266 FOUNDERS OF OCEANOGRAPHY 



demonstrate the connection between cause and effect in 

 detail. The plankton increase in spring cannot be due to 

 temperature, as the records of sea temperatures at Port 

 Erin show that they are as low, if not lower, in March, at 

 the time when the phyto-plankton is waking up to activity, 

 as at any time during the winter. But although the sea has 

 not yet commenced to warm up, the days are much longer 

 and there are more hours of sunlight, and it seems probable 

 that this great increase in phyto-plankton, one of the most 

 important phenomena of the ocean, depends primarily upon 

 the rapid increase in the amount of solar energy which 

 accompanies the lengthening days of early spring about the 

 time of the vernal equinox. But this rapid increase in 

 Diatoms is no doubt also aided by the relatively large amount 

 of carbon dioxide and other necessary food matters, including 

 silica for their shells, accumulated in the sea during the 

 winter. Gran and Gaarder's investigations in the Chris- 

 tiania Fjord show a connection between the plankton in 

 spring and the amount of oxygen in the water, and also 

 indicate some relation between the increase of plankton and 

 the presence of nutrient matters in the water. The rapid 

 disappearance of the Diatoms after their maximum may be 

 due to a combination of causes — the exhaustion of the 

 carbon dioxide and the silica in the water, the depredations 

 of the increasing numbers of Copepoda, young fishes, and 

 other diatom-eating animals, or even to the toxic effect upon 

 the water of their own metabolism in dense crowds. 



Moreover, the conditions that suit one Diatom apparently 

 do not suit another, and so we have a regular succession 

 of different generic forms appearing at different times, and 

 therefore under different conditions. The first to become 

 abundant are the winter and early spring forms — the 

 circular discs or drum-shaped species of Coscinodiscus and the 

 almost square or oblong bright yellow species of Biddulphia 

 (Plate XXII, Fig. 1 ) . These two genera are at their maximum 

 in March and early April in an average year. Then follow 



