ioo 



PROF. W. A. HERDMAN ON THE DISTRIBUTION 



There are certain differences in detail. For example, the total Diatom 

 curve at Plymouth has three maxima or crests, in April, August, and October. 

 At Port Erin the curve has only two crests, a much greater maximum in 

 spring and a variable and smaller one in autumn, while Diatoms are usually 

 wholly absent in August. 



On the other hand, there is a general agreement in regard to the distribution 

 throughout the year of many of the more abundant organisms. For example, 

 amongst Diatoms Coscinodiscus is a winter and early spring form, Biddulphia 

 flourishes throughout the winter from November to April or May, RMzosoleniq 

 is a summer form having its maximum in June, while Cluvtoceras and 

 Lauderia have two maxima, the one in spring and the other in autumn, in the 

 English Channel and the Irish Sea alike. Amongst Copepoda there seems 

 to be a general agreement along with a certain amount of difference in detail 

 which will be referred to below when discussing the species. 



I may recall that in November 1910 I read a paper before this Society * 

 in which I made a comparison between the summer (July) plankton on the 

 West Coast of Scotland and that of the Irish Sea, showing that in some of 

 the deep fjord-like highland sea-lochs green-coloured phytoplankton can be 

 obtained even in the height of summer, while a zooplankton may be found 

 living in abundance a few miles away. This, of course, would be impossible 

 in the Irish Sea, where a zooplankton and a phytoplankton do not occur 

 simultaneously. 



DIATOMS. 



The seven generic forms I have selected for consideration taken together 

 make up nearly the whole of the Diatom plankton of the year. No other 

 genus occurs in anything like such profusion as these. In April, for example, 

 when the Diatoms are usually at their climax, all the remaining genera 

 (at most 10 or 12) taken together make up only about one-thousandth, or less, 

 of the whole. Moreover, these common Diatoms often attain their greatest 

 profusion successively, not simultaneously, so that single genera, or it may 

 be single species of a genus, make up on occasions the bulk of the phyto- 

 plankton. For example, in May 1916 the month's average haul of Diatoms 

 was 7,171,789, while the average for the genus Chwtoceras taken alone was 

 6,947,333, leaving only 221,456 as the average of all the rest of the Diatoms. 

 On the last two individual hauls, taken on May 25th and 29th, the actual 

 numbers were as follows : — 



Chcetoceras alone 24,094,500 ... 19,461,600 



C. sociale alone 23,936,000 ... 19,396,000 



All other Diatoms together... 166,300 ... 228,900 



So that on these occasions, and they are examples of many, one species makes 

 up nearly the whole of the plankton. 



* Journ. Linn. Soc.Zool. xxxii. (1913) p. 23. 



