14 president's address. 



biologists * have pointed out that the warm waters of the West Indies 

 and Florida may be noted for the richness of their floating life for 

 periods of years, while at other times the pelagic organisms become 

 rare and the region is almost a desert sea. 



It is probable, on the whole, that the distribution and variations 

 of oceanic currents have more than latitude or temperature alone to do 

 with any observed scantiness of tropical plankton. These mighty 

 rivers of the ocean in places teem with animal and plant life, and may 

 sweep abundance of food from one region to another in the open sea. 



But even if it be a fact that there is this alleged deficiency in tropical 

 plankton there is by no means agreement as to the cause thereof. 

 Brandt first attributed the poverty of the plankton in the tropics to the 

 destruction of nitrates in the sea as a result of the greater intensity of 

 the metabolism of denitrifying bacteria in the wanner water; and 

 various other writers since then have more or less agi'eed that the 

 presence of these denitrifying bacteria, by keeping down to a minimum 

 the nitrogen concentration in tropical waters, may account for the 

 relative scarcity of the phyto-plankton, and consequently of the 

 zoo-plankton, that has been observed. But Gran, Nathansohn, Murray, 

 Hjort and others have shown that such bacteria are rare or absent 

 in the open sea, that their action must be negligible, and that Brandt's 

 hypothesis is untenable. It seems clear, moreover, that the plankton 

 does not vary directly with the temperature of the water. Furthermore, 

 Nathansohn has shown the influence of the vertical circulation in the 

 water upon the nourishment of the phyto-plankton — by rising currents 

 bringing up necessary nutrient materials, and especially carbon dioxide 

 from the bottom layers ; and also possibly by conveying the products of 

 the drainage of tropical lands to more polar seas so as to maintain the 

 more abundant life in the colder water. 



Piitter's view is that the increased metabolism in the warmer water 

 causes all the available food materials to be rapidly used up, and so 

 puts a check to the reproduction of the plankton. 



According to Van't Hoff's law in Chemistry, the rate at which a 

 reaction takes place is increased by raising the temperature, and this 

 probably holds good for all bio-chemical phenomena, and therefore for 

 the metabolism of animals and plants in the sea. This has been 

 verified experimentally in some cases by J. Loeb. The contrast 

 between the plankton of Arctic and Antarctic zones, consisting of large 

 numbers of small Crustaceans belonging to comparatively few species, 

 and that of tropical waters, containing a great many more species 

 generally of smaller size and fewer in number of individuals, is to be 



' A. Agassiz, A. G. Mayer, and H. B. Bigelow. 



