DISTRIBUTION OF DIATOMS AND COPEPODA IN THE IRISH SEA. 95 



Spolia Runiana. — III. The Distribution o£ certain Diatoms and Ce^epoda, 

 throughout the year, in the Irish Sea. By W. A. Herdman. F.R.S., 

 F.L.S., Professor of Zoology in the University of Liverpool. 



(With 21 Text-figures.) V ^ X 



[Read 1st November, 1917.] s%^ g/ ^ % ^ 



Reprinted from Journ. Linn. Soc, Bot. vol. xliv. (1918), pp. 173-204 



Part of the work* of the yacht 'Runa' for some years previous to 1914 

 consisted in taking periodic samples of the marine plankton at various 

 localities around Port Erin, at the south end of the Isle of Man, during the 

 two most interesting times in the annual cycle — viz., spring (March-April) 

 and autumn (July-September). During the remaining months, when the 

 yacht was not in commission, plankton gatherings in Port Erin bay were 

 taken with great regularity at the rate of six in the week, three at a time on 

 two occasions per week, two of the three hauls being horizontal and the 

 third vertical. This systematic plankton survey has been continued for fully 

 10 years (1907-1917 inclusive), and over 5000 f samples have been collected 

 and examined. The general results of this intensive study of the plankton of 

 a central area of the Irish Sea have been given in a series of reports J drawn 

 up in collaboration with Mr. Andrew Scott, A.L.S., and others, and published 

 by the Lancashire and Western Sea-Fisheries Committee; but the material 

 and statistics collected still contain much information which has not yet 

 been made use of. It is proposed in the present communication § to deal 

 with the records of the occurrence throughout the year in our district of a 

 few of the most abundant of the Diatoms and the Oopepoda which make up 

 the bulk of the phytoplankton and of the zooplankton respectively at those 

 periods of the year when they are most abundant. At the time of the 

 spring maximum (usually in April or May) a small silk tow-net hauled for 

 about 15 minutes through about half-a-mile of the surface water of the Irish 

 Sea will usually catch some millions of individual Diatoms (up to a couple of 

 hundred millions || on occasions), constituting probably, on the average, some 

 999,999 out of each million of organisms in the gathering f. This is almost 



* For Parts I. and II. of "Spolia Runiana" see Journ. Linn. Soc, Zool. xxxii. p. 163 

 (1913), and p. 269 (1914). 



t More precisely 5116, to the end of 1916. 



* Trans. Biol. Soc. Liverpool, xxii. (1908) to xxxi. (1917). 



§ I wish to acknowledge, with thanks, the help I have received in the preparation of 

 these plankton records from Mr. Andrew Scott, A.L.S., and from my secretary, Miss 

 II. M. Lewis, B.A. Mr. Scott took for me the excellent photo-micrographs of the plankton 

 from which most of the illustrations have been reproduced. 



|| Estimated by counting measured samples. 



% The average of a number of cases where smaller, but still very large, hauls of Diatoms 

 were taken is — Diatoms=about 99 per cent, of the total organisms present. 



LINN. JOUKN. ZOOLOGY, VOL. XXXIV. 8 



