PLANKTON OF THE GULF OF MAINE 



481 



In the western coastal zone, south of Cape Elizabeth, and in the basin generally, 

 the vernal period of vertical mixing is brief, its activity lessening as soon as the 

 combined effect of solar warming and of the freshening of the surface increases 

 the vertical stability of the water. This becomes very stable indeed by the early 

 summer with very little interchange taking place between the upper and deeper 

 strata from that time until into the autumn. But strong tidal currents keep the water 

 in the northeastern corner of the gulf in a state of more active vertical circulation 

 throughout the year, especially in the Bay of Fundy, along western Nova Scotia, 

 and locally on Georges Bank. In the Grand Manan Channel, an extreme example, 

 the water is kept practically uniform in temperature from surface to bottom, even 

 in midsummer. 



Planktonic diatoms, with their silicious frustules and without power of locomo- 

 tion, tend to sink unless kept afloat mechanically by some movement of the water. 

 Although sinking is more or less hindered by their spines, slime threads, disclike 

 outlines, etc., 9 - they are more liable to sink than other members of the phytoplankton 

 are, as Gran (1915, p. 136) has emphasized. 



The mechanical influence of the state of circulation of the water on the flotation 

 of diatoms or on small objects of any sort is obvious. Indeed, particles as heavy as 

 sand may be kept in suspension by active vertical currents, as just remarked; and 

 from what has just been said it is evident that diatoms are more apt to remain in 

 suspension in the coastal waters of the Gulf of Maine from midwinter on through 

 spring, when the water is actively mixing, than in summer. The flotation of diatoms 

 or of any of the unicellular planktonic organisms is likewise made more easy in winter 

 and early spring than in summer by the more viscous condition of the water during 

 the cold season. The importance of viscosity in this respect, first appreciated by 

 Ostwald (1903), is now so generally recognized (Steuer, 1910; Gran, 1912; Murray 

 and Hjort, 1912) that no general discussion of it is called for here. 93 It is in waters 

 such as those of the Gulf of Maine, where a cold winter alternates with a warm 

 summer in the sea as well as in the air, that seasonal differences in this respect are 

 greatest, because the viscosity of the water depends almost wholly on its temperature 

 within the range of salinities there obtaining (say 27 to 34 per mille) . The following 

 table is compiled, in a slightly modified form, from Krummel (1907, p. 282), Murray 

 and Hjort (1912, p. 690), and Murray (1913, p. 102). 



Viscosity for sea water of SO to 33 per mille salinity, 100 being that of distilled water at 0° temperature 



" For a summary of these arrangements for flotation see Steuer, 1910, p. 193. 



••Asa homely and extreme illustration of the effect of differences in viscosity in fluids familiar to every biologist, consider how 

 much more rapidly a round cover glass, resting on its flat surface (which we may conceive as representing a Coscinodiscus), will 

 sink in water than in ordinary sylol-balsam, fluids hardly differing in specific gravity but of which the latter is much the more 



