386 THE ALGAE 



The length of the inter-tidal exposure also affects the conditions in 

 rock pools and salt pans, the diurnal changes being greater in the 

 higher pools and pans where the length of exposure is longer. 

 With one type of tide there will be certain levels on the shore where 

 the inter-tidal exposure can be doubled, e.g. at 3-5 ft. in Fig. 206. 

 Similarly the submergence period may be drastically altered over a 

 short vertical range, e.g. just above 2-5 ft. (Fig. 206) it is about 10 

 hr., whereas just below 2-5 ft. it is about 30 hr. The effect of this 

 phenomenon in relation to critical levels (p. 407) may well be pro- 

 found. 



(^) Continuous exposure and submergence 



At levels lying between lowest high water of neap tides and ex- 

 treme highest water of spring tides there will be periods of two or 

 more days during the neap period of the tidal cycle when the level 

 will not be inundated at all. Similarly between highest low water 

 of neap tides and extreme lowest low water of spring tides there 

 will be days during the neap period of the tidal cycle when the level 

 will be continuously submerged. In the former case, desiccation, 

 temperature and salinity changes will be far more profound than 

 those that occur between two consecutive tides : in the latter case 

 continual submergence may affect the metabohc activities and the 

 compensation point (see p. 408). It is probable that the periods of 

 continuous exposure are of far greater importance than inter-tidal 

 periods, especially during the sporeling stages. Unfortunately very 

 Httle data on this issue has been collected. Gail (1920) has suggested 

 that it is the desiccation of young plants that prevents the appear- 

 ance of algae outside their usual zones, and it is a remarkable fact 

 that sporeHngs of fucoids are usually very strictly confined from an 

 early stage to the level they occupy as adult plants. 



A rather different exposure phenomenon can be observed around 

 large lakes. Very often evaporation in the summer lowers the water 

 level so that the winter and summer levels are different. This zone 

 of exposure between the two seasonal levels may be occupied by a 

 special algal community, or it may form a bare zone between the 

 winter water edge community and the lower simimer water edge 

 community. 



Diurnal variations in temperature may affect the metabolic 

 activities of the algae (see p. 393) and extreme variations in salinity 

 can also bring about death of some species (see p. 404). Water loss 



