126 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 C0 2 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 Brandt's 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 seasonul development of surface plankton. These may be 

 contributor}' 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. 



