BIGELOW: EXPLORATIONS OF THE COAST WATERS. 321 



This table shows that local differences were considerable from year 

 to year, both in volume and in number of copepods; for example, the 

 latter were three times as numerous in the Western Basin in 1914 as 

 in 1913, while at other localities the reverse was true, the counts 

 made off Lurcher's Shoal in 1913 being four times as large as those of 

 1914. As a whole, copepods were more numerous in the western half 

 of the Gulf (Stations 10250, 10253, 10254, 10255) in 1914, in the 

 eastern half in 1913. Plankton volumes were greatest in 1913, at all 

 but two stations. But the fact that the averages, both of copepods 

 and plankton volumes, differ but little, suggests that these local 

 differences do not indicate any general change in the amount of 

 plankton present in the Gulf from 1913 to 1915; though this is appar- 

 ently an increase from 1912 (1915). 



Microplankton. 



The records are of interest chiefly as illustrating the general char- 

 acters of the microplankton over the whole breadth of the continental 

 shelf east of Cape Cod, instead of for the Gulf of Maine only, as in 

 previous years (1914a, 1915), and for the information they afford as 

 to seasonal variations in the Gulf. 



During the summer of 1914, diatom plankton was encountered at 

 three widely separated localities (Fig. 97) viz., off Marthas Vineyard 

 (Station 10258); on the northern edge of Georges Bank (Station 

 10224); and near land off Penobscot Bay (Station 10250). These 

 hauls were as far apart in composition as they were geographically, 

 the former consisting of Rhizosolenia (chiefly R. hchetata semispina) ^; 

 the Georges Bank (Station 10224) haul chiefly of Guinardia flaccida, 

 with smaller numbers of Rhizosolenia; while the microplankton oft' 

 Penobscot Bay was mainly Chaetoceras, with smaller numbers of 

 Rhizosolenia and Thalassiosira. At all these stations there were a 

 few Ceratium among the diatoms. 



Mixed plankton (diatom and Ceratium) occupied the outer part of 

 the shelf south of Marthas \'ineyard and the southern edge of Georges 

 Bank, and the coastal zone in the northeastern corner of the Gulf, 

 both off Maine and Nova Scotia, the three diatom swarms thus 

 influencing the plankton over considerable areas (Fig. 97). 



At Stations 10253 and 10254, off Cape Ann, the plankton consisted 

 chiefly of an acantharian radiolarian, Acanthometron, and it reap- 



' For an account of the marine diatoms of northern waters, see Gran, 1908. 



