36 BULLETIN OF THE BUREAU OF FISHERIES. 
Atlantic (charts 220, 221, 222), it will be seen that during the months of June to Sep- 
tember, inclusive, the waters of Long Island Sound and those at the station just south 
of Buzzards Bay have a temperature several degrees higher than that of the first two 
stations to the eastward of these points. Farther yet to the eastward, however, the 
temperature again rapidly rises, owing to the presence of the Gulf Stream. The local 
relations will be discussed more fully in the next section .of this chapter. 
In addition to these great ocean streams, the local currents due to tides are very 
important in determining the fauna and flora of our waters. ‘Tidal currents of sufficient 
velocity to be reckoned with by mariners occur at considerable distances offshore 
and, when deflected and concentrated by features of the coast line or by shoals, their 
velocity may be very great. In Woods Hole Passage, for example, they attain the 
speed of 8 miles per hour at spring tide. Such rapidly flowing currents, where the 
water is shallow and the bottom rocky, must result in a very high degree of oxygenation 
of the water. Moreover, a rapid current, of course, bears a more abundant food supply 
to those fixed or slow-moving organisms which depend for their food upon minute 
particles brought to them passively, or, as is the case with plants, upon gases or other 
substances in solution. Accordingly, we find beds of mussels and luxurious growths 
of anemones, ascidians, hydrozoa, bryozoa, and alge in some of these tidal streams. 
On the other hand, tidal and other currents undoubtedly have a deleterious influence 
upon certain other organisms, which, through their agency, may become buried in sand 
or mud. 
_ But the most widely prevailing effect of the tides locally is the continual mixing of 
the warmer (in summer), less dense, and relatively impure water of the coast line with 
the unlimited reservoir of cooler and purer water offshore. An idea of the magnitude 
of this process may be gained by considering the rate of tide flow in Vineyard Sound. 
This is as high as 2.6 knots per hour in the middle of the channel at the time of maximum 
velocity of the current. It is stated that ‘“‘an object set adrift at the time of slack before 
flood will be carried 7 sea miles eastward before the reversal of the current, and an object 
set adrift at the time of slack before ebb will be carried 9 sea miles westward before the 
beginning of the flood stream.’’* Thus a certain part of the water at least travels a 
distance of one-half or more of the length of Vineyard Sound during a single phase of the 
tide. Owing to the retardation due to the friction of the shores and bottom, the mean 
sectional velocity would perhaps not exceed half the figures stated above. Even so, 
however, the water throughout the entire section would be displaced on the average to 
the extent of 3% nautical miles during the flood phase and to the extent of 4% miles during 
the ebb. ‘ 
There would thus be a net westerly movement of the water amounting to about 1 knot 
during each complete tidal cycle, or about 2 knotsin 24 hours. Were this the only factor 
concerned, it would thus require about eight days to completely replace the water of 
Vineyard Sound. In reality the ocean water brought in during the flood tide constantly 
mixes with that already present in the Sound, and this process of diffusion must result 
in a fairly rapid renewal of the latter, quite independently of the transfer of water result- 
ing from the predominance of the westerly current. It seems likely, therefore, that a 
week would much more than suffice to bring about a practically complete change of 
water in Vineyard Sound. Obviously, the conditions are much less simple in reality 
3 These data, though not the deductions which have been drawn from them, were furnished by the office of the Coast and 
Geodetic Survey. See also current diagram for Nantucket and Vineyard Sounds, in U. S. Coast Pilot, Atlantic Coast, pt. m, 
Pp. 152. 
