ZoBeU 



— 12 — 



Marine Microbiology 



ers recognize a dysphotic zone in the ocean between the euphotic and 

 aphotic zones as a region 80 to 200 meters below the surface which 

 receives some light but not enough for effective plant production. 



Penetration of sunlight : — There are several factors which influence 

 the depth to which sunlight penetrates sea water. These include surface 

 reflection, transparency of the water, the angle of incidence of light, and 

 the intensity of the incident light. The latter two factors are largely 

 functions of atmospheric conditions, the time of day, and latitude. Ordi- 

 narily, light penetrates tropical water to greater depths than water at 

 higher latitudes where the angle of incidence is greater. 



Data recorded by Sverdeup et al. (1942) indicate that an average of 

 65 per cent of the incident light (energy) in fairly transparent oceanic 

 water is absorbed in one meter. Only 17 per cent of the incident light 

 penetrates to a depth of 5 meters, 9.5 per cent penetrates to a depth of 

 10 meters, 3.7 per cent to a depth of 20 meters, 0.31 per cent to a depth of 

 50 meters, and 0.0057 P^J* cent penetrates to a depth of 100 meters. In 

 less transparent coastal waters only 0.5 per cent of the incident light pen- 

 etrates to a depth of 10 meters. Pure sea water, free of all suspended and 

 coloring matter, permits the penetration of 22 per cent of the incident 

 light to a depth of 10 meters, but only 3 per cent of the light penetrates 

 pure sea water to a depth of 100 meters. 



Considerable difference is found in the transparency of water from dif- 

 ferent parts of the ocean. According to Clarke (1936), the water in the 

 Sargasso Sea is more transparent than sea water in any other part of the 

 world. It is 2 to 4 times more transparent than water in the Gulf of 

 Maine which is characteristic of water on the continental shelf, and the 

 latter is 3 to 4 times more transparent than water in Woods Hole Harbor. 



There is a marked difference in transparency of water for different wave 

 lengths of light. For example, Clarke (1936) reports that the transpar- 

 ency of the Sargasso Sea is greater than that at Woods Hole Harbor by the 

 following factors: 6-fold for green light, 7-fold for red, 11 -fold for violet, 

 and 16-fold for blue. 



In general, green light, or that having a wave length ranging from 4920 

 to 5350 A, penetrates sea water to greater depths than the shorter or 

 longer wave lengths of the visible spectrum. Blue (4220 to 4920 A) and 

 yellow (5350 to 5860 A) are more penetrative than orange (5860 to 6470 A) 

 or violet (3900 to 4220 A). Red (6470 to 8100 A) is the least penetrative 

 part of visible light. Sea water is relatively opaque to ultraviolet radia- 

 tions. The decreasing intensity of orange, green, and blue light with 

 increase in depth is given in Table II. 



Table II. — Per cent of transmitted light of three spectral bands in water at Dixon En- 

 trance, Southern Alaska, taken from data by Utterback (jpjd): — 



