Domain of the Marine Microbiologist 13 



ators, pathogens, and a tangle of other interrelated factors (15, 

 39, 44). There are diurnal, seasonal, and geographic differences 

 in predominating species and population densities. The popula- 

 tion of diatoms and dinoflagellates ranges from nil to 10^ i^er 

 liter. In certain waters the nannoplankton population reaches 10" 

 per liter. Knight-Jones and Walne (23) found the small chrys- 

 omonad flagellate Chromulina pusilla in numbers ranging from 

 50 to 3500 per ml in English Channel waters. Rarely does the 

 standing crop of phytoplankton exceed 0.2 mg organic carbon per 

 liter in the open ocean (52). 



The abundance, kinds, and products of photosynthetic organ- 

 isms influence the occurrence of animals and bacteria in marine 

 environments. Directly or indirectly, animals and heterotrophic 

 bacteria depend for food upon organic matter synthesized by 

 plants. Thus the greatest number of bacteria and animals are 

 found in the topmost few hundi^ed meters of water and on the 

 sea floor to which the organic remains of plants and animals set- 

 tle. Various workers (25, 57) have noted a parallelism between 

 the abundance of bacteria and phytoplankton. It is an oversim- 

 plification of the situation, however, to say that the abundance of 

 bacteria bears a direct relationship to the abundance of plankton. 

 Bacteria are often most abundant in the wake of a plankton 

 bloom rather than during the bloom. As stated by Harvey (15), 

 the equihbrium between the standing crop of plants, bacteria, 

 herbivores, and carnivores is continually passing in and out of 

 balance. The annual production of zooplankton may be no more 

 than one-tenth the annual production of phytoplankton, upon 

 which they depend for food, but at times the standing crop of 

 zooplankton may exceed the standing crop of phytoplankton. Sim- 

 ilarly, the bacterial biomass sometimes exceeds the combined 

 standing crops of phytoplankton and zooplankton. Bacteria are 

 subject to predation by grazing zooplankton and a good man}- 

 other animals. 



Exceptionally large bacterial populations occur where land 

 drainage, sedentary algae, coral reefs, plankton blooms, mass 

 mortalities, convergences, or other conditions make for high con- 

 centrations of organic matter. Although dependent upon organic 



