506 LIFE IN THE OCEAN. 



activity of the organisms — for example, of the marine bacteria — 

 while they are nearly without effect upon the solubilities of the inor- 

 ganic matters which are adapted to becoming food for the vegetation. 

 It thus seems that the cause of the richness of cold waters and of the 

 poverty of warm waters should be sought in the difference of develop- 

 ment of the bacteria of putrefaction in the largest sense of the term, 

 and in the influence of these bacteria on the proportion of nitrogenous 

 compounds in the water. 



Among these bacteria, the nitrifying bacteria exercise their function 

 in arable soil only at a temperature above about 5"^ C. (41° F.). In all 

 probability there are in the sea other sorts, nitrifiers and denitrifiers, 

 able to accommodate themselves to other temperatures. Still, in the 

 present state of our knowledge, we may assume that bacteria cease to 

 act at the freezing point, or a little below that point. But if denitri- 

 fying bacteria can not perform their function in cold waters, it follows, 

 almost necessarily, that polar seas must be richer in nutritive substances 

 than tropical seas. In a large part of the polar seas the temperature 

 of the whole liquid mass from surface to bottom remains even in sum- 

 mer, near 0-^. North of a line extending from eastern Greenland to 

 Norway-, through Iceland and the Faroe Islands, the temperature at 

 the bottom is generallv below 0° C. South of this line the tempera- 

 ture of the deep waters of the Atlantic is certainly not much higher, 

 because the cold water of the polar seas flows into the deep regions 

 toward the equator. But at 1,000 meters (547 fathoms) the tempera- 

 ture is already 4° to 5°, and for depths of less than lOO meters, as well 

 as along the coasts, it is notablv higher, so that the bacteria here find, 

 precisely as in the productive laA'ers of tropical seas throughout the 

 whole year, conditions favorable to their life. In the temperate zones 

 the destruction of nitrogenous compounds is too limited during the 

 winter and it is only in summer that it becomes important. Finally, 

 in the Mediterranean the conditions of life of bacteria are still more 

 favorable than in the Tropics, because a bar across the Straits of 

 Gibraltar prevents the cold water from entering. Hence, even at 

 great depths (of about 4.000 meters), there is alwaj's a temperature 

 of 1:2^ to 16°, which explains the development of bacteria in the whole 

 liquid column observed and the consequent striking quantitative pov- 

 erty in plankton of the Mediterranean Sea. 



If we can not dismiss absolutely the idea of a denitrification that 

 can not be neglected in the ocean, it appears to me highly probable, 

 according to the observations hitherto made, that this decomposition 

 of the principal vegetable nutritive substances is preferentially accom- 

 plished in warm regions. 



