406 THE ALGAE 



(c) pH (Acidity) 



The pH of sea water normally ranges from about 7-9 to 83 and 

 the factor is of no great importance for the littoral algae though it 

 might be of significance in some of the higher rock pools where, 

 during the day, the pH may rise to 10. Most algae that have been 

 investigated tolerate a pH range of 6- 8-9- 6 and the pH of pools is 

 rarely outside this range. 



(d) Oxygen content 



The oxygen tension in sea water is normally low but sufficient 

 for metaboHc purposes. In pools, during the day the oxygen content 

 may rise substantially. 



(e) Nutrients 



This factor is of importance in controlling the seasonal periodi- 

 city of marine and fresh- water plankton (see p. 342). It is also of 

 importance in lakes where oUgotrophic and eutrophic waters differ 

 chemically, and this difference is reflected in the vegetation (see 

 p. 367). High level rock pools well supplied with bird droppings 

 commonly support a very rich flora of microscopic algae. 



D. Biological factors 



(a) Animals 



Competition between plants and animals may in certain cases be 

 sufficiently severe to eliminate some algae. It therefore operates as 

 a presence or absence factor. If Patella (limpet) is present in abun- 

 dance it may eat up the Enter omorpha felt and so stop Fucus spore- 

 lings from becoming estabhshed. The attacks of boring mollusca 

 are probably responsible for the eventual detachment of Durvillea 

 plants, and in Europe Helicion pellucidum is said to bring about the 

 detachment of Laminaria saccharina plants. On salt marshes the 

 mollusc Hydrohia ulvae can cause havoc among beds of Ulva, 



(b) Plants 



Many of the possible relationships here have not been worked 

 out. There is, of course, the host-parasite relationship, e.g. Jan- 

 czewskia, (p. 252); Harveyella^ (p. 241, etc.), and host-epiphyte re- 

 lationship, because some of the epiphytes appear to be restricted in 

 their host requirements. The estabhshment of Fucus sporelings 



