308 



data, though the statistical data (PL LI.) indicate the recur- 

 rence of three such pulses in this period. 



The volumetric- production is very small throughout this 

 whole period, rising above an estimated amount of .01 cm.^ per 

 m.'^ in only 7 of the 18 catches, and not exceeding .13 in any of 

 them. This results in monthly averages of .01, .02, and .07 re- 

 spectively for the three months ( see table on p. 292). These are 

 lower than those of any other years excepting only that afforded 

 by the single collection of February, 1 895. The cause of this slight 

 production is, I believe, the high water and increased current 

 resulting therefrom, which does not afford to the channel plank- 

 ton the time requisite for breeding a more abundant plankton. 

 Some corroboration of this view may be found in the fact that 

 the February collections in the high water of 1897 (PI. XL) like- 

 wise yielded minute quantities of plankton (average .04 cm. •'per 

 m.''), while the channel waters of 1898 (PI. XIL) in January 

 and the early part of February, and of January-March, 1899 

 (PI. XUL). produced at stages below tliat of overflow (8 ft.) a 

 more abundant plankton — .07 to 1.15 cm.' per m.^ of water. 

 High water wdth accompanying rapid current is thus deleteri- 

 ous to plankton production in channel waters, in midwinter at 

 least. It is noteworthy that this minimum production occurs 

 in the presence of nitrates in great excess, in fact in quantities 

 larger than those recorded at any other period of our records. 

 (cf. PI. XLIII.-XLV.). It is not therefore for lack of nitrates 

 and other products of decay that the plankton fails to develop. 



The data of the collections in the latter part of March in- 

 dicate a rising production as levels fall and temperature rises. 

 The direction of movement is upward, though the quantity at- 

 tained in this month is not great. 



The interval of collections and the quantities of plankton 

 obtained from March to September enable us to trace with some 

 probability the course of the recurrent plankton pulses of this 

 season. 



The April pulse has a duration of 32 days, — from March 30 

 to May 1, — with a maximum amplitude of 9,39 cm.^ on the 24th. 



