BIOTIC STREAM RESPONSES TO STREAM REAERATION 45 



The sphaeriid mussel, Sphaerium rhomboideum, also responds posi- 

 tively to intense conditions of pollution (Graph 9). Although immediately 

 inhibited by trade waste, the rapid population increase downstream resem- 

 bles that of the Oligochaetes. 



The sow-bug, Asellus, is recognized as an indicator of mild or improv- 

 ing conditions. They reach their population peak near the 15 mile level 

 (Graph 10) and are infrequent beyond 25 miles. 



Population peaks of the prominent bottom organisms have the follow- 

 ing linear order: Oligochaeta— 3.2 miles, Sphaeriidae — 7.7 miles, Chironomi-= 

 dae — 11, Asellus — 15, snails — 22, and various insects as a group — ^26 miles. 



Gbaph 11. Relationship of populations and numbers of species. 



The trend curve of total bottom population (Graph 11) is in contrast 

 to station variety expressed as number of species. It will be noted that 

 variety begins to decrease above the zero level. This appears to be due, 

 at least in part, to decreased velocity above Mill Dam and to the presence 

 of bark and chips accumulated on the bottom. Variety is decreased further 

 with the entry of trade wastes with only 3 species found in the distance 

 of 1.8 miles to Lower Dam. Variety increases somewhat below this point. 

 On soft bottom, the pre-pollutional variety is not regained at any down- 

 stream level examined, but on hard bottom variety is normal between 

 the 10 and 15 mile level. 



While there is considerable station-for-station fluctuation in total popu- 

 lation, the trend is for a striking population peak near the 9 mile level. 

 The following progressive decrease apparently results from declining food 

 supply and changing physical conditions. 



