DISTRIBUTION 



in the bathyscaphe found it to be a dark twihght at 500 feet and quite dark at 

 1,000 feet. 



The distribution of plankton according to depth can be investigated by 

 means of closing nets (p. 1 8) that sample the plankton only within certain 

 limits, and such samples can be examined in detail in the laboratory. Another 

 and recent method which has given valuable results is by viewing the plankton 

 from the window of a bathyscaphe. This method as yet can give no samples 

 for analysis so that the value of the observations will depend on the experience 

 of the observer. To get the best out of such an expensive dive it should be 

 manned b)' the scientist available with the most experience in the field to be 

 explored. Diving off Toulon to a depth of 2,100 metres, Professor Bernard of 

 Algiers noted the sequence of the abundance of plankton. The average number 

 of organisms he could see per second of descent is given in the table. 



This was in the intensely blue water of the Mediterranean, and as explained 

 on page 113 the surface plankton here is very sparse indeed compared with 

 the shelf water surrounding the British Isles and similar places. The increase 

 of life in the intermediate layers is here in part associated with the distribution 

 of coccolithophores, small plants that thrive in reduced light intensities (p. 

 41). Note, too the increase close to the bottom which will be explained 

 on page 125. 



Some planktonic animals are confined within certain salinity tolerances, 

 irrespective of temperature or depth. Much more subtle arc those distribu- 

 tions which are dependent upon some vitamin, hormone or other micro- 

 chemical constituent. Although often subtle, and as yet even quite inexphc- 

 able, these delicate niceties of distribution can be extremely important; so 

 often the plankton animals concerned are the food offish etc. Some of these 

 subtle factors are associated with the different water masses and are widespread 

 in their effect; others are extremely local and result in patchiness in the 

 plankton. 



We are used to patchiness in the plant and animal distribution on the land, 

 vaguely associating it with man's interference. Indeed often it is so caused, 

 but not all clumps of trees have been deliberately planted nor are all nettle 

 patches associated with man's habitation. Midges gather in clusters under a 

 tree and are absent a yard away. Plankton is patchy in just the same way, and 

 sometimes this may be due to a congregation of species, a kind of shoal as in 



119 



