Succession of Phytoplankton 263 



represent a drastic, unnatural modification of the flora and its 

 ensuing dynamics. 



Gross disruptions of historically established phytoplankton 

 cycles ( 14 ) are more readily attributable to sequential ( 15 ) or 

 more proti'acted hydrographic distmbances than to the vagaries 

 of antibiotic production. Minor departures from the customary 

 pattern of succession can likewise frequently be related to unusual 

 climatic conditions (7). "Red tides" and other blooms of extra- 

 ordinary intensity and duration, however, may represent instances 

 of environmental imbalance resulting from unchecked organic 

 substances. However the water mass size and its degree of phys- 

 ical insulation appear to be important in the generation and 

 maintenance of such blooms (47). 



It should also be noted that the nature, rate of succession 

 and the species involved generally appear to be relatively con- 

 stant irrespective of annual fluctuations in the magnitude of the 

 standing crop. Whereas if metabolites or changes in nutrient con- 

 centrations were the mainspring of succession, one would expect 

 the rate of succession and the degree of change in species com- 

 position to be approximately proportional to the density of the 

 populations producing these metaboUtes, or consuming the nu- 

 trients. 



It is revealing, then, that the most marked communal dis- 

 ruptions accompany the incursion of water masses, climatic de- 

 viations which may affect light intensity, water temperature and 

 stability, and "red tide" blooms which are accompanied by local 

 "atypical" hydrographic conditions. In other words, these are 

 environmental disturbances which produce changes in many 

 growth variables upsetting the previous equilibrium and leading 

 to community re-organization. These observations suggest that 

 rigorous environmental control of growth parameters normally 

 occurs preventing a given factor from disrupting phytoplankton 

 cycles. This control may account for the orderly, predictable 

 process of succession characteristic of a given area, rather than 

 an erratic, unpredictable succession that would be expected if 

 metabolites, light or nutrients solely determined succession. 



Let us examine what environmental controls ma\' be opera- 

 tive in counteracting the unilateral regulation of succession as- 

 suming metabolites are involved. 



