BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 



37 



than is implied in such a computation, for the rate of renewal is very different in different 

 parts of the Sound. The more central portion of the stream would enter to a much 

 greater distance than that close to shore, while the waters contained in various depres- 

 sions of the bottom (if we may judge from temperature considerations) are probably 

 renewed at a comparatively slow rate. 



In Buzzards Bay the change is in all probability much more slow, owing to the fact 

 that this body of water communicates with the ocean at one end only, and that its mouth 

 is very narrow in proportion to the total area of the Bay. Here there plainly can be no 

 such continuous displacement in one direction as was found to occur in Vineyard vSound, 

 and the renewal must be effected entirely through the mixture of waters resulting from 

 the ebb and flow of the tide. The amplitude of the tides is, however, considerably 

 greater in the Bay than in the Sound. Since the mean depth of the former is much less 

 than that of the latter, a proportionally larger degree of change must result from this 

 cause. The mean depth of Buzzards Bay, as computed from the 91 soundings indicated 

 upon the chart contained in the Atlantic Coast Pilot, part iii, is a little over 41 feet. 

 The average rise and fall of the tide in Buzzards Bay is about 4 feet. Thus the amount 

 of water brought in by the flood tide is equal to about one-tenth of the total volume 

 already contained in the Bay. To what degree this ocean water mixes with that already 

 present in the Bay, and, conversely, what proportion of the water which leaves the Bay 

 on the ebb tide consists of that which entered on the previous flood, would be impossible 

 to determine even approximately. Assuming that as much as one-half of this remains 

 behind, which seems an extreme supposition, then the entire Bay would require 20 tides 

 or 10 days to effect a complete renewal. On the whole, therefore, it seems likely that 

 the average rate at which the water is renewed in Buzzards Cay is not over half that 

 which obtains in Vineyard Sound. 



It is obvious, however, that this renewal of water would take place at quite different 

 rates in different parts of the Bay. Near the mouth the change is probably much more 

 rapid than the above figures would imply, while at its head the renewal of water is 

 probably far slower. Likewise the surface water is probably changed at a much more 

 rapid rate than are the lower strata. It must be remembered, also, that it is not pure 

 ocean water which enters either the Bay or the Sound, but coastal water, which has been 

 contaminated during previous ebb tides. Nevertheless, even such crude estimates 

 may be of service in showing the relative stagnancy of the two bodies of water under 

 consideration. 



A feature of great importance in determining the character of the local littoral fauna 

 and flora is the slight amplitude of the tides throughout the entire region. A table will 

 best illustrate the amplitude of the mean, spring, and neap tidesat 1 1 representative points. 



