occurrence of Daphnia was extremely patchy- - 

 temporally and spatially- -but some were pres- 

 ent in every sampling week. Ceriodaphnia , 

 though common in late July, August, and Septem- 

 ber, was scarce or absent the rest of the year. 



The next most abundant species were Sida, 

 Alona , and Chydorus . Sida was present from 

 July through November and scarce or absent 

 the rest of the year. Alona was most abundant 

 in late August, September, and October. 

 Chydorus peaked in mid-June and mid-July. 



The other zooplankton species, with the ex- 

 ception of Monospilus , occurred in summer 

 and fall but were never numerous. Monospilus 

 was present from October through early April 

 and absent during the summer. A complete list 

 of zooplankton collected and the average num- 

 ber per cubic meter for the survey year are 

 given in table 8. 



The numbers of zooplankton tended to in- 

 crease progressively downriver from stations 

 1 to 7 (fig. 14). 



Cyclopoids and Bosmina made up more than 

 75 percent of the population in all months ex- 

 cept November through February; their con- 

 tribution reached a low of 35.6 percent inearly 

 February (table 9). Zooplankton, low in the 

 summer, constituted a larger percentage of 

 the population in winter. Daphnia reached 24 

 percent of the population in early February, 

 whereas in early September it constituted 

 only 1 percent; calanoids went from 2 to 1 5 

 percent, Alona from 0.6 to 10 percent, and 

 Ilyocryptus from 0.1 to 9 percent in the same 

 September to February time interval. Thus, 

 the diversity of the zooplankton community is 

 greater in winter than in spring, summer, or 

 fall. 



Table 9. — Percentage of cyclopoids and Bosmina 

 in samples of zooplankton at stations 1 to 

 7, July 1967 through July 1968 



I 2 3 4 5 6 7 

 STATION 



Figure 14. — Relative abundance of zooplankton at each 

 sampling station, July 1967 through July 1968. 



COMPARISON OF DATA BETWEEN YEARS 



Limnological data collected in 1967-68 are 

 compared with data of 1954-55 from Sylvester 

 (1958) in table 10. In the 1967-68 data, the 

 concentration of dissolved oxygen is lower than 

 in the 1954-55 data (0.7 p. p.m. in Decemberto 

 2.0 p. p.m. in June). That this is not just a factor 

 of temperature is shown by the data on per- 

 centage of oxygen saturation. The river water 



11 



