PHYSICAL PROPERTIES OF SEA WATER 31 



hand, that the increased absorption in oceanic water is due at least in 

 part to selective scattering, because at short wave lengths the increase of 

 the extinction coefficients was greater than it was at longer wave lengths 

 (table 4). Kalle is of the opinion that selective scattering is of dominant 

 importance, but the question is not yet settled as to the mechanism 

 which leads to increased absorption in oceanic water as compared to pure 

 water. The fact that even in the clearest oceanic water the absorption 

 is greater than in pure water indicates, however, that finely suspended 

 matter is always present. One could state that the ocean waters always 

 contain dust. 



The increase of the extinction coefficients in coastal waters appears 

 to be due in part to another process. Clarke and James conclude from 

 their examination that in coastal water both suspensoid and ''filter- 

 passing" materials are effective in increasing the absorption, and that 

 each exerts a highly selective action, with greatest absorption at the 

 shorter wave lengths. These great absorptions at the shorter wave 

 lengths are demonstrated by Utterback's measurements (table 4). 

 Clarke does not discuss the nature of the ''filter-passing" material, but 

 Kalle has shown that, in sea water, water-soluble pigments of yellow 

 color are present. These pigments appear to be related to the humic 

 acids, but, since their chemical composition has not been thoroughly 

 examined, Kalle calls them "yellow substances." They seem to occur 

 in greatest abundance in coastal areas, but Kalle has demonstrated their 

 presence in the open ocean as well and believes that they represent a 

 fairly stable metabolic product related to the phytoplankton of the sea. 

 The selective absorption of these yellow substances may then be responsi- 

 ble, in part, for the character of the absorption in coastal water and 

 for the shift of the band of minimum absorption toward longer wave 

 lengths. 



It has not been possible anywhere to demonstrate any direct influence 

 of phytoplankton populations on the absorption, although it is probable 

 that very dense populations cut down the transparency. At present 

 it appears that the major increase of absorption of sea water over that of 

 pure water is due to two factors: the presence of minute suspended 

 particles, and the presence of dissolved "yellow substances." The latter 

 factor is particularly important in coastal waters. 



The Color of Sea Water 



The color of sea water as it appears to an observer ashore or on board 

 a vessel varies from a deep blue to an intense green, and is in certain 

 circumstances brown or brown-red. The blue waters are typical of the 

 open oceans, particularly in middle and lower latitudes, whereas the green 

 water is more common in coastal areas, and the brown or "red" water is 

 typical of coastal waters only. 



