PHYSICAL HYDROGRAPHY 



Compared with average meteorological con- 

 ditions of weather and mean temperature over a per- 

 iod of years, 1929 may be considered an average 

 year with conditions in the lake close to normal. 

 Until the ice leaves, the lake remains cold and homo- 

 thermous. In 1929 the last ice passed down the Nia- 

 gara River on May 2; in 1928 a considerable quantity 

 remained in the vicinity of Buffalo on May 15. As 

 the temperature continued to rise in June the response 

 was quite different in the eastern and middle sections 

 of the lake, the former warming more slowly. This 

 condition was reflected in both the plant and animal 

 plankton in which augmentation occurred progres - 

 sively later to the eastward. The maximum surface 

 temperature was reached in August, although due to 

 mixing by wind action the bottom temperature in- 

 creased through September. 



There was a slight and fairly even vertical tem- 

 perature gradient in May, and during the early sum- 

 mer a thermocline formed in both basins. An area 

 of cold bottom water of less than 12° C. extended 

 over about 45 percent of the total lake in July 1929 

 and covered a large portion of the Central Basin. By 

 August the area in the latter section had been reduced 

 to a relatively small area of about 235 square miles. 

 The depth of the thermocline was found to be in- 

 fluenced by the bottom, tending to follow the con - 

 tour in some localities. 



The thermocline in the Central Basin is short- 

 lived and probably remains for 2 or at most 3 months. 

 In 1929 heavy winds in early September thoroughly 

 mixed the water from surface to bottom. In years 

 of severe and frequent storms it is possible that no 

 thermocline may be found there. The biological im- 

 portance of this cold bottom water is uncertain. Ob- 

 servations on the summer population indicate that 

 both the benthic and the pelagic communities are 

 periodically subjected to such wide ranges of temper- 

 ature that it seems doubtful if this delayed warming 

 of variable duration exerts any very significant in- 

 fluence on summer non -migratory species. Such 

 cold water forms as Limnocalanus probably remain 

 longer in the Central Basin during years when the 

 thermocline persists far into the summer. 



A thermocline in the eastern area persisted 

 throughout the summers of 1928 and 1929. A cold 



water mass, with a temperature from 4° to 6° C. , 

 remained in the basin of the Deep Hole. In 1929 

 6° C. water covered an area of 1, 290 square miles 

 in May, but during the summer contracted to 410 

 square miles in September. A considerable portion 

 of the Deep Hole is thus covered by cold water 

 throughout the year and serves as a favorable habitat 

 for such "arctic relict" species as Triglopsis thomp - 

 soni, Mysis relicta, and Pontoporeia affinis. 



The movements of the cold bottom water in the 

 Deep Hole were first observed in 1928 when the east- 

 ern margin was found to have advanced 26 miles in 

 20 days and 2 weeks later had retreated again to the 

 west. Although sufficient facilities for establishing 

 the cause for this movement were not available dur- 

 ing the first season, when compared with meteoro- 

 logical data ". . . it would appear that the oscillation 

 of the cold bottom layer to the east in each case oc- 

 curred as the water, piled up at the eastern end of the 

 lake by westerly winds, was readjusting itself to the 

 normal lake level. The rise in the level in the east- 

 ern part of the lake was accompanied by a retreat of 

 the bottom layer, possibly due to the depression of the 

 eastern end of the pycnocline and the consequent 

 pressing back of the cold water mass. Thus a close 

 conelation occurred between the wind and the move- 

 ment of the water mass, the advance of the cold mass 

 taking place in the readjustment process after the 

 termination of the wind. " (Fish et a^. 1929, p. 196. ) 



Further investigation in 1929 indicated that when 

 the level at the eastem end of the lake rises over a 

 period of several days, even though the range does not 

 exceed 6 inches, an oscillation of the cold bottom 

 water to the west results. When the lake level becomes 

 normal again the bottom water returns by gravity to 

 its normal position, which in 1929 approximated the 

 40 -meter contour (6° C. water). All evidence in 

 1929 points to wind and not barometric pressure as the 

 principal factor controlling the oscillations. Wind 

 may also cause a north -south oscillation of the bot- 

 tom water but this was not observed as frequently as 

 the east -west movement which may be more cm: less 

 continuous in some years. The importance of this 

 cold bottom water on the fishery in eastern Lake Erie 

 has previously been discussed (Fish et al. 1929). It 

 is well known that schools of those species favoring 

 low temperatures move with the cold water mass, and 

 the fishermen set their nets accordingly, using the wind 

 as an indicator. 



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