PLANKTON OF THE GULF OF MAINE 37 



nit y as a whole — are not only charactersitic of the waters over the continental shelf, 

 but also of the neighboring parts of the ocean basin, and spread right across the 

 North Atlantic from the Norwegian Sea and Iceland, on the one side, to Newfound- 

 land and Nova Scotia, on the other (Herdman and Scott, 1908; Murray and Hjort, 

 1912). Passing southward from the region of the Grand Banks, however, the band of 

 cool banks water next the coast is a sort of cul-de-sac for them, with the tropical 

 water ("Gulf Stream") limiting their spread on the offshore side as definitely as the 

 coast line does on the inner side. 



The contrast in distribution between the neritic and oceanic elements of the 

 zooplankton of the Gulf, which I have just outlined, prevails throughout the sum- 

 mer, autumn, and winter; and although in spring neritic diatoms, such as Thalas- 

 siosira, appear in swarms over deep water as well as along the shore, when the rivers 

 are in flood and the outpouring of land water is evidenced far out from the coast by 

 lowered salinity, they are decidedly more abundant in the coastal zone than in the 

 basin even at the time of their widest dispersal, a fact discussed below in the general 

 account of the phytoplankton. Neither are larvae of coastwise origin of much more 

 importance in the plankton over the basin in spring (as exemplified by our tow 

 nettings of March, April, and May of the years 1915 and 1920) than in summer. 

 Probably this is because the water has hardly warmed appreciably by freshet season, 

 so that the vernal wave of reproduction has only begun on the part of the littoral and 

 bottom fauna. 



SEASONAL FLUCTUATIONS IN THE PLANKTONIC COMMUNITIES 



Seasonal fluctuations in the plankton are greatest in regions where neritic larvae, 

 or forms dependent on the bottom at some time of year, bulk large in the pelagic com- 

 munity, and in seas where the pelagic fauna or flora is largely recruited from extra- 

 limital sources by ocean currents, which may vary in strength or in origin from month 

 to month. In the Gulf of Maine the presence or absence of the various crustacean 

 larvae, or of fish eggs, may govern the composition of the catch for the particular 

 season close in to the land, as examples of which I may cite the swarming of Balanus 

 cyprids near the Isles of Shoals (p. 44) and of haddock eggs on Georges Bank (p. 44), 

 both in spring. This applies more generally to the North Sea, the Irish Sea, and the 

 Baltic than to the Gulf of Maine, where the communities of planktonic animals are, 

 as a whole, more oceanic ; and since few constant or even regularly seasonal members 

 of the zooplankton of the gulf are immigrants, but nearly all of them are endemic, the 

 seasonal cycle of the plankton is a simpler problem for us than for students of the 

 North Sea region. It can hardly be emphasized too strongly that very few immi- 

 grants, whether from the north, the south, or from the open ocean, penetrate the 

 Gulf of Maine in numbers sufficient to color its plankton community (Sagitta 

 serratodentata is an exception, p. 58), instructive though the regular or sporadic 

 occurrence of animals of exotic origin may be for the light they throw on the sources 

 of its waters. This question is discussed below (p. 51). 



In the case of the pelagic flora, a very pronounced alternation of the prevalent 

 planktonic types does take place from season to season, and one characteristic of 

 northern seas as a whole; viz, a tremendous flowering of diatoms in spring, giving 



