170 SIR W, A. HERDMAN: PLANKTON OF THE IRISH SEA. 
A comparatively small number of genera of Diatoms and Copepoda, half-a- 
dozen of each, are the dominant organisms of the plankton, and make up by 
far the greater part of the phytoplankton and zooplankton respectively, and 
these are the all-important organisms upon which the nutrition of higher 
animals and ultimately of the food-fishes from the sea depends. 
It is possible, moreover (as suggested by Hjort), that the survival of large 
numbers of newly hatched food-fishes in early spring, upon which will 
depend the prosperity of commercial fisheries a few years later, is determined 
by the amount of phytoplankton present at that particular time in the sea. 
It seems probable that the vernal increase in phytoplankton, one of the 
great phenomena of the ocean, depends primarily upon the rapid increase in 
the amount of solar energy which accompanies the lengthening days of early 
spring, especially about the time of the vernal equinox. Thus, general 
meteorological conditions are linked up with the commercial fisheries of 
several years ahead. The diatom maximum in spring is no doubt aided by 
the winter increase of carbon dioxide and other food-matters in the sea. - The 
rapid disappearance of the diatoms after the maximum may be due to some 
toxic effect upon the water caused by their own metabolism in dense crowds. 
It is impossible to draw numerical conclusions as to the population of 
large sea-areas from few and small samples of the plankton. 
Even series of vertical hauls taken at the same spot in rapid succession 
show so much variation from their mean that conclusions drawn from any 
one haul might be anything up to 50 per cent. wrong in either direction. 
We arrive, then, at the conclusion that the distribution of plankton in the 
sea is not uniform, and that many animals such as Copepoda are present in 
swarms or patches. 
EXPLANATION OF PLATE 7. 
Variations of Biddulphia sinensis and B. regia in the Irish Sea. From photo-micrographs 
by Mr. Andrew Scott. (For detailed explanation, see pp. 155 & 156.) 
