EMERGENCE O 



DEPTH OF SUBMERGENCE 



T 



SUBMERGENCE 



EMERGENCE 



SUBMERGENCE 



Fig. 214 Top: effect of light intensity upon photosynthesis of 

 exposed and submerged Hormosira banksii in relation to depth. 

 a=low light intensity; b, optimum light intensity; c= supra- 

 optimum light intensity. Bottom: effect of desiccation upon 

 photosynthesis (full Hne) and respiration (broken Une) of Hor- 

 mosira banksii when exposed. (After Trevarthen.) 



days. In regions where chop is considerable over much of the year, 

 or where cloud is persistent, the lower Umit of algae will be higher 

 then elsewhere. 



In the study by Stocker and Holdheide, who used Fucus spiralis 

 var. platycarpus and F. serratus, the difference between sunny and 

 cloudy days was extremely pronounced (Fig. 213), and it would 

 seem that on exposure the fall off in the assimilation rate on sunny 

 days is correlated with the water content, because the exposed 

 thaUi quickly dry up and cease to assimilate. An investigation into 



397 



