264 FOUNDERS OF OCEANOGRAPHY 



millions of individual Diatoms, constituting on the average 

 some 999,999 out of each million of organisms in the 

 gathering. Similarly, when the zoo-plankton is at its height 

 in summer, the same net may contain a gathering of Copepoda 

 numbering hundreds of thousands of individuals, making up 

 about 999 out of every thousand organisms present. At other 

 intermediate times of year the plankton is smaller in amount, 

 and of a mixed nature (PI. XVII, Fig. 2 ; PI. XXI, Fig. 4). 



It is evident that there is an annual planktonic cycle 

 (text-fig. 17) as follows: — After a winter minimum, the 

 spring maximum of phyto-plankton starts about March 

 (when the sea has still a low temperature), and increases to 

 a climax in April, May, or June, after which the Diatoms 

 rapidly diminish in number to their minimum in the height 

 of summer, when their place is taken by the Copepoda and 

 other animals of the zoo-plankton, to be followed by a 

 secondary lesser Diatom maximum in late autumn (Septem- 

 ber or October), after which the whole plankton diminishes 

 to the winter minimum. This cycle has been followed year 

 after year at several localities in North-West Europe ; but 

 further observations throughout the year are stUl required in 

 regard to tropical seas and the open oceans. 



In a series of observations carried on at the Port Erin 

 Biological Station during fifteen years, 1907-21 (when on 

 the average six plankton hauls were taken and examined^ 

 every week, amounting to over 7,500 samples in all), 

 it is found that the spring maximum for the total 

 plankton varies from April to June, and is in most years 

 in May ; and if the total plankton be analysed into its 

 three chief constituents (Fig. 17), Diatoms, DinoflageUates, 

 and Copepoda, they are found to succeed one another in 

 that order. For example, the Diatom maximum was in 

 March in 1907, in April in 1909, and in May in 1908 ; the 

 Dinoflagellate maximum was about a month later in each 



^ For a summary of the results, see " Spolia Runiana V," Journ. 

 Linnean Soc, Botany, July, 1922. 



