MARINE BIOLOGICAL STATION AT PORT ERIN. 37 



common constituents of the Plankton. It is possible, how- 

 ever, that in some parts of the ocean, far from land, the 

 Plankton may he distributed with the uniformity supposed 

 by Hensen. It is important to recognise that at least three 

 classes of locality exist in the sea in relation to distribution 

 of Plankton : — 



(1) There are estuaries and coastal waters where 

 there are usually strong tidal and other local currents, 

 with rapid changes of conditions, and where the Plankton 

 is largely influenced by its proximity to land. 



(2) There are considerable sea areas, such as the 

 centre of the North Sea and the centre of the Irish Sea, 

 where the Plankton is removed from coastal conditions, 

 but is influenced by various factors which cause great 

 irregularity in its distribution. These are the localities 

 of the greatest economic importance to man, and to which 

 attention should especially be directed. 



(3) There are large oceanic areas in which there may 

 be uniformity of conditions, but it ought to be recognised 

 that such regions are not those in which the Plankton is 

 of most importance to men. The great fisheries of the 

 world, such as those of the North Sea, the cod fishery in 

 Norway, and those on the Newfoundland Banks, are not 

 in mid-ocean, but are in areas around the continents, 

 where the Plankton is irregular in its distribution. 



As an example of a locality of the second type, 

 showing seasonal, horizontal and vertical differences in 

 the distribution of the Plankton, we may take the centre 

 of the* Irish Sea, off the south end of the Isle of Man. 

 Here, as in other localities which have been investigated, 

 the Phyto-Plankton is found to increase greatly about the 

 time of the vernal equinox, so as to cause a maximum, 

 largely composed of Diatoms, at a period ranging from 

 the end of March to some time in May. During this period 

 c 



