224 



Hydrographic changes affect the Cladocera by increasing the 

 amount of silt and flocculent debris in suspension, which, by ad- 

 herence to the swimming antennas and flotation processes of the 

 animal, tend to impede its movements and sink it to the bottom, 

 where it is removed from its normal feeding area and readily becomes 

 the prey of the larger organisms of the bottom fauna. Barren flood 

 waters also tend to displace and wash away in the increased current 

 the Cladocera which have developed in the stream, and to afford 

 both less food and less time for their further development. 



The occurrences of the total Cladocera fall into the type of 

 recurrent pulses, though with slightly less distinctness than in the 

 case of individual species of the group. Such pulses can be traced 

 in all seasons in which records were made at short intervals, and 

 suggestions of their occurrence appear in the less frequent records 

 of other seasons. Thus in July-December, 1897, (PI. IV.), there 

 are 6 well-defined pulses culminating at intervals of 3(1), 4(2), 5(1), 

 and 6(1) weeks. In 1898 (Table I.) the pulses are less regular in 

 the flood waters of the disturbed year. In 1896, when records were 

 frequent, we can trace pulses in March, May, June, July, August, and 

 September. The character of these pulses is well illustrated in the 

 vernal pulse of 1898 (Table I. and PI. IV.), culminating June 7 at 

 136,000. The species which share in this pulse are Alona affinis, 

 A. costata, A. quadrangularis , Bosmina longirostris*, Ceriodaphnia 

 scitula*, Chydorus sphcericus* , Daphnia hyalina*, D. cucullata*, 

 Diaphanosoma brachyurum, Leptodora hyalina, Macrothrix laticornis, 

 Moina micrura, Pleuroxus denticulate s, Scapholeberis mucronata, and 

 Simocephalus serrulatus. Of these, only the five marked by the 

 asterisk occur in numbers sufficient by our methods to delineate a 

 pulse. The other species are accordingly of little consequence in 

 modifying the form or location of the pulse. The June volumetric 

 pulse (Part I., PI. XII.) culminates June 14 at 6.99 cm. 3 per m. 3 , 

 though the record for June 7 is also high (5.28). The cladoceran 

 pulse culminates June 7 at 136,000. On this same day four of the 

 dominant species also reach their culmination, viz. : Bosmina 

 longirostris (62,800), Ceriodaphnia scitula (55,800), Daphnia cucul- 

 lata (3,400), and D. hyalina (11,600), the remaining 2,400 being 

 contributed by other species. Chydorus spharicus, which appears 

 this spring only in small numbers, attains its maximum (7,880) on 

 May 24, two weeks earlier, though the record for May 31 is also high 



