34 Marine Microbiology 



adsorption of the excreted metabolites and possibly also of nu- 

 trients occurs. Since according to Wood, particles (leptopel) are 

 often 100 to 1000 times as numerous as phytoplankton, adsorption 

 may be highly important in the dissemination of nutrients as far 

 as the smaller microorganisms are concerned. 



THE DISTRIBUTION OF PHYTOPLANKTON 



Bernard (Chapter 22) has demonstrated large occurrences 

 of Coccolithophores and blue-green algae far below the photic 

 zone in the Mediterranean Sea and in the western Indian Ocean. 

 As it has been generally considered that phytoplankton is limited 

 to the photic zone during its active state, a search at greater 

 depths than 150 meters has rarely been made by other phyto- 

 planktologists. I have failed to find any phytoplankton elements 

 at 300 m in the Coral Sea, in the few samples that I have taken, 

 but have made no observations below this depth. It is necessary 

 for the phytoplanktologist to probe deeper into the ocean to de- 

 termine the quantitative significance of Bernard's findings. In my 

 experience, the phytoplankton in the South West Pacific and 

 North East Indian Oceans shows a maximum growth in the vicin- 

 ity of 75 m depth, often expanding in daylight upwards to 50 m 

 and downwards to 100 m, the maximum occasionally occurring at 

 100 m and very rarely at 150 m. At night, phytoplankton is con- 

 centrated usually at or near the 75 meter level. In the southern 

 Tasman, but still in the East Australian Current, and in the sub- 

 Antarctic waters, the maximum growth occurs usually between 

 the surface and 50 m. The total phytoplankton population of the 

 photic zone is lower at night than in the day, and the diurnal 

 fluctuation is more marked at the surface. This diurnal fluctu- 

 ation does not show the same pattern every day, and maxima 

 and minima may occur at different times, so it is not possible 

 to allow for this variable by collecting samples at a fixed hour 

 and applying a conversion factor. We have confirmed this by 

 counting phytoplankton, total organisms and estimating the as- 

 similation of C^^, and the same story is told in each case. This 

 limits the value of quantitative studies made at a series of stations 

 in the ocean, but diurnal variation is likelv to be much lower 



