204 DISTRIBUTION OF DIATOMS AND COPEPODA IN THE IRISH SEA. 
microscopic contents of the sea represented by the phytoplankton annual 
curve, and the connection between the two will be seen when we realise that 
the alkalinity of the sea is due to the relative absence of carbon dioxide. In 
early spring, then, the developing myriads of Diatoms in their metabolic 
processes gradually use up the store of CO, accumulated during the winter 
and so increase the alkalinity of the water, till the maximum of alkalinity, 
due to the reduction in amount of carbon dioxide, corresponds with the crest 
of the phytoplankton curve in, say, April. Prof. Moore has calculated that 
the annual turn-over in the form of carbon which is used up or converted 
from the inorganic into an organic form probably amounts to something of 
the order of 20,000 or 30,000 tons of carbon per cubic mile of sea-water in 
the Irish Sea; and this probably means a production each season of about 
two tons of dry organic matter, corresponding to at least ten tons of moist 
vegetation, per acre—which shows that we are still very far from getting 
from our seas anything like the amount of possible food-matters that are 
produced annually. 
Testing the alkalinity of the sea-water may therefore be said to be merely 
ascertaining and measuring the results of the photosynthetic activity of the 
great phytoplankton rise in spring due to the daily increase of sunlight. 
Other possible causes, more or less related to the above, have been suggested 
—such as Brandts hypothesis that the fluctuations in the phytoplankton 
depend upon the accumulation, and then the exhaustion, of necessary 
inorganic food-matters in the water, such as nitrogen or phosphorus com- 
pounds or silica ; and the view of Nathansohn, Gran and others that vertical 
currents, carrying up food-matters from the deeper water, have a powerful 
effect upon the seasonal development of surface plankton. These may be 
contributory causes or may be effective locally, or on occasions; but it seems 
probable that a widespread phenomenon of enormous amount such as the 
vernal increase of phytoplankton must depend upon an equally widespread 
and powerfully-acting cause such as the rapid increase in the amount of 
solar light energy which marks the lengthening days of the year in early 
spring. 
