96 



PROF. W. A. HEBDMAN ON THE DISTRIBUTION 



a pure sample of Diatoms — a " monotonic phytoplankton." Similarly, when 

 the zpoplankton is at its height in late summer (usually September) the same 

 net may contain almost a pure gathering of Copepoda numbering some tens 

 to hundreds of thousands of individuals (up to 214,000), and making up 

 perhaps 999 out of every thousand organisms present — a " monotonic 

 zooplankton." But we may still regard the gathering as a zooplankton if 

 over 50 per cent, of the organisms are animals — on account of their greater 

 bulk. 



Moreover, these very abundant Diatoms and Copepoda belong in each case 

 to very few species, so that one can select about half-a-dozen species of 

 Copepoda which constitute by far the greater part of the summer zooplankton, 

 and about the same number of Diatoms which similarly make up the bulk 

 of the spring phytophinkton. These few species, belonging to these two 

 very widely separated groups, thus come to be the most significant organisms 



Fig. 1. — "Ilensen,'* "Nansen," " Funnel " and other plankton nets 

 drying after use on the yacht. 



in relation to the annual metabolic cycle of our seas and the food-supply 

 from our coastal fisheries. Consequently it is of both scientific and economic 

 importance to obtain such data as seem possible from our long series of 

 observations, extending over a decade, as to the occurrence of these dominant 

 factors in the plankton. No doubt there are exceptional years with unusual 

 occurrences which will have a disturbing effect, but the ten or eleven years' 

 results ought to give us an average of some value. 



We have endeavoured, in our work from the yacht, as the result of many 

 experiments, to make use of a standard net in a constant manner so that the 

 hauls should be approximately comparable. Our two horizontal tow-nets of 

 fine-meshed miller's silk (No. 20 and No. 9) measure 35 cm. (about 14 inches) 



