72 Types of Aquatic Environment 



grouped in two classes: first, those that are shallow 

 enough for complete circulation of their water by wind 

 or otherwise at any time ; and second those deep enough 

 to maintain through a part of the summer season a 

 bottom reservoir of still water, undisturbed by waves or 

 currents, and stratified according to temperature and 

 consequent density. In these deeper lakes a thermo- 

 cline appears during midsummer. In the lakes of New 

 York its upper limit is usually reached at about thirty- 

 five feet and it has an average thickness of some fifteen 

 feet. Our lakes of the second class may therefore be 

 said to have a depth greater than fifty feet. 



Lakes of this class may differ much among them- 

 selves according to the relative voltime of this bottom 

 reservoir of quiet water, Lakes Otisco and Skaneateles 

 (see map on page 65) serve well for comparison in this 

 regard, since they are similar in form and situation and 

 occupy parallel basins but a few miles apart. 



*In feet, measured by Secchi's disc. 



tin cc. per liter of water. Alkalinity by phenoltbalein test is indicated 

 by the minus sign. 



The figures given are from midstommer measure- 

 ments by Birge and Juday. At the time these observa- 

 tions were made both lakes were alkaline at the surface, 

 tho still charged with free carbon dioxide at the bottom. 

 Apparently, the greater the body of deep water the 

 greater the reserve of oxygen taken up at the time of 

 the spring circulation and held through the summer 

 season. Deep lakes are as a rule less productive of 

 plancton in summer, even in their surface waters, 

 because their supply of available carbon dioxide runs 

 low. It is consumed by algae and carried to the bottom 



