482 BULLETIN OF THE BUREAU OF FISHERIES 



With the temperature of the upper strata in the eoastal waters of the Gulf only 

 about 1 to 0.5° at its annual minimum when the vernal flowerings of diatoms com- 

 mence, but rising to upwards of 18° off Massachusetts Bay and even to 20° locally in 

 the center of the gulf in August, the viscosity decreases, say, by 40 per cent (from 

 about 100 to about 60) during the spring and early summer. Consequently, other 

 things being equal, a diatom would sink four-fifths faster in midsummer than during 

 the first days of spring. Other tilings are not equal, however, because the specific 

 gravity of the water as well as its viscosity decreases with the rising temperature and 

 with diminishing salinity of spring. Thus, the surface stratum is not only a thinner 

 fluid but a lighter one absolutely in summer than in winter, which makes for a still 

 greater disparity between the tendency of diatoms to sink in the cold and in the warm 

 seasons. 



It would, perhaps, be safe to say that differences in specific gravity of the water 

 and in its viscosity would necessitate twice as active vertical circulation to hold any 

 given object in suspension in summer as in early spring. As we have seen, however, 

 (p. 481), the reverse actually obtains, the active vertical mixing characteristic of spring 

 giving place to a condition of comparative vertical stagnation in midsummer, con- 

 sequent on the increasing vertical stability of the water, which must increasingly hinder 

 the flotation of diatoms in the gulf, just as happens in the fresh-water lakes described 

 by Whipple (1905). Thus the seasonal cycle of viscosity and of vertical circulation 

 combined tends to put a period to the seasonal multiplication of the species of diatoms 

 which are characteristic of spring by increasing their tendency to sink. 



In the preceding pages I have tried to show that on theoretic grounds the 

 gulf, taken as a whole, offers its most favorable environment for planktonic diatoms 

 in spring, because of the following combination of circumstances: The supply of 

 two of the nutrients on which it is probable that diatoms chiefly depend — nitrogen 

 and silica — is then greatest. (European analyses suggest that this also applies to 

 phosphoric acid.) The circulation of the water then tends to bring up a supply of 

 nitrogen compounds and of dissolved phosphates most actively from below, the 

 high viscosity of the water then most favors the flotation of diatoms, and the increas- 

 ing strength of the sunlight from late winter on increasingly favors the processes 

 of photosynthesis. It is probable that for abundant flowerings of diatoms all 

 these requirements must be satisfied. Conversely, fluctuations in the amount of 

 any one of the essential foodstuffs may govern the amounts of diatoms actually 

 present at any given time or place, and may even terminate the flowerings if it fall 

 below the requisite minimum. 



The parallelism that has actually been shown to exist between the fluctuations 

 in the concentration of silica in the sea water of Massachusetts Bay and of the diatoms 

 there (p. 47fi, fig. 134) makes this our most suggestive illustration. Without the 

 accumulation of this substance (which takes place during the winter when there are 

 few diatoms to make use of it) the tremendously productive flowerings which we have 

 encountered in spring probably could not take place, any more than they could 

 unless there were enough nitrogen in available form to nourish them. But after the 

 flowerings have abounded for a few weeks in this particular location they so reduce 

 the sxipply of silica (as the analyses show) by converting it into an unavailable 



