ECOLOGICAL FACTORS 



385 



(e) Silt load 



In some areas, e.g. mouth of the River Severn in England, Bay 

 of Fundy in Nova Scotia, the load of silt carried by the sea exerts a 

 distinct effect upon the fauna and flora. Certain organisms are un- 

 able to tolerate silt deposition, in the case of algae photosynthesis 

 presumably being the main metaboUc factor concerned. Silt load 

 can therefore operate as a presence or absence factor. 



(/) Inter-tidal exposure 



This is the exposure suffered at any level between two consecu- 

 tive tides. Submergence-exposure curves have been constructed 

 for a number of areas and as they vary with the type of tide, i.e. 

 spring or neap, it is clear that tidal phenomena alone cannot deter- 

 mine zonation, though it is evident that they represent a major 

 causal factor. Exposure will operate indirectly in so far as it affects 

 water loss, temperature changes of the thallus, saHnity of the cells. 

 The degree of atmospheric humidity is also of importance during 

 the periods of exposure. It is this factor, together with that of tem- 

 perature, which prevents algae from occupying the shore in the 

 tropics and enables them to form a dense covering at high latitudes. 



-- 23 hrs. -->► 



Fig. 206 Tide curves at San Francisco and the relevant hours of 

 exposure at different levels. (After Doty.) 



