356 ECOLOGICAL FACTORS, ETC. 



arriving gradually towards a state when the factors controlling the 

 zonation of fucoids on the seashore are really becoming understood, 

 but much more still remains to be discovered, especially in respect 

 of the species occupying the lower belts. 



Rock pools are commonly encountered on most rocky coasts, and 

 for this reason it is perhaps desirable that they should be mentioned 

 here. Klugh (1924) studied a series of six pools on the coast of 

 New Brunswick and concluded that the factors which may affect 

 the flora are (i) character of the bottom, (2) depth of the pool, 

 (3) amount of wave action, (4) temperature, (5) salinity, (6) ^H, 

 which probably depends to a large extent on the proportion of 

 chlorophyllous organisms in the pool, and (7) light. Many algae are 

 tolerant of a wide pH but there is often only a narrow range in 

 which they will develop to their greatest extent, whilst it is also 

 possible that the percentage of iron in the liquid medium or solid 

 substrate may at times be a limiting factor. Klugh considered that 

 temperature was the most important factor operating in tide pools 

 and Johnson and Skutch (1932) arrived at a similar conclusion. 

 Whilst a pool is exposed the temperature of such a small body of 

 water may rise considerably, and then when the tide returns the 

 cold sea water will lower the temperature very suddenly. An 

 examination of the flora showed that Rhodophyceae tended to be 

 more abundant in shaded pools whereas Chlorophyceae and 

 Phaeophyceae were relatively more abundant in the exposed pools. 



Biebl (1937) has recently published the results of a study of 

 seven rock pools on the English south coast, in which particular 

 attention was paid to the influence of different concentrations of 

 seawater on the cells of various species of the Rhodophyceae. As a 

 result of this study he concluded that light, temperature, pH and 

 seawater concentration were the important factors operating in tide 

 pools, though they are subject to some qualification. Temperature is 

 more likely to be effective in determining the northern limits of 

 species rather than actually causing damage, because it was dis- 

 covered that warming up to 26° C. over a period of 24 hours has no 

 effect on most Rhodophyceae, and changes of 12° C. could occur on 

 a hot day without causing any damage. Spondylothamnion multi- 

 fidum, for example, apparently cannot tolerate temperatures lower 

 than +3° C. and so reaches a northern limit on the English south 

 coast. Most of the algae investigated tolerate a^H range of 6-8-9-6 



