Succession of Phytoplankton 269 



succession is readily tempered and applied throngh interactions 

 with physical growth factors. 



HOLOCOENOTIC CONCEPT OF PHYTOPLANKTON 

 SUCCESSION AND DYNAMICS 



Ectocrine substances undoubtedly influence succession. 

 However, the over-riding influence of the environment normally 

 preventing the independent action of such growth regulators has 

 been demonstrated. This environmental control is exerted along 

 the following lines: 1) the production and liberation of metab- 

 olites are governed by the prevailing environmental conditions; 

 2) metabolite liberation appears to be most copious when least 

 likely to provide competitive advantages; 3) stringent physical 

 and biological restraints tend to curb the accumulation and at- 

 tenuate metabolites upon liberation; and 4) evolutionary adapta- 

 tion of naturally occurring communities has probably moderated 

 the capacity of species for "chemical warfare." 



Although the role of ectocrine substances in succession was 

 emphasized in the previous section, essentially the same con- 

 clusions result if tlie unilateral regulation of succession by light, 

 temperature or nutrients is applied. Environmental control of 

 these physical growth factors is most apparent in the frequent 

 coupling of variables resulting in certain physiological or ecolog- 

 ical responses. Some of these interactions, and their effects in- 

 clude: 



Temperature — Salinity 



Ritchie (41) has demonstrated that the growth of marine 

 fungi is related to the combined influence of temperature and 

 salinity. This interaction also governs the distribution of higher 

 algae (8) and dinoflagellates (30). 



Temperature — Light 



There are numerous examples of temperature— light inter- 

 actions. Findenegg (11) clearly demonstrated that the seasonal 

 occurrence of certain limnetic phytoplankton species is deter- 

 mined by tliis interaction, permitting the following classification : 



