JOHN H. RYTHER 389 



In the temperate regions of the ocean, the relatively unproductive 

 winter season is normally followed by a spring "flowering" or "bloom" 

 of phytoplankton, which consists predominantly of diatoms. This 

 spring maximum, which usually develops into the largest population 

 of the year, becomes limited by the supply of available nutrients and 

 ma)' pass through its entire cycle of growth and decline in a few 

 weeks. As the nutrient level falls and the temperature rises, the num- 

 bers of diatoms decrease and many species may disappear entirely 

 from the plankton. 



By late spring, the dinoflagellates appear in significant numbers, 

 and while the\' seldom attain the abundance of the spring diatom 

 bloom, they often persist as the dominant member of the plankton 

 community throughout the summer months. 



The sequence of events described above occurs in temperate regions 

 in the open ocean (Herdman, 1922), in coastal and slope waters 

 (Gran and Braarud, 1935), and in estuarine situations (Gaarder and 

 Gran, 1927; Marshall, 1947; Braarud, 1945). 



While northern waters support larger populations of dinoflagellates, 

 the number of temperate species is extremely small compared with 

 the number of tropical forms ( Sverdrup, Johnson, and Fleming, 1942 ) . 

 This is particularly true of the liighly specialized Dinophysiales, 

 which are characteristic of tropical seas (Fritsch, 1935) but also ap- 

 plies to the ubiquitous genus Ceratium of which Peters ( 1932 ) found 

 33 of 55 South Atlantic species confined to warm water. 



The dense populations of dinoflagellates which create "red water" 

 conditions are known only in the tropics or in temperate water dur- 

 ing the warmer (and usually the warmest) time of the year. This 

 subject will be discussed in more detail in another section. Dino- 

 flagellate luminescence shows a similar seasonal periodicity in the 

 temperate parts of the ocean. Allen (1939) reported that luminous 

 displays by these organisms are common in the La Jolla region during 

 the summer months, but never occur between October and May. 



Such fragmentary physiological evidence as is available concerning 

 the temperature relations of dinoflagellates appears to support the 

 view that they are predominantly a warm-water group. Barker ( 1935), 

 who is one of the pioneers in developing successful culture methods 

 for dinoflagellates, observed optimal temperatures for the growth of 



