BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 447 



The movement of the ice along the shore and through the channels, whether due 

 to the rise and fall of the tide, to storms, or to tidal currents, serves to scrape bare the 

 large stones and bowlders, wherever they are exposed, so that they are frequently 

 almost or entirely free from algae in the spring when the ice disappears. These effects 

 are particularly evident on the exposed shore line of the upper portion of Buzzards 

 Bay and in portions of Vineyard Sound, where the rocks in the winter are not only bare 

 of algae, but also at times free from the common barnacle (Balanus balanoides) which 

 covers their surfaces in the summer. This action of the ice along exposed shores and 

 channels thus prevents or greatly reduces the littoral growth during the winter, when 

 the conditions are most favorable for the development of a very characteristic flora, 

 with species of the rockweeds (Fucaceae) as the most conspicuous forms. If it were 

 not for these facts we should expect in the winter heavy fringes of rockweeds along the 

 shore, for these grow luxuriantly where they are not exposed to the scraping of the ice, 

 as, for example, along the shore of Cuttyhunk and elsewhere in the lower portion of 

 Buzzards Bay and the westerly portion of Vineyard Sound. 



The scraping effects of ice on a particular group of rocks may be better understood 

 by comparing chart 267 of Spindle Rocks with chart 274 and the charts that show the 

 coming in of the spring and summer floras after the ice has disappeared (charts 268, 269, 

 and 270). Rocks which are perfectly bare after the winter become thickly covered 

 during the spring and summer with algae characteristic of these seasons. 



5. DEPTH OF WATER. 



Buzzards Bay and Vineyard Sound are relatively shallow bodies of water. As 

 may be seen from chart 227, at only a few stations was a depth greater than 18 fathoms 

 obtained. There were a number of stations with a depth between 14 and 17% fathoms, 

 but by far the larger number in the middle regions of both Bay and Sound were between 

 8 and 14 fathoms. The Bay in general gradually deepens toward the lower portion, 

 but the Sound, on the contrary, shows no marked progressive deepening toward the 

 western end. 



The depth at which algae will grow is determined chiefly by the penetrating power 

 of light and consequently varies in different seas according to the relative amount of 

 sunshine during the year and the clearness of the water. Rosenvinge (1898, p. 233) 

 places 20 fathoms as about the limit of growth for algae in northern seas where, how- 

 ever, the proportion of cloudy and foggy days is very large. Borgesen (1905, p. 700) 

 found the limit of growth around the Faroes to be between 25 and 30 fathoms. In 

 southern seas, where there is a very large proportion of sunny days and more direct 

 penetration of the sun's rays, as in the Bay of Naples and off the Balearic Islands (Rod- 

 riguez 1888) in the Mediterranean, deep-water algae have been reported to grow at 50 

 to 100 fathoms. Most of the species at these great depths belong to the Rhodophyceae, 

 but there are many of the Phaeophyceae in water deeper than 50 fathoms, and several 

 species of the Chlorophyce3e are found at 20 to 60 fathoms. 



With respect to the amount of sunlight during the year Woods Hole probably stands 

 somewhat midway between the conditions over northern seas and those of the south. 

 It certainly has both in winter and summer a large proportion of fair and sunny days. 

 Consequently there are no parts of either Buzzards Bay or Vineyard Sound included in 

 the limits of the survey that are too deep for certain algae. The dredgings of the Survey 



