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 tha year, as is proven by the rich flowerings which take place thcr.- 

 late in the spring and in early summer (p. 306). 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. 4S1). 

 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 



