58 THE SEAS 



most important. If there were not a constant reserve of 

 these in the bottom deposits, the sea would soon become 

 denuded of them ; as it is the amount of silica, to take but 

 one example, in solution near the surface is greatly reduced 

 in the spring and early summer when the diatoms are 

 increasing most rapidly. In the winter when the diatom 

 flora is reduced and there is a mixing of the waters at 

 various depths and a fresh dissolution of silica from the 

 bottom, the surface waters become fully charged with 

 silica ready for the demands of the coming spring. 



Life on the Sea Bottom 



The animals which inhabit the bottom of the ocean are 

 called the benthos, to distinguish them from the drifting 

 and actively swimming animals known as plankton and 

 nekton respectively. Extensive investigations by means 

 of dredges and trawls have shown that the benthos 

 varies greatly in different regions both in quantity and 

 quality. Geaerally speaking animals are most numerous in 

 coastal waters, and become less and less so the further we 

 pass from the land into deeper water, though there is 

 animal life, and often of a unique and interesting kind, 

 even in the abyssal regions with red clay deposits. The 

 greatest number of different species is found in the tropics, 

 especially around continents and in the neighbourhood of 

 coral islands, but the temperate and polar seas make up for 

 the paucity of variety of species by the far greater 

 numbers of individuals of those kinds which occur there. 



Differences in fauna are chiefly the result of differences in 

 depth, temperature, salinity and food, and of the interactions 

 of these factors. The influence of the last is universallv felt, 

 that of salinity is apt to be local, the result of currents or 

 great influxes of fresh water from rivers, that of temperature 

 is especially effective in the shallower waters near the coast 



