PLANKTON OF THE GULF OF MAINE 483 
form (that is, their own shells) that the water becomes unable to support their 
active multiplication. 
It is obvious that the water of the coastal zone north of Cape Ann, along the 
coast of Maine, and in the Bay of Fundy must continue fertile for diatoms until 
much later in the year, as is proven by the rich flowerings which take place there 
late in the spring and in early summer (p. 396). On theoretic grounds this regional 
difference may have any or all of several causes. First, and probably most important, 
is the discharge from the rivers, richer in nitrogen, phosphorus, and silica than 
the sea water with which it mixes. The importance of river waters as carriers of 
dissolved nutrients is so great that the regions immediately off river mouths might 
be expected to be richest in diatoms. Though this is not strictly the case in the 
Gulf of Maine, fuller knowledge may show a closer correspondence between the 
outpourings from the rivers and the vernal diatom flowerings than is now apparent. 
Certain facts point in this direction, especially the general parallelism between the 
season of spring freshets and melting snow, on the one hand, and the date of appear- 
ance of the diatom flowerings off different parts of the coast, on the other. Thus, 
generally speaking, it is off the mouths of the most southerly group of large rivers — 
Merrimac, Piscataquis, and Saco — between Cape Elizabeth and Cape Ann, where 
the flood waters from the land are felt earliest in the spring, that the diatoms flower 
earliest in great numbers. There is no important influx of river water into the gulf 
south of this, and the expansion of the diatom flowerings around Cape Ann into 
Massachusetts Bay corresponds roughly with the probable expansion of the "spring 
current” of land water to the southward past the cape. 
The large rivers east of Cape Elizabeth— Kennebec, Penobscot, Machias, St. 
Croix, and St. John’s — -come into flood later in the season; correspondingly, the 
augmentation of diatoms commences later in the season along this part of the coast 
than farther west and south. 
As the outflow from the rivers diminishes in late spring and summer, the sea 
water might be expected to remain richer in silica, phosphorus, and nitrogen near 
their mouths than elsewhere — i. e., close along the stretch of coast between Cape 
Elizabeth and Nova Scotia, which includes all the localities where we have actually 
found notably rich diatom flowerings in summer (p. 392) . In line with this is the fact 
that Fritz (1921a) did not find it necessary to include silica among the nutrients 
which she added to sea water at the mouth of the St. Croix River in order to obtain 
abundant growth of several genera of diatoms there. 
The viscosity is likewise more favorable for the growth of planktonic diatoms in 
the northeastern part of the gulf than in the southwestern in summer, in inverse 
ratio to the local differences in temperature, the Bay of Fundy at 10 to 11°, for 
example, offering a much more favorable medium for the flotation of diatoms than 
Massachusetts Bay at 16 to 18° in the proportions given in the viscosity table (p. 481). 
A similar regional difference exists, with respect to the vertical circulation of the 
water, during the warm months of the year, this being least active in the southwestern 
part of the gulf where the tidal currents are weakest, and most active east of Mount 
Desert, to culminate in complete and constant stirring of the water from surface to 
