333 



Terminology with Reference to Degree of 

 Pollution and Tolerance 



The terms used in the present paper to describe the varying de- 

 grees of tolerance exhibited by the small bottom animals do not differ 

 essentially from those used by Forbes and Richardson, 1913: i. e., 

 septic, pollutional, contaminate, and clean-ivater, in order of decreas- 

 ing pollution. For the purpose of greater flexibility and at the same 

 time better suitability of application to the animals themselves, I have 

 substituted the word tolerant, with qualifying adjectives, for the word 

 contaminate as used in the 1913 paper. After the comparatively few 

 cases of species in the present bottom fauna with authentic previous 

 records of pollutional or unusually tolerant habit*, the tests of tolerance 

 used have been (1) survival under conditions of much lower than 

 normal oxygen supply; (2) association with other authentic pollutional 

 or unusually tolerant species; or (3) survival in areas where species 

 of known cleaner preferences have wholly succumbed. It can not be 

 too strongly urged that at the best we do not find in the actual field 

 habitats the hard and fast lines perhaps too easily suggested by the 

 use of these and other group terms by authors. The great flexibility 

 called for in classifications of the kind is particularly well illustrated 

 in the recent species lists from the bottom muds of Peoria Lake, which 

 may be said to have been neither septic at the worst nor clean at the 

 best, either in 1920 or 1922; by the occurrence there side by side in 

 both years of tubificid worms and midge larvae characteristic of the 

 septic zones of the upper Illinois, and of a number of species of sphae- 

 riid snails thought of usually heretofore as of clean-water habit, though 

 now shown to have more than the ordinary amount of tolerance. 



The approximate position of the various subdivisions of Peoria 

 Lake in the zonal system above recognized may be outlined as follows, 

 basing conclusions on the chemical, plankton, and bottom-fauna data 

 of 1920 and 1922. 



/. Pollutional Zone: — Chillicothc to lozver End 

 of Upper Peoria Lake. Distance 7.5 miles 



The dissolved oxygen in midsummer both in 1920 and 1922 ranged 

 here normally between 0.5 and under 3 p. p. m. Though below the 

 septic zone, in the most proper sense, this is still the zone of vast num- 



• Includes Tubifcx tubifcx ; CMronomua pluvwaus, and a very few others. 



