

 at 6 wave lengths within the visible spectrum. 

 Curves show the percentage of incident light (loga- 

 rithmic scale) which would remain after passing 

 through the indicated thickness of water. 



1 Mean wave length of range for each color as 

 employed by Birge and Juday (1931). 



2 As determined for the designated wave length 

 by James and Birge (1938). 



3 Extinction coefficient = k in the equation I/Io 

 = e^kL where the light is reduced from I„ to I by 

 a stratum L meters thick, e = 2.7. 



unequal quantities of energy at the differ- 

 ent wavelengths and these are absorbed at 

 unequal rates as the light penetrates into 

 the water. The result is that after passing 

 through successive meters of water the spec- 

 tral composition of the light present be- 

 comes rapidly and profoundly altered. The 

 infrared and red components are reduced 

 to small quantities within a very few meters 

 and the ultraviolet, which was initially of 

 small magnitude, soon drops to a minute 



7000 



8000 



3000 A 4000 5000 6000 



WAVELENGTH 



Fig. 2. The spectral distribution of solar energy 

 at tlie earth's surface is given by the uppermost 

 curve. When sunlight is passed through successive 

 meters of pure (distilled) water, the reduction of 

 intensity and the change in spectral distribution 

 which result are indicated l)y the curves beneath 

 (from Birge and James 1939). 



fraction of the whole. As a consequence, 

 after sunlight had traversed 100 meters of 

 distilled water nothing but the blue com- 

 ponent with a little green and violet would 

 remain, as is demonstrated by the family 

 of curves in Fig. 2. These curves represent 

 the uppermost limit possible for the trans- 

 parency of any body of water. 



In most natural waters tlie rate at which 

 light penetrates is further decreased by the 

 presence of varying amounts of suspended 

 particles and dissolved material. The great 

 diversity which results in the transparency 

 of natural waters may be appreciated by 

 comparing the curves of light penetration 

 for certain typical lakes and oceanic areas 

 in various parts of the world (Fig. 3). As 

 will appear presently, only those measure- 

 ments can be fairly compared wliich were 

 made within the same part of the spec- 

 trum. In the figure observations witli pho- 

 tometers sensitive to the yellow-green re- 

 gion (maximum 5500 A) are presented. 

 Sargasso water is seen to be nearly as trans- 

 parent as distilled water in this spectral 

 region and light is reduced to 1 per cent of 

 its surface value at about 100 meters. Be- 

 yond tlie edge of the continental shelf 100 

 miles or more from the coast in both the 

 Atlantic and the Pacific the transparency is 

 such that the 1 per cent value is reached at 

 about 50 meters. In coastal waters the 

 same value occurs at between 30 and 15 



