PLANKTON OF THE GULF OF MAINE 87 



Scotia, but in 1914 the neighborhood of Lurcher Shoal proved far less productive 

 than the deeper basin near by. 



Were all parts of the gulf equally favorable for the existence and multiplication 

 of animal plankton, the catches of the vertical hauls might be expected to vary in 

 direct ratio to the depth — that is, to the amount of water filtered by the net — and, 

 speaking broadly, there usually is more plankton below any given unit of the sea's 

 surface in moderately deep water (say 50 meters or more) than in very shoal water. 

 Notwithstanding the comparative barrenness of the greater part of the coastal zone, 

 however, the regional differences in the abundance of plankton in the Gulf of Maine 

 do not correspond closely to the depth; nor can they be correlated with the distance 

 from the coast, per se, because we have repeatedly found the plankton very plentiful 

 in moderate depths both near land, as in Massachusetts Bay, and close in to Cape 

 Sable, and as far offshore as Georges and Browns Banks, while, on the other hand, 

 some of our deep hauls have proved unproductive in spite of the considerable length 

 of the column of water fished through. Such, for example, was the case in the Eastern 

 Channel and the neighboring part of the basin in July, 1914. In fact, the vertical 

 hauls made in the southeastern deep of the gulf in summer (July 23, 1914, station 

 10225, and June 25, 1915, station 10298), have both proved extremely barren, with 

 only 30 to 70 cubic centimeters per square meter in spite of the considerable depths of 

 the hauls (175 to 260 meters), showing that both in June of 1915 and July of 1914 the 

 rich zone was bounded on the east by much less prolific waters. It is on the strength 

 of these hauls that I have laid down the demarcation between the two zones on the 

 accompanying chart (fig. 38), but the volume of plankton present in the water varies 

 so widely from season to season and from year to year that the lines must not be 

 drawn too finely in plotting its regional variations, and the future alone can show 

 whether it is regularly characteristic of the summer season for such a barren wedge 

 to separate the rich waters to the north from the equally prolific shallows of Georges 

 and Browns Banks. 



The presence of more than 200 times as much animal plankton beneath each 

 square meter of the surface of the sea at the mouth of Massachusetts Bay on July 20, 

 1916, as in water nearly twice as deep in the Grand Manan Channel on August 19, 

 1912 (only a trace), and the fact that there were 200 cubic centimeters per square 

 meter in 85 meters of water on the northeastern edge of Georges Bank on July 24, 

 1914, but only 50 cubic centimeters per square meter that same day in the Eastern 

 Channel, 15 miles distant, where the depth was 220 meters, illustrate the contrast 

 between productive and barren waters. 



Vertical hauls in the Massachusetts Bay region, the only part of the gulf where 

 our data warrant even a tentative account of the quantitative, fluctuations that take 

 place during late summer and autumn, suggest a diminution in the volume of zoo- 

 plankton during the late summer followed by an autumnal increase, which was so 

 considerable in 1915 that there was over twice as much plankton per square meter 

 in water only 80 meters deep by the end of October as we had found at a neighboring 

 station in 140 meters depth two months previous. 



