45 



November the species does not again appear in the records during 

 the year. In 1898 there is an unusual midwinter pulse on January 

 11 of 146,280, followed by a decline and irregularities due to the ris- 

 ing winter flood (Pt. I., PI. XII.). At the middle of March a rapid 

 increase ensues, culminating April 26 at 891, 648,000 ^on-the declin- 

 ing spring flood. A decline to 197,683,200 is found at the close of a 

 week, and it is accelerated by the secondary spring flood, which 

 attains the overflow stage of 15 feet in the closing days of May 

 (Pt. I., PI. XII.). With the decline of this flood in June a second 

 pulse appears, increasing from 15,080 on May 26 to 336,194,880 on 

 June 14, and at the end of three weeks the species practically dis- 

 appears from the plankton. A few scattered entries appear during 

 the summer and fall, and a minor pulse of 10,500 appears on Decem- 

 ber 20, followed by a decline in the next month. 



This species in our waters exhibits a well-defined vernal pulse 

 towards the end of April at about 60, but no autumnal pulse 

 appears when this temperature recurs. , There is a slight indica- 

 tion of a minor midwinter pulse at the minimum temperatures 

 of the year. This occurrence of a midwinter pulse was noted 

 by Whipple and Jackson ('99) in the reservoirs of the Brooklyn 

 water- works, and in the same paper its seasonal distribution in 

 Fresh Pond, Lake Cochituate, and Wenham Lake, Massachusetts, 

 is given for the years 1890-97, in the majority of which a mid- 

 winter pulse commensurate in magnitude with the vernal pulse is to 

 be found. Autumnal pulses are of infrequent occurrence, the vernal 

 pulse being the most frequent but not constant. In European 

 waters no such long-continued examination of the seasonal distribu- 

 tion of this organism has as yet been reported. Apstein ('96) finds 

 two pulses per year in Ploner See in May and the last of July ; and 

 two in Dobersdorfer See, one in April and one in October, separated 

 by midsummer and midwinter minima. Lauterborn ('93) finds that 

 this species in the " Altwasser" of the Rhine attains its maximum in 

 June and again increases in October. In the backwaters of the Elbe, 

 Schorler ('00) reports Astenonella as abundant in April, June, July, 

 and October, but refers the organisms to the preceding species. The 

 existence of the vernal pulse only in our waters is thus somewhat 

 unique, and the cause of the phenomenon probably lies in some 

 environmental conditions, perhaps in our peculiar bacterial and 

 sewage contamination of the autumn. Our vernal pulses appear on 



