Succession of Phytoplankton 267 



predictable succession might be expected if metabolites were pro- 

 duced and operated independently of physical growth param- 

 eters. 



It is obvious tliat the impact of a growth regulator upon a 

 community or species is one of degree. For example, lethal phy- 

 totoxins are potentially more important in succession than those 

 only moderately toxic. Yet it is significant that antagonism be- 

 tween species is related to their relative population densities and 

 the extent of acclimatization. Rice (37) has demonstrated that 

 a species suddenly exposed to an antagonist is usually more in- 

 tensely inhibited than if both species had grown together over 

 an extended period. Lucas (26) appropriately points out that 

 evolutionary adaptation to metabolites must have occurred be- 

 tween species growing together over interminable generations. 

 That phytoplankton species adapt to diverse environmental con- 

 ditions is demonstrated by the responses of natural communities 

 (52) and the diatom Bicldidphia mobiliensis (18) to various 

 light levels. Additionally, it is commonly experienced that past 

 environmental conditions detemiine the growth of species upon 

 sub-cultine. It might be that species adaptation to metabolites 

 represents an additional control over the exclusive regulation of 

 succession by these potent substances. A similar conclusion might 

 apply to the other growth parameters. It follows that should a 

 "wild" metabolite be introduced into, or emanate within, an 

 historically established community, a floristic catastrophe such 

 as a "red tide" or PJiaeocystis bloom may occur— an abnormal 

 disruption of succession (27). 



Let us now consider the nature of the various growth regula- 

 tors and their potential role in succession. There are two major 

 groupings of phyto-stimulatory substances: I) those that only 

 supplement existing growth, i.e., not required for normal growth; 

 and II ) those substances required for growth. Since growth pro- 

 gresses independently of the a\'ailability of substances in cate- 

 gory I, they are probal)ly of minor significance in succession. For 

 example, it is doubtful that an auto-stimulated species can signif- 

 icantly influence succession. It does not create a niche for itself 

 since it already occupies it. Therefore, it might prolong its 

 presence and/or promote abundance through the competitively 



