president's address. 19 



when dissected out and examined under the microscope, reveals a 

 surprising assemblage of the smallest protozoa and protophyta, less 

 than 30 micro-millimetres in diameter, which would all pass easily 

 through the meshes of our finest silk nets. 



The latest refinement in capturing the minutest-known organisms 

 of the plankton (excepting the bacteria) is a culture method devised 

 by Dr. E. J. Allen, Director of the Plymouth Laboratory." By diluting 

 half a cubic centimetre of the sea-water with a considerable amount 

 (1500 c.c.) of sterilised water treated with a nutrient solution, and 

 distributing that over a large number (70) of small flasks in which 

 after an interval of some days the number of different kinds of organisms 

 which had developed in each flask were counted, he calculates that 

 the sea contains 464,000 of such organisms per litre; and he gives 

 reasons why his cultivations must be regarded as minimum results, 

 and states that the total per litre may well be something like a million. 

 Thus every new method devised seems to multiply many times the 

 probable total population of the sea. As further results of the quan- 

 titative method it may be recorded that Brandt found about 200 diatoms 

 per drop of water in Kiel Bay, and Hensen estimated that there are 

 several hundred millions of diatoms under each square metre of the 

 North Sea or the Baltic. It has been calculated that there is approxi- 

 mately one Copepod in each cubic inch of Baltic water, and that the 

 annual consumption of these Copepoda by herring is about a thousand 

 billion ; and that in the 16 square miles of a certain Baltic fishery 

 there is Copepod food for over 530 millions of herring of an average 

 weight of 60 grammes. 



There are many other problems of the plankton in addition to 

 quantitative estimates — probably some that we have not yet recognised — 

 and various interesting conclusions may be drawn from recent planktonic 

 observations. Here is a case of the introduction and rapid spread of 

 a form new to British seas. 



Biddulphia sinensis is an exotic diatom which, according to Osten- 

 feld, made its appearance at the mouth of the Elbe in 1903, and spread 

 during successive years in several directions. It appeared suddenly 

 in our plankton gatherings at Port Erin in November 1909, and has 

 been present in abundance each year since. Ostenfeld, in 1908, when 

 tracing its spread in the North Sea, found that the migration to the 

 north along the coast of Denmark to Norway corr-esponded with the 

 rate of flow of the Jutland current to the Skagerrak — viz. , about 17 cm. 

 per second — a case of plankton distribution throwing light on hydro- 

 graphy — and he predicted that it would soon be found in the English 



" Jorirn. Mar. Biol. Assoc, xii. 1, July 1919. 



c 2 



