1898] NOTES AND COMMENTS 297 



Eichter in 1891 that there existed in many lakes, during the sum- 

 mer, a layer of water in which an extremely rapid decrease in 

 temperature took place. For instance, the difference in the tem- 

 perature of the water of a lake from the surface down to perhaps 

 ten metres might only be one or two degrees, whereas in the next 

 metre or two the decrease might amount to ten degrees or more. 

 Below this, again, the temperature wovild decrease comparatively slowly 

 and uniformly to the bottom. The layer showing the great jump in 

 temperature was christened the " Sprungschicht," and its position was 

 found to vary with the season, gradually sinking as the year advanced. 



References to the " Sprungschicht " or Thermocline have been 

 made by many subsequent writers, but Prof. Birge seems to be one 

 of the first to have shown how important the phenomenon may be 

 to the plankton organisms in certain cases. In Lake Mendota, for 

 example, he finds that the formation of the Thermocline is followed 

 by the practical stagnation of the whole mass of the water lying 

 below, and as the latter is obviously shut off from direct exposure 

 to sun and air it soon becomes unfit to support the majority of the 

 inhabitants of the lake. In less than a month, therefore, after the 

 appearance of the Thermocline, i.e. before the middle of July, from 

 90 to 95 per cent, of all the Crustacea present in the lake are 

 found above the 9 -metre level, which represents approximately the 

 middle of the Thermocline at that date. This state of things con- 

 tinues during July, August, and part of September, except that, as 

 the Thermocline moves gradually downwards, the Crustacea have 

 more room in which to develop, and so, by the middle of September, 

 the high percentages mentioned can only be obtained by collecting 

 to a depth of about 12 metres. Two of the effects of this practical 

 exclusion of life from the lower water during the summer are, that 

 the total number of Crustacea in the lake is far smaller than it 

 would otherwise be, and that the perennial species which are un- 

 favourably aflPected by heat, being unable to retire into the cooler 

 water, decline in numbers. Perhaps also the small number of 

 periodic species in the lake may be due to the fact that they have 

 little chance to develop when the habitable water above the Thermo- 

 cline is so completely occupied by the perennial forms. 



The Thermocline is closely connected with another peculiarity 

 of Lake Mendota, and that is the enormous increase of the vegetable 

 plankton in the late summer and autumn, for it appears certain that 

 this is rendered possible by the liberation of the products of decomposi- 

 tion stored in the deeper water, as the Thermocline moves downwards. 



As regards the production of the Thermocline, Prof. Birge be- 

 lieves that, in Lake Mendota at least, it is due to the concurrence of 

 gentle winds and hot weather, and not to the day and night differences 

 in temperature of the surface water, as is stated by Pdchter and others. 



