ECOLOGICAL FACTORS 393 



fall off. An interesting phenomenon is the changes in air temperature 

 that can be observed (Doty, 1957) immediately above the surface 

 of the water. With a rising tide such temperatures can be approxi- 

 mately 5° C. higher than that of the water. With an ebbing tide 

 evaporation brings about cooHng of the shore so that there may be 

 a narrow zone with a temperature below that of seawater and 5° or 

 6° C. below that of the air. It seems unlikely that this phenomenon 

 plays any part in determining zonation. 



Most workers also agree that temperature is a very important 

 factor in rock pools and probably also in salt pans. Whilst a pool is 

 exposed the temperature of such a small body of water may rise 

 considerably, especially in the case of pools at high levels, and then 

 when the tide returns the cold sea water will lower the temperature 

 very suddenly. An examination of pool floras has shown that 

 Rhodophyceae tend to be more abundant in shaded pools or parts 

 thereof, whereas Chlorophyceae and Phaeophyceae are relatively 

 more abundant in the exposed pools. 



Biebl (1937) studied some rock pools on the EngUsh south coast, 

 in which attention was paid to the influence of temperature as well 

 as of other factors. Temperature probably rarely causes actual 

 damage, except perhaps at high level pools, but it may cause res- 

 piration to be so speeded up that kataboHsm is not made good by 

 anaboHsm. Biebl found 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 sharply without causing any damage. 



Work with Fucus vesiculosus and Hormosira hanksii (Fig. 220) has 

 shown that respiration rate decreases with lowering of the tempera- 

 ture but that with the former species it is still measurable even at 

 - 15° C. (Kanwisher, 1957). 



The highly dissected appendages on the long *boa' fronds of 

 deep-water plants of Egregia laevigata (see p. 190) show a very 

 marked response in apparent assimilation rate to temperature 

 changes. Juvenile 'leaves' and basal 'leaves' behave very diflferently: 



Phs. as ml Ozlgm. dry wt.l5 min. 



