296 



imately two months' duration, reveals an almost complete ex- 

 termination of the plankton, the amount given in the table,. 01 

 cm.^ being only an expression for an amount beyond the reach 

 of our methods of measurement. As shown in Plates VIII. 

 and IX., there was prolonged and quite stable low water from 

 Oct. 15, 1894, till the flood of the last week of February of the 

 following year which carried away the ice. The concentration 

 of sewage under such conditions was shown by the stench of 

 the water, by the departure of fish into tributary backwaters, 

 and by the death of many not escaping. Unfortunately no 

 chemical analyses of river water at this season are now availa- 

 ble, and the chemical conditions can only be inferred from 

 those in later years at times of briefer ice blockade, higher 

 water, and presumably less contamination. For example, in 

 January, 1898, following the low water of 1897, we find under 

 ice of three weeks' duration (PI. XL, XII., and XLIV., XLV.) 

 great excess of free ammonia and chlorine, and high albuminoid 

 ammonia, organic nitrogen, and oxygen consumed — all, in- 

 dices of contamination. The ice sheet upon a contaminated 

 stream must also profoundly affect the equilibrium of oxygen 

 and carbon dioxide dissolved in the water, and thus directly 

 influence the life of all constituents of the plankton. It is there- 

 fore not surprising that these unusual conditions should ex- 

 terminate all but the most resistant members of the plankton. 

 The catch consisted almost entirely of flocculent debris (zo- 

 ogloese?) with a few minute filaments of bluish green alga of 

 uncertain affinities, while the usual plankton was represented 

 by only 43 individuals, representing 14 + species, as follows. 



Protozoa : 



Diffugia sp. (deformed?) 1 



EpistyJis sp., heads 3 



Carchesium lachmanni, head 1 



Ciliate, indeterminate 1 



Heliozoan 1 



