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 \ery 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 algae 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 agengy, 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 3H nautical miles during the flood phase and to the extent of 4K miles during 

 the ebb. 



There would thus be a net westerly movement of the water amoOnting to about i knot 

 during each complete tidal cycle, or about 2 knots in 24 hours. Were this the only factor 

 concerned, it would thus require about eight days to completely replace the water of 

 Vinevard Sound. In reaUty 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 westeriy current. It seems likely, therefore, that a 

 week would much more than suffice to bring about a practically complete change of 

 water in Vinevard Sound. Obviously, the conditions are much less simple in reality 



o 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, 

 p. 153. 



