96 H. JONSSON 



-2 fathoms, down to a depth of 20 fathoms; and finally the 

 elittoral region stretches from the 20-fathom contour downwards. 

 This division, unchanged in its main features, is generally employed. 

 The boundary between the littoral and sublittoral regions is, I think, 

 commonly supposed to be rather sharply defined in places with 

 tides, and less sharply defined where tides do not occur. 



By this division into three regions the algal Benthos is divided 

 into three belts of different depth. Although the boundary lines thus 

 drawn may be described as floristic boundaries, as regards many 

 species, yet they cannot always be regarded as natural limits of 

 vegetation. If natural limits of vegetation are to be drawn, several 

 factors must be taken into consideration. From depth-records, pure 

 and simple, a somewhat clear idea may naturally be formed of the 

 conditions of light, but not of the salinity and warmth. If the 

 conditions of light, salinity and warmth in those layers of water 

 where the algal vegetation lives were known all the year round it 

 would be easy to characterize the limits of the vegetation. The 

 great importance of the salinity to algal vegetation is well-known 

 and is emphasized by Rosenvinge (63), Svedelius (71), Borge- 

 sen (12) and Kyi in (45), amongst others. 



The ecological factors in the coastal waters round Iceland are 

 not so well known that I am able to draw the natural limits of 

 the vegetation by means of them. My starting point is, therefore, 

 the vegetation itself, and from the appearance of the vegetation it 

 is possible, to a certain extent, to form an opinion as regards the 

 ecological factors, in the same way as an opinion may be formed from 

 these concerning the appearance and composition of the vegetation. 



The marine algal vegetation divides itself into several zones as 

 the Benthos does in fresh water. The divisions between the zones in 

 the sea are very distinct : some species seem to be spot-bound or 

 very sensitive to changes of level; other species may occur in two 

 or several zones; but it depends especially, I think, on the quality 

 of the water, the intensity of the light and, where there are tides, 

 on the period of exposure (in the littoral zone). 



By studying the vertical distribution of the species and asso- 

 ciations, I have come to the conclusion that the marine algal vege- 

 tation may be divided into three zones almost parallel one with an- 

 other: the Littoral Zone, the Semi-littoral Zone and the Sublittoral Zone. 



The Littoral Zone understood in a more restricted sense is 

 identical with the upper littoral zone and extends almost to the 



