628 Marine Microbiology 



summer and fonii large populations which are reduced by selec- 

 tive grazing. Evidence has been obtained that the large Coscin- 

 odiscus species require a biennial cycle to complete their develop- 

 ment, which is retarded by the arctic environment. 



In summary, the arctic phytoplankton shows a heterogeneous 

 taxonomic composition, mostly cosmopolitan with less numerous 

 endemic arctic forms. The universal rhvthm of successions in 

 cold seas involving spring Pennatae and summer Centriceae is in 

 its main outline similar throughout the northern latitudes, with 

 the respective maxima occurring much closer together in the 

 shorter seasons of the far north. The absence of Pennatae in seas 

 with no ice and the absence of Cyanophyceae in the arctic are 

 fundamental biological characters of these seas. 



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