452 BULLETIN OF THE BUREAU OF FISHERIES. 
The study of Spindle Rocks (pages 476-479) has shown in a rather surprising way the 
degree of change which takes place on a small mass of rocks over a 12-month period. 
The northerly species which would be most likely to invade the Bay and Sound dur- 
ing the favorable winter season would be forms that reproduce rapidly through large 
crops of spores and mature so quickly that several generations may develop during the 
season. The tidal currents of the region would serve to distribute such species very 
widely, even though the favorable season might be short. 
8. SALINITY OF THE WATER. 
There are no fresh-water streams of importance in the immediate vicinity of Woods 
Hole to affect markedly the salinity of its waters, which are not much less dense than 
the open sea, having an average density of about 1.024 (the density of water in the north 
Atlantic being from 1.027 to 1.028). In the westerly portion of Vineyard Sound and 
lower portion of Buzzards Bay the density is somewhat greater, having been found at 
one point as high as 1.0243 (November, 1907). In the extreme upper portion of Buz- 
zards Bay the density is considerably less than at Woods Hole, having been recorded 
as. low as 1.0212 (March, 1908). Details of the observations on density made by the 
survey are presented in section 1, chapter 11, pages 52-54. 
The lower density of the upper portion of Buzzards Bay is evidently due to the 
proximity of a number of small streams that empty into the head of the Bay, but these 
are too far removed from Woods Hole to influence materially the salinity of the water 
. at that point. The swift tidal currents of Vineyard Sound keep its waters fairly uniform 
in density. It is not probable that density is a factor of importance in determining the 
distribution of alge in the deeper waters of the Bay and Sound, and it certainly is not 
to be compared with the two chief factors of temperature and the character of the bottom. 
The only bodies of brackish water in the immediate vicinity of Woods Hole are 
those of small ponds or areas of salt marsh which are connected with the sea by channels 
and rendered saline in various degrees by the inflow of tides or during storms. Such 
brackish waters support characteristic floras totally unlike those of the Bay and Sound 
proper, well illustrated by the Lyngbya salt-marsh association and the Enteromorpha 
salt-marsh association (see page 456). 
