LINNEAlf SOCIETY OP LONDON. 29 



high peak at the end of September. Omitting, however, the 

 single catch of September 30th (which is due in the main to 

 lihizosolenia semisjpina) the peak is reduced to less than one-third 

 its former height. A remarkable feature of this September hump 

 is the sudden character of its appearance and disappearance and 

 its short duration (six days). An inspection of the temperature 

 curve of the year for the water of the Bay, shows that the sudden 

 increase in the phytoplankton coincided with the maximum in 

 temperature; and our weekly weather records at the Biological 

 Station show at that same time a week of fine calm weather with 

 easterly breezes (S.E. and E.S.E.). I have noticed the same 

 phenomenon in previous years, both at Port Erin and on the 

 west coast of Scotland, which seems to indicate that if weather 

 conditions be suitable at the end of autumn the phytoplankton 

 may suddenly increase so as to constitute a second jnaximum in 

 the year, the first being in spring ; but that this possible " maxi- 

 mum " may be so modified in time and in amount by temperature 

 and wind as to be unrecognisable. In 1906 it was very much 

 more marked at Port Erin than in 1907, and lasted longer. 



The phytoplankton minimum for the bay occurs in August, no 

 Diatoms being taken from August 9th to August 23rd, though 

 nettings were taken on all days included between these dates save 

 three. 



As an example of a sudden change in the plankton, we may 

 compare the surface hauls taken in the bay on October 1st and 

 14th. The total quantities of the two gatherings were 1*5 and 

 1]*5 i-espectively ; on the 1st, Diatoms were relatively abundant 

 (over 91,000) ; by the 14th they had disappeared. But Sagitta 

 and various larvae, and especially Copepoda, had greatly increased 

 in number by the latter date. The adult Copepoda in all numbered 

 only 1,045 on the 1st, while they reached 27,790 by the 14th ; 

 younger forms and Nauplii had also become much more abundant. 

 By November, however, the Diatoms were back in quantity, and 

 Copepoda had begun to decrease again. 



The Dinoflagellata rise to a maximum in April later than the 

 Diatoms, and may have a second period of sudden increase in the 

 autumn if weather conditions are favourable. 



Ceratium tripos is the most abundant species of Dinoflagellate 

 in the Irish Sea, and is present practically all the year round in 

 considerable abundance (up to 7753 per haul) at the Isle of Man. 

 Our 650 gatherings in one year showed C. tripos on 492 

 occasions. 



The curve for Ceratium tripos agrees in general with that for 

 the total Dinoflagellates, but differs markedly from those both of 

 Diatoms and Copepoda. The spi-ing maximum in the Dino- 

 flagellates is later thivn that of the Diatoms, but precedes that of 

 the Copepoda. Then again the September hump of the Dino- 

 flagellates is earlier than that of the Diatoms, and much earlier 

 than the October maximum of Copepoda. On the whole the 

 annual curve for the Dinoflagellates lies intermediate between 

 those for Diatoms and Copepoda. 



