n] RHYTHMICAL CHANGE IN THE SEA 47 



We see then that a great number of fishes and 

 invertebrates reproduce in the spring and summer 

 months and that the plankton increases largely in 

 bulk because of the immense numbers of eggs and 

 larvae so produced. But the increase in the mass of 

 the plankton during the spring months which is so 

 remarkable a phenomenon in the sea is due far less 

 to the appearance of these larvae than to the repro- 

 duction of the pelagic unicellular organisms. Just 

 as each species of fish and invertebrate has its own 

 spawning time, so each species of diatom or protozoan 

 making up the permanent plankton also reproduces 

 at a particular time. The stimulus to the reproduc- 

 tion of the higher animals is the temperature change 

 in the spring of the year, and this has also something 

 to do with the reproduction of the diatoms and pro- 

 tozoa, but it appears that the change of sunlight and 

 salinity are also factors of great importance in the 

 latter cases. During the first two months of the 

 year there are very few diatoms in the sea in 

 Liverpool Bay, but sometimes about the beginning of 

 March certain spring species begin to appear, and 

 attain a maximum of abundance during that and the 

 next month, while at the same time the animal 

 plankton, which was characteristic of the sea before 

 this time becomes temporarily less. The spring 

 diatoms (which are best represented by the two 

 genera Biddulphia and Coscinodiscus) become very 



