62 Puget Sound Marine Sta. Pub. Vol. 1, No. 9 



We thus find four factors which seem to be largely responsible for 

 the abundance of the algal vegetation on San Juan Island: (a) The 

 character of the shore line; (b) The movement of the water caused by 

 currents and tides; (c) The absence of fresh water and debris from 

 streams; (d) The absence of floating ice and extremes of temperature. 

 Modifications or variations of the above factors also help to account for 

 the distribution of the algae along the shores of San Juan Island. 



In addition to these factors we must consider two others which 

 have to do with the distribution of algae. Those species which grow 

 above low tide must be able to withstand desiccation. The writer 

 has shown that those algae which grow highest above the low tide line 

 can withstand more desiccation than those which grow nearer it (11). 

 Those plants which grow below the low tide line are growing in de- 

 creased light. Light is probably the most important factor in the dis- 

 tribution of algae at different depths below the water's surface. 



Probably the earliest attempt to classify algal vegetation was in 

 1836, when J. G. Agardh (1) first recognized the three zones of algae 

 along the coasts of Scandinavia. He named these the zone of Green 

 Algae, the zone of Brown Algae, and the zone of Red Algae. Kjellman 

 (9) in 1877 defined the littoral, sublittoral and elittoral zones. Later 

 Rosenvinge (15) in 1898 modified the boundaries of these zones, and 

 B^rgeson (2) in 1905 agreed with him. According to these workers the 

 littoral zone includes all algae which grow between the highest high 

 tide line and the lowest low tide line. The sublittoral zone includes all 

 algae which grow below the highest low tide line. From this it follows 

 that between the two zones there is a band which overlaps and has algae 

 which may belong in either littoral or sublittoral zone, as the exact line 

 of separation is somewhat variable. This study is limited to the littoral 

 zone and that portion of the sublittoral zone which extends down to 

 and includes the Nereocystis beds. 



Those areas or zones of vegetation which are dominated by one or 

 more species are here termed "Associations." Within each association 

 may be found one or several smaller groups which are again dominated 

 by other than the dominant species of the association. These smaller 

 groups are termed "Societies" by B0rgeson (2), and the same term 

 is applied to such groups here. The units of vegetation which are here 

 called associations are larger than the 57 associations into which Davis 

 (5) divides the algal vegetation of Woods Hole and vicinity, but the 

 writer does not think that the five associations of algae of San Juan 

 Island are large or distinct enough to warrant raising them to the rank 

 of formations, nor to subdivide them into smaller associations. 



