404 MARINE DINOFLAGELLATES AND RED WATER CONDITIONS 



explained by concentrations of the organisms themselves, either by 

 means of their active motility, or by passive floatation, should they 

 become less dense than sea water. 



Gran and Braarud (1935) observed that Peridinium triquetrum was 

 often found in maximum concentrations at the surface, but that this 

 organism could occur at any depth from to 25 meters. They con- 

 cluded that it could seek out the level at which the general conditions 

 for growth were most favorable. Halse ( 1950 ) demonstrated marked 

 vertical diurnal migrations in several dinoflagellates, the character- 

 istics of which differed considerably from species to species. Thus 

 Ceratium fusus and C. tripos rose to the surface at night and sank to 

 the lower depths during the day, while Gomjaulax polyedra and Pro- 

 rocentrum m^icans showed the opposite response to light, rising in the 

 daytime and sinking at night. 



Concentration of dinoflagellates by means of such migrations may, 

 at times, create red water conditions. Hirasaka (1922) observed red 

 water in Gokasho Bay, Japan, in which the entire bay was apparently 

 affected, in contrast to the usual situation in which red water is re- 

 stricted to patches or streaks. According to this author, the organisms 

 were confined to a band no more than 3 to 4 ft thick which appeared 

 to migrate diurnally, the red color being most conspicuous in the 

 late afternoon. 



In many cases, however, the red water organisms remain at the 

 surface of the water at all times. Woodcock ( personal communication ) 

 collected samples of Gyinnodinium brevis during the 1946-47 red 

 tide off Florida and observed that the organisms remained at the 



