324 



MARINE BIOTIC COMMUNITIES 



average low-tide line also extend several meters (8-10) below low 

 tide (Petersen, 1918:18; Wisncr and Swanson, 1935). They are es- 

 sentially subtidal, as are all other described communities of this type. 

 The use of the term intertidal is misleading, as are most habitat 

 designations for communities, as it implies that all aggregations ob- 



A Gastropod -echinoderm Community 

 ( Strongylocentrotus - Argobuccinum biome ) 



Ecotone 



A Clam -worm Community 

 (Macoma-Paphia biome) 



Fig. 74. — A diagram showing the relations of three major marine communities 

 at the north shore of the Straits of Juan de Fuca in the northeast Pacific. 

 Starting at the left the Strongylocentrotus-Argobuccinum biome has its upper 

 limit above the level of the extreme low tides (E), and at the level the aver- 

 age of low tides (D) this boundary is very sharp. The Balanus-Littorina (BL) 

 biome is differentiated between the mean of the lower tides (D) and the mean of 

 the higher tides (A) but does not reach the mean of high tides (B). Toward the 

 right, the Balanus-Littorina biome is bounded at its bottom by sand and its strip 

 becomes narrower. The Macoma-Paphia biome reaches downward through about 

 8 meters with a wide ecotone between it and the Strongylocentratus-Ai-gobuc- 

 cinum biome. For convenience in making the diagram, it is shown as ending 

 abruptly at a ledge of hard rock at about 6 meters. C indicates the upper limit 

 of this community which is about half way between high and low tides, making 

 it essentially a subtidal community and at average low tide only 1^/^ meters 

 (C-D) of the 8 meter belt is exposed. 



served between the tide lines are distinct communities, whereas this is 

 not true. The barnacle-gastropod-mussel community might be called 

 intertidal, but since it is evidently adjusted to, and probably requires, 

 tidal rhythm it seems best to call it a tidal community. The primary 

 division with which we have to deal is, however, the two life-form 

 groups noted on page 323, which for convenience may be referred to as 



