MARINE ALGAL VEGETATION 27 



2. THE OCEAN. 



As regards the ocean, the chief points are its movements, tem- 

 perature and salinity. 



A. The Movements of the Ocean. 



These are tides, waves and currents. All these movements 

 of the ocean are of very great importance to the life of the algae. 



a. Tides. By the alternate rise and fall of the tide a part of 

 the shore is laid bare, and the vegetation growing there must then 

 be capable of maintaining life in the air for a longer or shorter 

 period. Those plants which grow highest up in the zone thus left 

 dry, are exposed during the greater part of the period between the 

 one flood-tide and the next, or for about 10 11 hours in every 12. 

 The plants occurring lowest down in the zone, on the other hand, 

 are not exposed during spring-tide for more than one hour in every 

 12, and they are submerged the whole time during neap-tide. The 

 upper limit of the algal vegetation is, moreover, dependent on how 

 high the tide rises, i. e. the height of the flood-tide. 



The Height of the Flood-tide. The following data regarding 

 the height of the flood-tide are taken from "Den islandske Lods" 

 (1903) and from the alterations and additions to it which have been 

 published. The height of the flood-tide is greatest in SW. Iceland 

 and least in E. Iceland. The height of the flood-tide at spring-tide 

 is recorded as being about 14 feet from Reykjavik (SW. Iceland), 

 10 11 feet from NW. Iceland, 5 5Ys feet from the north coast 

 and 5 feet from E. Iceland. 



In many places there is a great difference as regards the height 

 of the flood-tide during the spring and neap tides. To illustrate 

 this more fully I give the following figures from some localities on 

 the different parts of the coast: 



Spring-tide Neap-tide 



South Iceland, Vestmannaeyjar 8 10 feet 4 feet 



Eyrarbakki 10 6 



South-west Iceland, Reykjavik cir. 14 4 



Stykkisholmur 12 6 



North-west Iceland, Dyrafjordur 11 56 



North Iceland, Akureyri 5*/2 - l x /2 - 



East Iceland, Djiipivogur 7 l /z - 2 l /2 - 



From Elli5aey near Stykkisholmur in SW. Iceland the height 

 of the flood-tide during spring-tide is recorded as being 14 feet and 



