JOHN H. RYTHER 395 



its inclusion as oil droplets in their protoplasm and may possess 

 structural adaptations of various types which increase their surface 

 area and hence retard sinking (Gran, 1912; Russell, 1927; Sverdrup, 

 Johnson, and Fleming, 1942). Gross and Zeuthen (1948), on the other 

 hand, attribute the buoyancy of diatoms to their ability to maintain 

 extremely low concentrations of divalent ions in their cell sap. Accord- 

 ing to these authors, under suitable physical conditions plankton 

 diatoms have a specific gravity equal to that of sea water and do not 

 sink, but at temperatures of 20° C or above, this equilibrium does not 

 exist and the cells settle to the bottom. In general diatoms may be 

 said to thrive in cold, fully saline ocean water at times of the year 

 when there is considerable mixing of the surface water. 



In temperate or northern summer conditions, the density of the 

 water decreases as its temperature rises. For example, an increase in 

 temperature from 0-25° C decreases the viscosity of sea water by 

 one-half ( Sverdrup, Johnson, and Fleming, 1942 ) . In addition, vertical 

 mixing is usually at a minimum in summer. Under these conditions, 

 the diatoms find it increasingly difficult to remain in the upper water 

 layers, and in Gran's opinion, their populations are unable to maintain 

 themselves in the euphotic zone. 



In the tropics, this situation usually prevails throughout the year 

 and provides a definite disadvantage to the diatoms. For the same 

 reason, they may encounter suboptimal conditions in the low-salinity 

 (and hence low-density) waters of many coastal and estuarine situa- 

 tions. 



The dinoflagellates, on the other hand, possessing the advantage of 

 motility, are able to maintain themselves in water of low density with 

 comparative ease and are relatively independent of vertical mixing. 

 Furthermore, they, too, are often assisted by morphological adapta- 

 tions which increase their surface area and thereby retard sinking. In 

 the tropical forms, these structures far surpass similar features of the 

 diatoms. Thus Ceratium is reputed to have the ability of prolonging 

 or shedding its horns as it moves into warmer or colder water masses. 

 (Fig. 1). Others, like Ornithocercus, grow wide, wing-shaped mem- 

 branes (Fig. 2). Some species, such as Triposolenio, have asymetri- 

 cally arranged appendages so that, as soon as they stop swimming and 

 start to sink, they are quickly oriented horizontally in the water, 



