28 Nature of Aquatic Environment 



beneath the surface. Such a disc has been found to 

 disappear at very different depths. Witness the fol- 

 lowing typical examples: 



Pacific Ocean 59 meters 



Mediterranean Sea 42 meters 



Lake Tahoe . 33 meters 



"Lake Geneva . 72T meters 



Cayuga Lake 5 meters 



Pure Lake (Denmark), Mar 9 meters 



Pure Lake (Denmark), Aug 5 meters 



Pure Lake (Denmark), Dec J meters 



Spoon River (111.) under ice 3.65 meters 



Spoon River (111.) at flood 013 meters 



It is certain that diffused light penetrates beyond the 

 depth at which Secchi's disc disappears. In Lake 

 Geneva, for example, where the limit of visibility is 

 2im., photographic paper sensitized with silverchloride 

 ceased to be affected by a 24-hour exposure at a depth 

 of about 100 meters or when sensitized with iodobromide 

 of silver, at a depth about twice as great. Below this 

 depth the darkness appears to be absolute. Indeed it 

 is deep darkness for the greater part of this depth, 90 

 meters being set down as the limit of "diffused light." 

 How far down the light is sufficient to be effective in 

 photosynthesis is not known, but studies of the distri- 

 bution in depth of fresh water algae have shown them 

 to be chiefly confined, even in clear lakes, to the upper- 

 most 20 meters of the water. Ward ('95) found 64 

 per cent of the plancton of Lake Michigan in the upper- 

 most two meters of water, and Reighard ('94) found 

 similar conditions in Lake St. Clair. Since the inten- 

 sity of the light decreases rapidly with the increase in 

 depth it is evident that only those plants near the sur- 

 face of the water receive an amount of light comparable 

 with that which exposed land plants receive. Less than 

 this seems to be needed by most free swimming algae, 



