480 BULLETIN OF THE BUREAU OF FISHERIES 



under further oxidization, compounds favorable to the growth of diatoms result. 

 At the same time the vertical currents bring diatoms up to the surface from the 

 bottom, where they or their spores had previously been resting, prevented from 

 growth by darkness and lack of available food substances. Once near the surface, 

 they multiply rapidly under favorable surroundings, and this multiplication con- 

 tinues either until the available supply of nutrient substances is exhausted or until 

 a cessation of the vertical currents allows them to settle once more below the fertile 

 and illuminated stratum, when they lie over until the next period of vertical circula- 

 tion. 



In the same way the state of stability of the water, joined to the effects of the 

 river freshets, influences the distribution and availability of the food substances on 

 which diatoms depend for their nutrition in coastal seas such as the Gulf of Maine. 

 As Nathansohn (1906) pointed out, wherever there are upwelling currents these 

 may be expected to bring a rich supply of nitrogenous compounds up to the surface 

 from the deeps, where they accumulate from the decomposition of the rain of dead 

 plankton. In the Gulf of Maine local upwellings are a characteristic event along 

 the western and probably along the northern coasts in spring, following offshore 

 winds (Bigelow, 1914a, p. 394). But here and in shoal boreal seas generally the 

 active vertical mixing by tides, by dominant currents, and by winds, which takes 

 place whenever or wherever the water possesses little vertical stability, is no doubt 

 more effective in dispersing accumulations of dissolved nutritive compounds 

 through the upper strata of water than are the more definite upwellings, because 

 more widespread, given an accumulation of organic detritus on the bottom. 



The analyses of nitrogen in samples of mud and sand (p. 472) prove this last 

 requisite fulfilled for the area of the Gulf of Maine as a whole; probably most 

 abundantly so around the coastal zone, where submarine vegetation (Zostera and 

 al^re) and animals (bottom dwellers as well as planktonic) die and decay in vast 

 quantity. Atkins (1923 and 1925) has also emphasized the importance of vertical 

 circulation in making available to the phytoplankton of the upper illuminated layers 

 the dissolved phosphates that accumulate in the deeper strata. As Gran (1912, 

 p. 379) has pointed out, it is in areas where the summer and winter temperatures of 

 the surface differ most that vertical circulation is most active during the brief period 

 (or periods) when vertical stability is lost (a period generally coinciding with the 

 lowest surface temperature), and our first winter's work proved that the Gulf of 

 Maine is a typical example of this. 



The physical aspect of this subject has been touched upon in earlier papers 

 (Bigelow, 1914a and 1917) and will be discussed in the third part of the present 

 report. 90 It will therefore suffice to note here that the whole coastal zone of the Gulf 

 and the water over its offshore banks, down to a depth of at least 100 meters, is in 

 such an active state of vertical mixing at the end of the winter and during the first 

 davs of spring (when the temperature is lowest for the year and just before the river 

 freshets lower the surface salinity appreciably) that it often carries sand in suspen- 

 sion, 91 not to speak of light flocculent material. 



•"Section ? of Part II. Vol. XL, Bullet'n ofihe Bureau of Fisheries. 

 n i 

 20163). 



•"Section ? of Part II. Vol. XL. Bullet nnf the Bureau of Fisheries. 



" In 1920 we had instances of this on Georges Bank in February (station 20047) and on German Bank on Apr. 15 (station 



