than the rule. Usually the water is nearly uniform in temperature from sur- 

 face to bottom. During; a period of rising teraperatures and gentle winds a 

 thermocline may be established, to he destroyed in a few days and replaced by 

 another at a higher temperature range. The available evidence indicates that 

 there were two periods of thermal stratification in 1929 and three in 1930. 

 At most of the stations where a thermocline was observed, it was small in 

 vertical extent and in range of temperature. In two of the observed periods 

 of stratification, the thermocline remained established long enough to bring 

 about marked changes in the content of dissolved gases, as at Station 60 on 

 May 30, 1929. and at Station 37A on August 5, 1930. But on the whole, thermal, 

 stratification may be regarded as a minor factor in the aquatic environment 

 of Western Lake Erie. At the east end of the lake stratification persists 

 throughout the summer period. For a discussion of the situation there, the 

 reader may refer to Parmenter (1929) • 



Seasonal Changes in Temperature 



I 

 The data collected during this investigation are incomplete in that ' 

 they do not cover the months November to March, and in that they were taken 

 too infrequently at any one station to show all of the changes which occurred. 

 However, the records of several stations are complete enough to show the prin- 

 cipal changes during the period April to October. 



The mean temperature for each date on which temperatiires were taken 

 at Station 1$8 in 1929 is shown in Table 8 and Fig. 8. l^fhen the first read- 

 ings were taken, on May 30» the mean temperature was 11.6". Between May 20 

 and July 10 the rise was rapid, and the highest mean temperature for this 

 station (22.5") was attained on the latter date. During midsummer the record 

 is somewhat atypical in that it fails to show a period of rising temperature 

 in late July. This was noted on July 30 at Station 8F (Table 9) and at a 

 number of other stations for which data are not presented. A study of all 

 the available data indicates that the maximum temperature of the season 

 occurred in late July, and that the high point of early September was about 

 the same as that of early July. After mid-September the temperature declined 

 rapidly at Station 1^8 and on October 22 it was nearly as low as on May 20. 



At Station 8F (Table 9 and Fig. 9) only one peak, the one of late July, 

 shows on the record. It may be seen that this station lagged behind Station 

 l58 during the warming period of early summer and during the cooling period 

 of early autumn. This more rapid warming and cooling of the water at Station 

 158 than at 8F is readily explained by the fact that the latter station is 

 about three meters deeper than the former, and consequently there is a greater 

 mass of water to be warmed and dooled. A similar difference in rapidity of 

 warming and cooling may be found in comparing Station l5^ with 8F for 1930 

 (Tables 10 and 11). If we compare Statiorrl58 and 37A for 1930 (Fig. 10), 

 we find there was a distinct lag during the warming period at Station 37A, 

 which is about $ meters deeper than Station l58. However, during that part 

 of the cooling period for which data are available, the lag was very slight. 



52 



