PHYSIOGRAPHY OF THE SEA-SHORE 25 



content of the water also was higher on the north side, where 

 vegetation is abundant, and often, in this vegetation, reached 

 nearly twice saturation. On the south side the oxygen 

 content rarely reached saturation. Herdman quotes Moore 

 (19 1 5) as to the greater alkalinity of the sea in spring than 

 in summer, and considers that testing the water for alkalinity 

 may be said to be merely ascertaining and measuring the 

 results of the photosynthetic activity of the great seasonal 

 phytoplankton rise in spring due to the daily increase of 

 sunshine. 



Light. — The effect of light on life processes is known 

 to be profound, and the abundant illumination of the shore 

 zone is one of the chief reasons for its suitability as a haunt of 

 life. With increasing depth the amount of light which 

 penetrates rapidly diminishes, the red rays being absorbed 

 the first, then the yellow and so on, absorption occurring 

 in the order of the spectrum. The degree of illumination 

 varies considerably, being modified by local climate and by 

 physiographical features ; similarly the extent to which light 

 may penetrate varies with the transparency of the water. 

 In particular, the distribution and character of plant life 

 are governed by light ; in the sea beyond a certain depth 

 plant life is entirely absent. Algae living at a given depth 

 receive light of different quality from that received at a 

 higher or lower level. It was once thought that pigments 

 supplementary to the chlorophyll enabled the plant to utilise 

 light rays of a quality useless to the latter, and that con- 

 sequently the power of an alga to grow at a given depth 

 depended on the presence of a particular pigment. While, 

 however, it is an undoubted fact that algae living at different 

 levels tend to differ in colour (for instance, at a certain depth 

 red algse predominating sometimes almost to the exclusion of 

 other forms), this explanation cannot now be accepted without 

 considerable modification (Sauvageau, 1920). To speak of 

 the succession of colours in the narrow tidal zone as being 

 explicable on this basis is inaccurate. 



Temperature. — Coastal waters in the temperate regions 

 display very considerable seasonal and even daily variations. 



