246 THE SEAS 



which reach maturity during the course of the summer 

 months do so at the expense of the great flowering of 

 drifting plants that occurred in the spring. These for 

 some reason die down in the summer, to be followed by a 

 smaller and shorter outburst in the autumn. To explain 

 these seasonal changes we must go back still further, and 

 find the causes for these curious maxima of plant abundance. 

 It is common knowledge that to obtain good crops on 

 land we must manure the ground ; of first importance in 

 manure for plant growth are those chemical bodies, the 

 phosphates and nitrates. Sea water contains these in 

 minute quantities in solution. At the beginning of March, 

 and indeed throughout the previous winter months, these 

 salts are present in greatest quantities, but it is not unti' 

 the spring that the diatoms are able to utilize this manure 

 to any great extent ; for plants are dependent for their 

 activities upon light, and during the winter the sun's 

 rays are very feeble, and owing to the sun's low altitude 

 little of its light penetrates into the water and much is 

 reflected from the surface. In the spring months however 

 the sun mounts in the sky and its light increases in strength. 

 The sun's energy now becomes available for the plants* 

 activities and they start to grow. With the large available 

 fund of manurial salts the growth is rapid ; but the food 

 supply is not inexhaustible and within a month it is almost 

 all used up (Fig. 52) and the great crop perforce dies 

 down. Throughout the summer months there may be 

 very small sporadic outbursts of plant life where the w^ater 

 has perhaps been temporarily enriched in nutrient material 

 by the presence of large shoals of animals whose excreta 

 manure the surrounding water, but the renewed vigour of 

 the diatoms in the autumn still remains to be explained. 

 The cause of this has been found to lie in the physical 

 properties of the sea water. During the hot summer 



