10 Papers from the Marine Biological Laboratory at Tortugas. 



GENERAL CONSIDERATIONS AND PREVIOUS WORK. 



It is generally conceded that the plankton of tropical and subtropical 

 seas is far less in quantity than that found in colder waters.^ The zoo- 

 plankton depends ultimately for its food on the phyto-plankton ; hence any 

 factor limiting the growth of the phyto-plankton which was capable of 

 exercising its influence in tropical and not in temperate or arctic waters 

 might offer an explanation of this phenomenon. It has been shown by 

 various investigators that this factor is not temperature, light, or salinity, 

 and it has been suggested that the explanation may lie in the relative 

 deficiency, in tropical seas, of the nitrates or nitrogenous compounds so 

 essential for all plant life. A matter of common observation in support of 

 this view is the remarkable scarcity of algal growth in the shallow waters of 

 tropical shores as compared with that in temperate regions, and the fact 

 that in the tropics, wherever sewage or other nitrogenous waste is poured 

 into the sea, a free growth of algse is found. 



There is at present no really reliable and accurate chemical method of 

 estimating the combined nitrogen in sea-water, hence the above theory 

 can not be directly put to the test. On the other hand, the existence of 

 denitrifying bacteria in temperate waters has long been known, and it 

 would seem a fair deduction that should this bacterial destruction of nitrates 

 take place with greater intensity and completeness in tropical than temper- 

 ate waters, an explanation of the relative scarcity of phyto-plankton in the 

 former would be offered. This suggestion was first made by Brandt (3) 

 in 1901, and is universally known as " Brandt's hypothesis." He enunciated 

 it as follows: 



If the denitrifying bacteria of the sea, like the closely investigated denitrifying bacteria 

 of the land, develop a strongly disturbing activity at higher temperatures, only a relatively 

 small production (of phyto-plankton) would take place in the warm seas in spite of much 

 more favorable conditions, according to the law of the minimum, owing to the great dis- 

 turbance amongst the indispensable food substance; whilst, in the cold seas, more nitrogen 

 compounds would be at the disposal of the producers owing to the retardation or suppression 

 of the disturbing process. (From the published English translation.) 



The presence of denitrifying bacteria has been demonstrated in Kiel 

 Bay by Baur (i), along the Dutch coast by Gran (9), in the open waters 

 of the North Sea and Baltic by Feitel (7) and Brandt (2), and in 1909 I 

 identified several of the species described by Gran in samples of water 

 obtained from the western part of the English Channel. All these deni- 

 trifying species have a higher temperature optimum than that of their 

 natural environment and this is obviously a point strongly in favor of 

 Brandt's hypothesis. 



The chief difficulty in the way of putting the hypothesis directly to proof 

 lies in the fact that at present no accurate method of determining the nitrate 



' For the most recent work, and full discussion of this subject, see The Depths of the Ocean, by Murray 

 and Hjort, p. 366 et seq., 1912. London. 



