geii in tilt' hyixjlimnioii. Of this luiiubcr, Si species 

 are confined to the littoral and sublittoral zones, 7 

 to the profundal zone, and 10 occur throughout 

 (Miller 1941). 



The bottom mud of outrojihic lakes commonly 

 consists of a thin, ui)])cr, brown, detritus layer of 

 newly de])osited organic matter that has drifted down 

 from above : a relatively thick gray layer containing 

 many fecal pellets and much organic matter, as well 

 as diatoms : and a relatively barren bottom layer. In 

 England's Lake Windemere, 85 per cent of all bot- 

 tom organisms occur 6 meters below the surface, 

 and 100 per cent 12 meters below tlic surface in 

 the upper layers (Humphries 1936). 



Maximum populations of insect larvae in eutro- 

 phic lakes are ordinarily reached during the winter. 

 Minimum populations occur during late spring and 

 summer, both in the littoral and profundal zones, 

 because many immature insects have completed their 

 development and emerged, as adults (F.ggleton 1931, 

 Ball and Hayne 1952). Although relatively few gen- 

 era make up the bottom fauna, populations may at 

 times be enormous. Chaoborus larvae alone have 

 been recorded in populations of 97,000 individuals 

 per square meter, and Tcndipcs larvae at 26,000 indi- 

 viduals per square meter (Deevey 1941). 



re(|uires special adjustments by organisms. Some 

 bacteria are truly anaerobic, an(l perhaps some ani- 

 mals are, too, but most forms simply accumulate an 

 o.xygen debt that is rejiaid when the autumnal over- 

 turn takes place. It is of interest that the annelid 

 worms and those midge fly larvae that tolerate the 

 lowest o.xygen concentrations ])ossess hemoglobin in 

 the blood, the i)igment which has the greatest capacity 

 and efficiency in transporting o.xygen at low tensions. 

 Tubificid worms extrude farther from their tubes 

 and wave their tails more vigorously for a time as 

 the oxygen content becomes reduced. The nightly 

 excursions of Chaoborus larvae into the oxygenated 

 epilimnion is certainly an opportunity for replenish- 

 ment of their oxygen needs. A considerable propor- 

 tion of the larvae migrate out of the profundal zone 

 during the spring, and do not return until autumn 

 or early winter (Wood 1956). The copepods Cyclops 

 bicuspidata. Canthocamptus staphylinoides. and per- 

 haps others, encyst and lie on the bottom during the 

 summer period (Moore 1939), although this action 

 has not definitely been related to any particular en- 

 vironmental factors (Cole 1953). Some midge fly 

 larvae also form cocoons inactive. 



NEKTON 



Summer stagnaliDn period 



The low concentrations or complete disappear- 

 ance of oxygen in the hypolininion of eutrophic lakes 

 for periods of several days or weeks in the summer 



The nekton of lakes consists principally of 

 fish. There is an interesting small shrimp, Mysis 

 relic ta. found in the deeper waters of many northern 

 lakes of North America that is often included with 

 the nekton. This species is believed to be a relic of 



DEPTH IN FEET 

 100 150 200 250 300 350 400 450 500 550 600 . 1500 



'0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 

 DEPTH IN METERS 



10 Variation with depth in abundances of 



lisms in oligotrophic Great Slave Lake (F 



Lakes 73 



