334 



bers of Tubificidae, including Tubifex and several species of Limno- 

 drilus ; as well as of Chironomus plimiosiis. Practically the only snails 

 surviving here recently are those that are more than ordinarily tolerant 

 — the list including several species of Sphaeriidae, but only one mem- 

 ber of the family Viviparidae (Cainpeloma subsoUdum) . On the 

 plankton side this is the zone of blue-green algae, these organisms, both 

 filamentous and non-filamentous types, .giving a blackish blue color 

 to the plankton samples, easily visible to the unaided eye immediately 

 after treatment with alcohol-formalin preservative, in the case of col- 

 lections made either in July, August, or September, and as far south 

 as Spring Bay Narrows or even the upper end of the middle lake. 

 Rotifer tardus was common in the plankton in the summer of 1920, 

 while shelled rotifers were rare. 



2. Sub-poUulioiial or Contaminate Zone; or Zone of Tolerant 



Bottom Animals: — Head or Center of Middle Peoria 



Lake to foot of Loiver Peoria Lake 



Distance about 12 miles 



The dissolved oxygen ranged usually from around 2.5 to more 

 than 5 p. p. ni. through the summer season, both 1930 and 1922. The 

 blackish blue color given the upper lake plankton by the blue-greens 

 begins to disappear rapidly at or just below Mossville ; the blue-green 

 organisms thereafter giving way to great increase in chlamydomonads 

 and other chlorophyll-bearing forms. Shelled types of rotifers of the 

 family Brachionidae increase rapidly down the middle lake and Rotifer 

 tardus tapers out almost altogether at Peoria Narrows. In the bottom 

 fauna, sphaeriid snails increase in variety in the middle lake, but the 

 Viviparidae and Amnicolidae remain absent, both there and in the 

 lower lake. In the lower lake, areas or zones of heavy local pollution 

 and of unusually fast current cause mixed conditions ; and there are 

 otherwise in this portion of the river more or less insensible gradations 

 between conditions nearly clean, contaminate, and poUutional. 



1 IvDUOGR.APHICAL SUBDIVISION 



The bottom collections made in the summer of 1922 were all made 

 under substantially average low water conditions. Although we went 

 several times within between 50 and 100 feet from shore with the 

 dredge, few depths under five feet were found even at that distance ; 



