l6] VEGETATIONAL TYPES OF SEAS 539 



vegetation is mainly composed of Bacteria, which occur in great 

 abundance in the upper layers of bottom sediments and, although of 

 course microscopic and scarcely evident in their physical effect, are 

 believed to play significant roles in determining the character of the 

 deposits. Thus they form humus and precipitate compounds of 

 calcium, iron, and manganese. They are mostly motile rods and 

 comma-shaped forms, and are much more often coloured but less 

 frequently spore-forming than is the case with terrestrial types. 



The greatest numbers of marine Bacteria have been found in 

 coastal waters where life is most prolific. In the upper waters, and 

 especially within the uppermost 50 metres, there are often a consider- 

 able number attached to floating organisms and other particulate 

 matter; but the main concentration is on the bottom — especially 

 just below the mud- water interface. Here as many as an estimated 

 420,000,000 cells per gram of wet mud have been observed. More 

 or less teeming populations of this nature appear to be practically 

 world-wide, marine Bacteria being capable of development at un- 

 usually low temperatures, and ocean depths being relatively stable 

 and not too low in this respect. Thus even in the Arctic Ocean, 

 Bacteria in great numbers may be responsible for the colours of 

 bottom deposits, including those at very considerable depths, and 

 it has been estimated that at latitude 82° 42' N. in the North Polar 

 Basin there are from 3I to 7 tons of bacterial matter per cubic 

 kilometre of sea water, while farther south, in Barents Sea, the 

 uppermost 4 cm. of bottom mud are estimated to contain 20 gm. 

 of Bacteria per square metre. 



The presence of numerous saprophytic Bacteria attached to the 

 planktonic organisms of the uppermost layers of the ocean, whether 

 or not such Bacteria be considered actually planktonic, results in 

 prompt decomposition of much of the dead material before it can 

 sink to great depths, it being claimed that a considerable proportion 

 of the mineral nutrients are returned to the water within or a little 

 below the euphotic zone. Nevertheless sufficient elaborated material 

 reaches even very deep ocean beds to support a teeming population 

 of, particularly, animals and Bacteria. The Bacteria are chiefly found 

 in a thin layer of surface ooze in which is concentrated a large pro- 

 portion of the organic detritus — dead bodies and parts thereof — 

 which is constantly sinking and supplying food for them as well as 

 for competing animals. These bottom Bacteria are in general more 

 numerous in fine than coarse deposits, and occur principally near 

 the surface of such deposits. Thus although viable cells have been 



