PLANKTON OF THE GULF OF MAINE 



91 



On the whole, it is in late winter and early spring, when the physical characters 

 of the sea water are most uniform vertically and when its vertical stability is least, 

 that the zooplankton of the Gulf of Maine and of other boreal seas most nearly 

 approaches vertical uniformity of distribution. At this season, as illustrated by the 

 March cruise of 1920, the volumes of zooplankton present in the water are so small 

 in all parts of the gulf, and the depth of water through which it was distributed at 

 the more productive localities is so considerable, that the volume per cubic meter 

 (by direct calculation) was only 0.7 to 1 cubic centimeter even where the plankton 

 was densest — for instance, in the eastern and northeastern troughs of the basin, in the 

 Eastern Channel, and over the northeastern and southeastern parts of Georges Bank. 

 It ranged down from this to a minimum of practically nothing in the deep water in 

 the southeastern corner of the gulf, the average for all stations being about 0.4 cubic 

 centimeters, which is something less than half the summer average by the lowest 

 possible estimate. Nor is it likely that this calculation seriously understates the 

 density of aggregation of the zooplankton for any large portion of the gulf in March, 

 because there was little evidence of vertical stratification during that month. 



Zooplankton volumes per cubic meter, March, 1920 



With the advance of the spring the concentration of the plankton is augmented 

 both by the increase in the total amount present in the gulf, just remarked, and by 

 its stratification at one level or another. Not only does the first of these factors 

 raise the volume per cubic meter to 2 to 4 cubic centimeters at the very least by 

 midsummer in such prolific though rather shallow regions as the waters off Cape 

 Cod, the neighborhood of Cape Sable, and the eastern part of Georges Bank, 43 but 

 stratification may result in a far denser concentration of the plankton at some 

 particular level while rendering other strata of water far more barren than the 

 ostensible volumes per cubic meter (as derived from the usual calculation) would 

 call for. We have encountered this phenomenon in its most extreme form in the 

 deeper parts of the gulf, but experience has shown that a greater or less tendency on 

 the part of the zooplankton, as a whole, to congregate at some particular level is to 

 be expected anywhere in the gulf in summer, leaving the shoaler as well as the deeper 



* s Plankton volumes per cubic meter, calculated from our summer and autumn hauls, have been published already; those for 

 the year 1913 in Bigelow, 1915, p. 326; for 1914 and 1915 in Bigelow, 1917, pp. 310 and 314; and for 1916 in Bigelow, 1922, p. 136. 



