478 Dynamics of the Ecosystem 



zontal cunents moving differentially at various levels would secondar- 

 ily bring about a patchiness in horizontal distribution. Subse- 

 quently, the scope of the theory has been extended to include other 

 types of exclusion of one population by another. Dense diatom con- 

 centrations might be inimical to copepods either mechanically or 

 chemically— possibly through antibiotic effects. Such effects have 

 been demonstrated for oysters and cladocerans in certain instances 

 as described in Chapter 8. 



At about the same time, Harvey and co-workers ( 1935 ) came for- 

 ward with the theory of grazing, according to which copepods are 

 believed to reduce the diatom population locally by the intensity of 

 their feeding activity. At first sight these theories seemed mutually 

 exclusive, but more extensive investigation has shown that the in- 

 fluences involved in both may be in operation simultaneously without 

 conflict. Copepods depend upon suitable abundance of diatoms 

 for food, but they may avoid thick concentrations of diatoms and, by 

 grazing around the margins of diatom populations, may accentuate 

 the patchiness in the distribution of the latter. The timing of the 

 supply of diatom food in relation to the appearance of new broods of 

 copepods is also critical (Clarke, 1939). Investigation of the pro- 

 duction of Calanits finmarchicus in the Clyde Sea area gave evidence 

 that those broods of this copepod which hatched during periods of 

 diatom scarcity failed to reach maturity, whereas broods appearing 

 when suitable diatom food was available developed successfully 

 (Marshall etal., 1934). 



Another possible source of food for copepods as the key herbivores 

 in the oceanic food cycle is the nannoplankton. Lohmann drew at- 

 tention long ago to the abundance of such representatives of the 

 nannoplankton as the green flagellates, peridinians, other protozoans, 

 and certain bacteria (Sverdrup et al., 1942). These organisms may 

 occur in concentrations of millions of cells per liter, and, since they 

 are able to divide several to many times a day, they represent a large 

 and rapidly replaced potential food source. The fact that certain 

 copepods at least can sieve these extremely small organisms from the 

 sea is shown by the successful culturing of Calanits on flagellates 1.5 

 to 3 ju, in diameter (Raymont and Gross, 1942); and Calanus can de- 

 rive at least some nourishment from bacteria (Clarke and Gellis, 

 1935). Nannoplankton is undoubtedly of particular importance in 

 the feeding of young stages. 



Dissolved organic matter and fragments of dead organisms as de- 

 tritus have also been suggested as important food sources for primary 

 consumers in the sea. Analyses have shown that more than three 



