PHYSICAL AND CHEMICAL FACTORS IX THE METABOLISM OF LAKES 



13 



jiliases making- in all eijilit or nine subdi- 

 visions. 



In practice, observations are rarely so 

 complete as to show the exact limits of these 

 periods. Moreover, in successive years the 

 cycle may vary rather widely ; for example, 

 Welch (1935) records a variation of two 

 months in the time of stratification in lakes 

 of northern Michip-an. Thus records of 

 both actual and average duration of the 

 periods are desirable and since complete 

 observations over a period of years are 

 available for relatively few lakes, it is the 

 more important that they be recorded and 

 compared. The accompanying- chart (Fig. 

 3), is suggestive of the type of records to 



Hutchinson's (1938) calculation of hypo- 

 limiiial oxygen deficits per day of stagna- 

 tion may be cited as a significant applica- 

 tion of exact data as to seasonal cycles. 



The calculation of the amount of heat 

 acquired and lost by a lake during the year 

 was developed by Birge (1916) into a sys- 

 tem of comparable heat budgets. His an- 

 nual heat budget is the amount of heat 

 needed to warm the lake from its winter 

 minimum to the summer maximum and his 

 summer heat income is the amount to raise 

 it from 4° C to the summer maximum. The 

 latter was also referred to as wind distrib- 

 uted heat, but in view of the observations 

 of Ricker (1937) and others as to the depth 



Jan. Feb. Mar. Apr. Ma/ June Jul/ Au^. Sept. Oct. Nov. Dec. 



W<a5k<25/u L. 

 Saskatchewan 



L.Nipissing 

 Ontario 



L.t^endota 

 Wisconsin 



Paul Lake 

 Br. Columbia 



lc« 



\ce — 



Winfer Stagnation 



Compl&tc ] Partial 



Ice 



Wi'nfer 6fog'nahon Vamaic Summer Sta^njstion^ 



Ice 



Coi-nplate j Varttot 



Summer 5to^. 



Complzte ;P 



Autumn C\rcu\W5tag 



Complete ,ParC 



Corrtplete 



\A/inter Stagnation Vernal Ch-c^ Summer Stagnation 



Complete 



Jan. Feb. Mar Apr. Ma/ June Jul/ Auj. Sz^t. Oct Hov. Bee. 



Fig. 3. Comparison of tlie thermal cycles of four lakes. 



which we refer and includes the available 

 data for AVaskesiu Lake (Rawson 1936) 

 and three other lakes. In each ease the 

 values represent average conditions over 

 a period of at least five years. The periods 

 are those defined by Ricker (1937). 



The duration of the periods of circula- 

 tion and stagnation influences in a pro- 

 found way the whole ecology of the lake 

 and is of great practical importance in our 

 consideration of the metabolic exchanges. 



of direct radiation it might be well to avoid 

 the reference to wind. 



Although the heat budget would seem to 

 be a useful means of comparing thermal 

 conditions in different lakes, it has received 

 comparatively little attention. A recent 

 exception to this is Strom (1938) who finds 

 the summer heat income of special use in 

 differentiating mountain lakes from low- 

 land lakes in the same geographic region. 



According to Strom (1938). the summer 



