PLANKTON OF THE GULF OF MAINE 39 



The most striking event in the seasonal cycle of the zooplankton of the Gulf 

 of Maine (if a negative one) is that a very decided decrease, amounting on occasion 

 almost to complete disappearance of the pelagic fauna, takes place early in spring 

 over the whole area of the gulf, coincident with the tremendous vernal flowering of 

 diatoms (p. 3S5), an event the precise date of which varies locally and from year to 

 year. The quantitative aspect of this change is discussed elsewhere (p. 82), but it 

 also exerts an adventitious influence on the qualitative composition of the plankton, 

 for with all its members sharing in the impoverishment, the rare as well as the com- 

 mon, the less abundant forms practically disappear and the scanty catches become 

 extremely monotonous. 



We first observed this impoverishment in Massachusetts Bay during the late winter 

 and early spring of 1913, when the zooplankton fell to so low an ebb, quantitatively, as 

 the water began to warm from its winter minimum, that the total volume of the 

 catch of a net about 1.2 meters in diameter, towed for half an hour at 40-0 meters on 

 March 4, was only about 15 cubic centimeters. In this catch an occasional Pseudo- 

 calanus elongatus, 12 Sagitta elegans, 9 Tomopteris catharina, an odd Euthemisto, 

 and some haddock eggs were the only variants detected among the Calanus finrnar- 

 chicus, of which the general mass consisted. On April 3, following, the net yielded 

 only a few dozen copepods,one Euthemisto, and two Clione, with a few unrecognizable 

 siphonophore bells and Balanus nauplii; while the catch of planktonic animals 

 made on April 14 was no more varied (a few Calanus, one Tomopteris, one 5. elegans, 

 one Beroe, one young Staurophora, and a few Balanus nauplii), whereas the water 

 was thick with diatoms on both these occasions. 



Subsequent experience during the spring of 1920 has shown that this vernal 

 impoverishment of the zooplankton, which takes place to a greater or less degree 

 in the upper strata of water over the entire area of the gulf, is especially characteris- 

 tic of the coastal belt aDd of Georges Bank, where it culminates in March. It in- 

 volves no qualitative alteration in the plankton, however, for the spring community, 

 sparse though it be near land, is of essentially the same type as the more abundant 

 pelagic population of midsummer, with the same groups and species (notably Calanus 

 finrnarchicus) predominant. Practically all the common oceanic animals of mid- 

 summer except Sagitta serratodentata, which is a seasonal immigrant (p. 320), may be 

 found represented in late winter and spring, if a sufficient mass of plankton be ex- 

 amined from any given locality in the gulf, though many are so rare then that the 

 net is more apt to miss than to catch them. Winter adds few extralimital visitors 

 to the local pelagic fauna, never (in our experience) enough to give a distinctive 

 aspect to the plankton. 



The essential qualitative unity between the zooplankton of summer and that of 

 spring may be illustrated by the horizontal hauls off Cape Elizabeth on March 4, 

 1920 (station 20059), which yielded Calanus finrnarchicus (dominant), Sagitta elegans, 

 Thysanoessainermis, Th. raschii, haddock and plaice eggs, Pleurobracbia, and Tomop- 

 teris catharina, although the water was then so barren that the vertical net caught 

 nothing at all (p. 82). The typical boreal fauna was still more fully represented 

 on the same day off Penobscot Baj^ (station 20057), although the plankton was hardly 

 denser there numerically, viz, by C. finrnarchicus (dominant), Pseudocalanus, 



