INVESTIGATION OF THE PLANKTON OF THE IRISH SEA, 145 
the two dominant groups, the Diatoms and the Copepoda, up to the end of 
1916, and further observations have only confirmed what was then stated. 
The spring maximum of phytoplankton, starting in March when the sea has 
still a low temperature and increasing to a climax in April, May, or June, 
the diatom minimum at the height of summer in July or August, the 
secondary lesser maximum in autumn (generally September or October), 
very variable both in extent and in constituent organisms, and, finally, the 
winter minimum have been recorded for every year and need no further 
demonstration. I am now, however, in a position to show, in the following 
table, the variation, both in amount and date, over the whole series of 
15 years, of the great spring maximum—by far the most striking feature of 
the annual plankton cycle ; and, moreover, to state that in most years this 
phytoplankton rise in the curve can be resolved into two elevations, an 
earlier formed mainly by species of Chwtoceras and a later due to species of 
Rhizosolenia. There are also some further details in regard to the distri- 
bution in time throughout the year of various groups of the plankton that 
can now be added :— 
TABLE I, 
Year. Pl. max. | Mth. aver, Record za Diat. max. Dino, max. | Copep. max. | 
er "A Ap. | 215 | 5l ^o Mar. May | Oct. 
| 08...... | May 113 | 23:5 May June | Sept. 
| 09...... ^0 May 24 41 Apr. May | Aug | 
| 10... | Apr. 63:3 129 Apr. July | July | 
11...... May 46 | 602 May June | July | 
12...... Apr. & Ju. 375 | 64 June May | June 
13...... May 401 722 May July | July 
l4...... May 348. 885 May May | Jy. & Sept. 
15...... May 635 | 1165 May June Aug. 
16...... | June 61:8 | 175 | June July Sept. 
17...... /—— May 77-6 1765 May Aug. | — Aug. 
18...... |o May 858 158 May July | Aug. 
19...... | May 36 | 99 May July | July 
20.00... May 453 | 102 May June | Sept. 
21...... | May 40 | 140 May June | June 
Table I. shows, for each of the fifteen years (1) the month in which the 
maximum of the total plankton occurred, (2) the monthly average in cubic 
centimetres of the hauls taken in that month, (3) the record (the highest) - 
haul of plankton in cubic centimetres, (4) the month of the Diatom maximum, 
(5) the month of the Dinoflagellate maximum, and (6) the month of the 
Copepod maximum for each year. 
