INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 151 
In that year the spring maximum was in May, a single haul on May 16th 
giving over 60 e.cm. and containing over 54 million diatoms. The diatom 
maximum was in May, the dinoflagellate maximum in June, and the Copepod 
maximum in July. 
At the time of a mixed plankton a sudden increase in the volume of the 
catch does not necessarily mean an increase in the number of the organisms 
prevalent at the time, or even an increase in the total number of organisms. 
For example : 
Diatoms. Copepoda. 
April 18th... ... us 45 e.em. — 238,000 and 7651 
April 21st ... 0.0... 2275 eem. = 194,000 and 2403 
The rise in volume in this case was due to a eomparatively small number 
of much larger organisms, such as medusæ, polychæte larvæ, fish eggs, etc. 
On the other hand, a sudden rise in the following month was due to an 
inerease of the organisms prevalent at the time :— 
May 4th .. ... .. 1 c.em.— 115,450 diatoms. 
May 10th ... ... ... 20°5 com. = 2,268,050 ,, 
It is evident, then, that to draw conclusions merely from the quantity of 
the catch (in c.cm.) may be deceptive, and that it is necessary in all cases 
to make a microscopic qualitative examination, so as to ascertain the 
organisms that are present, and then estimate their approximate numbers. 
DOMINANT ORGANISMS OF THE PLANKTON. 
Diatoms. 
In *Spolia Runiana,” III. (1918), I drew attention to the fact that the 
dominant organisms in the plankton which constitute the greater part of 
the summer zooplankton are about half-a-dozen species of Copepoda, and 
similarly in the case of the spring phytoplankton about the same number of 
genera of diatoms. ‘These comparatively few species belonging to these two 
very different groups thus come to be the most significant organisms in 
relation to the annual metabolie cycle of our seas and the food-supply from 
our coastal fisheries. 
Of the half-dozen generic groups of diatoms involved, the two most 
important genera in our seas are certainly Chetoceras and Rhizosolenta. 
They are the only forms * that ever reach hundreds of millions per haul, and 
in 14 of the 15 years under consideration they are by far the most abundant 
organisms present in the plankton. Moreover, it is these two groups of 
diatoms that enable us to analyse the vernal maximum into two distinct 
elevations, the Chwtoceras rise being earlier (April-May) and the Rhizo- 
solenia rise later (usually in June). It is therefore of some importance to 
consider the record of these two dominant genera throughout the series 
of years in more detail. 
* With the exception of Asterionella in 1913. 
