126 



PROBLEMS OF LAKE BIOLOGY 



of removing- the organisms from the sub- 

 stratum by the use of screens and has re- 

 sulted in the arbitrary separation of the 

 bottom organisms into macroscopic and 

 microscopic faunas. Up to the present 

 time, practically all the work has been con- 

 cerned with the macroscopic forms. Only 

 a few investigations of considerable dura- 

 tion and magnitude which are confined to 

 the microscopic fauna alone are known to 

 the speaker and results from them gener- 

 ally remain to be published. 



The benthic fauna in general, in com- 

 pany with the rest of fresh-water forms, 

 has been aptl,y characterized by Carpenter 

 (1928) as "an immigration fauna." It is 

 possible that the ancestors of some of the 

 present-day insect members of this group 

 may have sprung from swamp inhabiting 

 forms of a long past age but it is commonly 

 agreed that the aquatic adaptation of cer- 

 tain insects today has been secondarily ac- 

 quired. This seems certainly true for the 

 benthic forms. And what is true of the 

 insects in this connection is quite as valid 

 for the other groups. The bottom dwellers 

 of our lakes today trace their ancestry 

 either to terrestrial forms or to early in- 

 vaders from the sea. Carpenter, although 

 agreeing in general to the immigration the- 

 ory mentioned before, suggests the inter- 

 esting possibility of a fresh-water origin for 

 the Protozoa and perhaps for life in 

 general. 



However that may be, it is obvious that 

 any forms which might have invaded fresh 

 waters from the land, secondarily or other- 

 wise, would first have populated the littoral 

 regions. No great experience with the bot- 

 tom animals of our lakes is needed to con- 

 vince any one of the clear relationship be- 

 tween the littoral and the sublittoral fauna. 

 The picture is not quite so clear when one 

 considers the question of the origin of the 

 profundal benthic fauna. Without taking 

 the time here to attempt a defense of the 

 position the speaker confesses to the con- 

 viction that the true profundal benthic 

 fauna is a selection fauna, recruited from 

 the sublittoral and littoral zones and re- 

 duced today to a few hardy and highly 



adaptable species through the medium of 

 many purges at the hand of the seasonal 

 cycles. 



One of the problems, which from the 

 early days of the present century has at- 

 tracted students of this subject, has been 

 that of the distribution of the animals over 

 the lake floor. It was not so many years 

 ago that evidence was first offered showing 

 that the distribution was not uniform from 

 shoreline to abyss, but that it was, instead, 

 clearly marked by modes and peaks into 

 zones. It has been amply demonstrated 

 within the last 10 or 15 years that there is 

 both a qualitative and a quantitative vari- 

 ation of the benthic fauna with depth. 



The enormous diversity of lakes provides 

 exceptions to almost any statement one may 

 make about them but in general it is true 

 that quantitatively the fauna increases 

 with deptli down to an optimum level some- 

 where within the lower littoral or upper 

 sublittoral and then decreases with depth 

 to a minimum within the deepest regions. 

 Some of the exceptions referred to will be 

 considered in a later paragraph. Qualita- 

 tively there is a somewhat similar situation, 

 but the decrease within the profundal is 

 tremendously accentuated in a qualitative 

 sense. As has been stated before, only a 

 few species persist into the lowest levels of 

 the lake floor, but these few kinds may 

 under certain circumstances provide enor- 

 mous populations. 



Probably the most striking characteristic 

 of depth distribution of the benthic fauna 

 is the concentration zone. When the 

 speaker first proposed this term (Eggleton 

 1931) the following statement was made: 

 "... there was a distinct zone in which 

 the individuals of the benthic fauna were 

 considerably more numerous than immedi- 

 ately above or below. This feature of the 

 dejith distribution, which repeatedly oc- 

 curred at certain seasons of the year, will 

 be referred to hereafter (in this paper) as 

 the concentration zone." The data pre- 

 sented in several papers which had ap- 

 peared previously actually show a well 

 developed concentration zone to have been 

 present in the waters investigated, but 



