FEOM WEST COAST OF SCOTLAND AND THE IRISH SEA. 



33 



plankton maximum (Table, col. F). The accompanying diagram (fig. 7) 

 shows, e.g., the cm-ve for the whole plankton in the year 1908, as given by 

 a very lai'ge series of gatherings *, extending over every month, taken across 

 Port Erin Bay. The high points in April and May, and then again, but to 

 a less extent, in September and October, show the influence of the vernal 

 and autumnal phyto-plankton maxima, and the effect would, of course, be 

 still more marked in the curve showing the Diatoms alone. Taking the 

 average of the last three years in Port Erin Bay, we find that the monthly 

 averages for the total plankton begin low in January and February, rise in 

 March and still more in April, reach the maximum' in May, drop rapidly 



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through June, July, and August to the summer minimum, rise a little in 

 September and October to form the autumnal maximum, and fall aoain in 

 November and December to the mid-winter minimum. The autumnal 

 maximum is always very much less than the vernal^ and is sometimes scarcely 

 apparent. Both are caused by a very marked increase in the phyto-plankton, 

 chiefly Diatoms (see lists in Table, p. 35) : and the species which are the 

 most abundant and characteristic at the two seasons are, for the most part, 

 distinct f. In summer, when the phyto-plankton is practically absent, the 

 zoo-plankton reaches its maximum ; but, in bulk, even at the maximum, the 

 zoo-plankton, except on rare occasions, is small compared with the sprino- 

 gatherings of phyto-plankton (see columns D and E). A phyto-plankton 

 gathering in the Irish Sea is practically only obtainable in spring or in late 

 autumn ; and if the gathering be a very large one (like D in the Table), it is 



* See Trans. Biol. Soc. Liverpool, vol. xxiii. p. 244 (1909). 

 t See op. cit. vol. xxii. p. 202 (1908). 

 LINN. JOURN. ZOOLOGY, VOL, XXXII, 3 



