446 BULLETIN OF THE BUREAU OF FISHERIES. 



lation and settling of finer material. A detailed description of the bottom characters 

 will be found in section i, chapter II, pages 29-33, and the peculiarities are graphically 

 presented on chart 227. 



The luxuriance and to a large extent the nature of the algal vegetation depends 

 upon the character of the bottom. Rocky, stony, and shelly bottoms are the most 

 favorable for the attachment of algae and, in general, support the heaviest growths of 

 marine vegetation. Sandy and muddy bottoms are less favorable and are generally 

 very barren, although some species are confined to such situations. It is clear that 

 the shifting nature of sand and mud, frequently stirred by tides and storms, presents 

 conditions very unfavorable for the germination of algal spores, which quickly become 

 covered by sediment. Sandy or muddy bottoms are, however, apparently necessary 

 for the development of extensive beds of the eel grass, Zostera marina. 



3. THE TIDES AND TIDAL CURRENTS. 



As stated before, the tides at Woods Hole and adjacent portions of Buzzards Bay 

 and Vineyard Sound are of relatively slight amplitude. There is considerable variation 

 at different points in the Bay and Sound and in the harbor of Woods Hole, due to the 

 peculiarities of the tidal currents in the region. At Woods Hole, on the Sound side, 

 and in Vineyard Haven the average tide is 1.7 feet, at Gay Head it is 3 feet, in Buzzards 

 Bay at Woods Hole 4.1 feet. With such small tides it is clear that the strip along the 

 shore habitable for a littoral algal flora — that is, a flora above the lowest tide mark — could 

 not be very broad. It is generally only a few feet wide, and one notices at once in this 

 region that the receding tide fails to expose broad stretches of rock, sand, or mud in the 

 manner characteristic of the coast north of Cape Cod, where the tides are much greater. 



The arrangement of the land that bounds Vineyard and Nantucket Sounds is 

 responsible for the remarkable tidal currents that flow east and west in Vineyard 

 Sound, and in and out of Buzzards Bay through the channels of Woods Hole, Robin- 

 sons Hole, and Quicks Hole. These tidal currents must be very effective in distribut- 

 ing algal spores, and it seems probable that the rapidity with which algal vegetation 

 springs up after each change of season (as over areas scraped clear by floating ice) 

 must be due, at least in large measure, to the tidal currents. It is certain that any 

 alga which develops large crops of spores has by such means the opportunity of dis- 

 tributing these very rapidly throughout practically all of the waters of this region. 

 This factor must be of considerable importance in securing the almost universal presence 

 of some species that can grow under a wide range of life conditions, as well as the 

 appearance of others at distantly separated stations. 



4. THE EFFECT OF ICE. 



The upper portions of Buzzards Bay are at times during the winter more or less 

 completely frozen over, and small harbors, such as Little Harbor at Woods Hole, may 

 have a thick covering of ice. Sheltered portions of the coast, wh::h are not exposed 

 to surf or stong tidal currents, are fringed with ice. There is also much floating ice in 

 the Bay and Sound consisting of large cakes which come from the breaking up of larger 

 masses. This floating ice is swept by the tides back and forth in the Bay and Sound 

 and through such channels as Woods Hole. 



