262 Marine Microbiology 



exclusively by a single factor. Rather, the responses of cultured 

 and natural populations and the gradual seasonal changes in 

 growth parameters suggest that phytoplankters have specific 

 tolerances as defined by Shelford's Law of Tolerance (31, p. 26). 

 This is reflected in the classification of phytoplankton as eury- 

 thermal or stenothermal, euryhaline or stenohaline, and so forth. 



Biological interrelationships are also potentially important 

 in succession. Lucas (25, 26, 27, 28) has instanced numerous 

 examples of biotic interplay in formulating an "ectocrine" theory 

 which assigns a general ecological role to organic substances in 

 the ocean, including the possible mediation of phytoplankton 

 succession. Tliis important concept has stimulated many dis- 

 coveries of the ecological role of metabolites (45), including the 

 well-documented observations of Droop and Provasoli that many 

 phytoplankton species require vitamins. Metabolic products, 

 therefore, may be involved in succession through their growth- 

 regulatory properties which would determine the presence or 

 absence of species. 



The ultimate influence of metabolites, as with other growth 

 factors, is regulation of cell division irrespective of whether they 

 are phyto-toxic or phyto-stimulatory. It remains to be deter- 

 mined, however, whether such metabolites act independently of 

 other growth factors, or whether metabolites are only one of a 

 combination of variables whose collective, simultaneous and in- 

 terdependent operations govern succession and the cyclic abun- 

 dance of phytoplankton. Much of the following discussion will 

 concern the application of the ectocrine concept to succession 

 primarily because it is particularly amenable to bringing out 

 certain aspects of environmental regulation of succession and 

 growth variables in general. 



SUCCESSION AND NATURAL CYCLES 



Only succession generated by the autochthonous production 

 of organic substances, or physical environmental changes asso- 

 ciated with natural and normally occurring climatic and biological 

 cycles will be considered. Succession induced l)v extraneous fac- 

 tors such as domestic pollutants (2) or the nitrogenous-rich ef- 

 fluent of duck farms (44) are of relatively local interest and 



