Aiiicrirdii FisJicries Society 



227 



t« in|H'r;iliiiv. l)riic;illi whicli lies the cold watci'. consist iii.u- of a 

 ti-ansitio)i layer — the Ihcrinoclinc — in which the tcni|)('ratiii-c is 

 I'apidl.v rallin,^-, and hclow this the mass of the cold water. whoM' 

 ti'in|)eratni'e oi-(linai'ily falls rather slowly with the depth until 

 the hottoni of th.e lake is i-eaclied. 'Idle thickness of the upper 

 lliyei- \ai-ies with the size (d' the lake, troiii ten oi- tw(d\e feet 

 to thirty oi- I'ortv feei. It is pi'esent as a definite and permanent 

 layei- at a date xai'yini;' with the area of the lake from late April 

 to the middle of duly. It iiicivases in thickness after the cool- 

 in,u of the lake lie,uins hut does not chau,i;-e mu(di hef<iiv that 



This upper layei' is suhject to the dii'ect action of the wiiul ; 

 is kept in circulation, and may he saturated with oxyucu, or 

 nearly so: luit the only new supply of oxyu'eu which the lower 

 watei' can .u'aiu must come to it indiri'ctly fi'oui the ui)])er stra- 

 tum. This condition of pei'inaneii! sti'ati Ileal ion of the water 

 comes on at tlie time when the life oi' the lake and its conse(|Ueiit 

 uee(l foi- o.\y<i-cn ai'e risini^- to the maximum, with the increasin>z' 

 warmth of stimmei' and tlu' de\-elopment of life. Tlu' von- 

 sumption of oxy^'en for the pui'poses of decomposition is also 

 at a maximum. The se])ai'ation of the \i)\vvr water fi-om the 

 atmosphei'c in summer hy a thick layei' )f wai'iu watei' is there- 

 fore a much moi'e serious tli'n,u' than the separation of the water 

 from the aii' in winter hy ice. !ii winti'r the demand for oxyii'en 

 is at a luinimum and the slock coutaiiKil in the water is at a 

 maximum. In summer lioth of these conditions are exactly vc- 

 \-('i'sed. It i^ therefoi'e necessai-y for us to in(|uii'e as to the 

 means wdiich the lake has foi' ahsorhin^u' oxyucii from the aii' and 

 its iiK'ans of t raiisportin,u' the ,u'as fi'om the surface to the i)lace 

 wdiei'e it is to hi' used, and to note the elhciency of these pro- 

 cesses as com])ai-ed with thi' call for ox\\ueii in the summer life of 

 the lake. 



The ahsorption and disti'ihut ion of oxy.uen constitutes one of 

 the fundamental pi'ohlenis of life f<u- any lai'.a'e and active or- 

 pmisui. The dillicultv of solving- the pnihh in is increjised hy the 

 fact that no lai\u'e resei've stock of oxy,i:en can he maintained. 

 In the case of a human heinu' tlu're may he a food sui)ply in the 

 tissues sutticient to sustain life for we(d<s, e\-en thonaii no new 

 sui)i)l\- is takt'U in. There is water (Miouu-h in the hodv to main- 



