SPECIAL PROBLEMS OF SHORE LIFE 75 



THE PROBLEM PRESENTED BY THE VARIABILITY IN TEMPERATURE 

 AND SALINITY OF COASTAL WATER 



It is well known that, at a comparatively small depth in 

 the sea, life proceeds in surroundings of exceptional stability ; 

 as one approaches the shore, however, the range of varia- 

 tion becomes more marked till, between tide-marks the 

 water, owing to its shallow depth, and the influence of sun, 

 air, and fresh water from the land, is subject to wide changes 

 both in temperature and salinity. 



The Effects 0! Varying Temperature. — According to 

 Mobius (quoted by Semper, 1881), the coastal waters of 

 North European countries display considerable seasonal and 

 even daily variations in temperature. These variations may 

 be observed not only in surface water, but not infrequently 

 at considerable depths. In the Gulf of Kiel, for instance, 

 the mean monthly temperature varies, at a depth of 

 16 fathoms, between a maximum of 64*4° F. and a minimum 

 of 347° F. Hedley (1915) gives the highest known water 

 temperature in Sydney Harbour as 75-5° F. (July, 1887), 

 and the lowest as 50° F. At Woods Hole, on the eastern 

 coast of the U.S.A., the range is from below freezing point 

 to 70° F. ; San Francisco Bay ranges from 69° F. to 42° F. 



Local climate accounts for much variation in the amount 

 of heat and cold affecting the tidal region ; the particular 

 aspect and type of substratum are also of importance. The 

 higher and more landward portions are naturally most 

 affected. Rock surfaces become greatly heated unless 

 heavily overgrown with algae ; rock pools, if deep and shaded, 

 will be comparatively unaffected, while shallow pools near 

 the upper tidal limits evaporate rapidly. Boulders, etc., 

 will provide shelter not afforded by sand. Wind plays an 

 important part in aiding desiccation, and the salinity of the 

 water is of some consequence. 



In tropical seas the variation at the surface, even between 

 winter and summer heat, is relatively very slight — at the 

 Philippines, for instance, not more than 2° C. In northern 

 seas, so small a variation only occurs at a great depth. It 



