PLANKTON 259 



recorded large hauls of plankton in the Humboldt current 

 off the west coast of America, and during the " Challenger " 

 expedition some of the largest quantities of plankton were 

 found in the equatorial Pacific, and Diatoms were found to 

 be as abundant in the Arafura Sea (lat. 10° S.) as in the 

 Antarctic. Murray and Hjort found in their Atlantic 

 expedition that Coccolithophoridae, separated from the sea- 

 water by the centrifuge, were very abundant in tropical 

 seas, and they found large quantities of Crustacea at deeper 

 zones in the tropics. Moreover, it is common knowledge 

 that on occasions vast swarms of some planktonic organism 

 may be seen in tropical waters. The yellow alga Trichodes- 

 mium may cover the surface over considerable areas of the 

 Indian and South Atlantic oceans ; and some pelagic 

 animals such as Salpae, Medusae and Ctenophores are also 

 commonly present in abundance in the tropics. Then, 

 again, American 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 warmer water ; and various 

 other writers since then have more or less agreed that the 

 presence of these denitrifying bacteria, by keeping down to 



