PLANKTON OF THE GULF OF MAINE 83 



The few data available suggest that April sees a general augmentation in the 

 amount of animal plankton across the southern half of the gulf from the mouth of 

 Massachusetts Bay to the coastal bank off Cape Sable, including the eastern part of 

 Georges Bank. Over this zone the plankton volumes per square meter averaged 

 about 100 cubic centimeters during the second and third weeks of that month in 

 1920; but north of a line from Cape Cod to Cape Sable, where diatoms were flowering 

 freely (p. 385), our hauls, horizontal as well as vertical, certainly yielded no larger 

 amounts of animal plankton in April than in March and an unmistakable decrease 

 in the amount of zooplankton took place from March to April in the northeastern 

 part of the basin coincident with the local flowering of diatoms. However, the 

 swarms of microscopic plants which are then present make quantitative, measure- 

 ments of the larger forms diflieult or even impossible, both by clogging the meshes 

 and by overshadowing the copepods, etc., in the catches of the tow nets. 



Unfortunately we have not been able to follow the planktonic cycle through the 

 whole of any one spring. But if the May state of 1915 represents the normal sequence 

 to the April state of 1920 (a reasonable working hypothesis unless shown to be false), 

 the zooplankton increases to volumes of 200 to 235 cubic centimeters off Massachusetts 

 Bay and northward toward Cape Elizabeth, on the one hand, and in the eastern basin 

 off German Bank, on the other, during the last half of April and first half of May, 

 as tabulated elsewhere (Bigelow, 1917, p. 312), an increase caused by the tremendous 

 production of copepods which succeeds the vernal flowering of diatoms (p. 41). 

 In fact, it will probably be no exaggeration to set the average volume of zooplankton 

 per square meter by the last of May at 100 or more cubic centimeters for the whole 

 gulf outside the 50-meter contour and north of the Cape Cod-Cape Sable line, 39 with 

 the exception of the coastal zone from Penobscot Bay eastward, where the water 

 still remained extremely barren on May 11 and 12 (volumes of 10 to 20 cubic centi- 

 meters at stations 10275 and 10276). 



Except for this barren zone, where the catches have been so small as hardly to 

 be measurable, the gulf as a whole probably supports a greater mass of animal plank- 

 ton during the last week of May and the first part of June than at any other season, 

 though we have few quantitative records for the latter month. The considerable 

 Dumber of vertical hauls made in July and August during the summers of 1912 to 

 1916 (listed in table on p. 84) make it possible to outline with some confidence 

 the major geographic variations in the amount of zooplankton present in the guff 

 in midsummer. 



During the summer of 1914, which may serve as representative, the animal 

 plankton was most plentiful (volumes of 100 cubic centimeters or more per square 

 meter) in three distinct and separate regions, which I have described elsewhere 

 (Bigelow, 1917, p. 308, fig. 91) — first, over a belt running diagonally across the gulf 

 from the Massachusetts Bay-Cape Cod region to the northeast corner of the basin 

 off the mouth of the Bay of Fundy, as outlined on the accompanying chart (fig. 38) ; 

 second, over the northeast corner of Georges Bank; and, third, from Cape Sable out 

 across the northern channel to Browns Bank, which, on the evidence of the hori- 

 zontal hauls, should include German Bank, because of the Pleurobrachia which wo 



'» We have no quantitative data (or May and June from Georges Bank. 



