558 



mer rise in August. No trace, however, of the phenomenon of 

 recurrent volumetric pulses-so prominent in our fuller records 

 — appears in his infrequent data. His statement, "Den ganzen 

 Winter hindurch ist hier constant das Planktonvolumen unmess- 

 bar gering" (italics are his) has no counterpart in winter pro- 

 duction in our backwaters. 



The plankton of the Elbe and its backwaters has been in- 

 vestigated by Schorler ('00), but no volumetric determinations 

 were made of its channel plankton because of the difficulties 

 occasioned by the current and by detritus in suspension. The 

 quantity of plankton per m. 3 in three of the backwaters was de- 

 termined in twelve instances in April-October. There are in 

 the data suggestions of a May-June maximum and of one in 

 August, and the quantities are generally higher than the aver- 

 ages in our backwaters, though the greatest amount, 146 cm. 3 

 per m. 3 , falls below the highest of our records. The channel 

 plankton of the Elbe was found generally to contain fewer 

 species and individuals than its contiguous backwaters. 



The plankton of the Oder has been examined by Zimmer 

 and Schroder ('99), but no volumetric determinations were 

 made. Regarding the quantitative conditions, Zimmer makes 

 the general statement that these are influenced by tempera- 

 ture. The quantity in December-February is very small, rises 

 in March, and again, to a greater degree, in May, but attains 

 the maximum for the year in August, declining rapidly after 

 the middle of September, and reaching the winter minimum in 

 December. 



A comparison of the seasonal course of production of 

 plankton in the various lakes that have been explored in re- 

 cent years, with those of our several stations, does not promise 

 any profitable results, since most of the quantitative work is 

 limited in seasons, or has been made at such long intervals as 

 — in the light of our results — to raise some question as to the 

 representative value of such collections for comparison. In 

 addition to this, the very great divergence in the annual plank- 

 tographs of such localities as have been examined, including 



