Succession of Phytoplankton 261 



condition which also obscures the underlying causes of this 

 phenomenon. 



Culture studies have demonstrated that phytoplankters have 

 specific temperature, light, salinity and nutrient requirements, 

 indicating that these factors are involved in succession. One can 

 easily apply biogeographical data to the problem of succession 

 on the probably valid supposition that those environmental factors 

 governing species distribution also influence their temporal oc- 

 currence within an area. Accordingly, temperature may regulate 

 succession^ since the distribution of many species, such as the 

 arctic diatoms Thalassiosira htjalina and Bacterosira fragilis, ap- 

 pears to be temperature-dependent (48). 



The salinity-dependent distribution of many organisms (54, 

 48) may be reflected in the succession of populations occurring 

 in environments where significant variations in dilution may 

 occur. 



Phytoplankton species can be classified as "sun" (helio- 

 philic) or "shade" ( umbriphilic ) loving species, suggesting that 

 light may also be important in succession. Among the pelagic 

 arctic diatoms, Fmgilaria oceanica, Achnanthes taeniata and 

 Nitzschia frigida have been found in active growth on the under- 

 side of ice (vide 49). Steemann Neilsen (51) distinguished be- 

 tween oligophotic, mesophotic and euphotic species of Ceratitim 

 on the basis of their vertical distribution. Findenegg (11) demon- 

 strated that light determined both the vertical distribution and 

 seasonal occurrence of certain limnetic species. 



Rodhe (42) has shown experimentally that certain limnetic 

 phytoplankters vary in their tolerance to levels of basic nutrient 

 concentration. This observation lends credence to Peter's (32) 

 belief that the distribution of certain marine Ceratium species is 

 controlled by phosphate and nitrate concenti'ations. Accordingly, 

 nutrient levels and the degree of "saprobism" characteristic of 

 the species may influence succession. 



The inadequacy of reliably ascribing a given successional 

 change to any of the above factors, despite their involvement, 

 suggests that only in extreme instances is succession governed 



^ Succession will refer to the appearance or disappearance of a species or 

 community. 



