Huff : RESPONSE OF MICKO-ORGANISMS TO COPPER SULPHATE 415 



Stagnant, and organisms in large numbers were never found below 

 a depth of twenty or twenty-five feet. 



The great increase of one species of diatom, Stcphanodiscus 

 niagarae, during the period of circulation, was not entirely unex- 

 pected. On the contrary, it seems a little strange that some of the 

 other diatoms so common during the summer did not show a sim- 

 ilar increase. It is generally known that diatoms increase during 

 the period of circulation of water that follows a period of stagna- 

 tion. Definite reasons for this phenomenon can not be given until 

 we know the amounts of various materials, organic and inorganic, 

 they require for food, as well as the conditions of temperature, 

 light, and air most favorable to their growth and reproduction. It 

 is known that some species of diatom are saprophytic,^ and it is 

 probable that many of them are able to utilize albuminous or similar 

 nitrogenous compounds from the water in which they live. Dr. 

 Maximilian Marsson" is authority for the statement that they can 

 absorb carbon compounds from dissolved organic matter, also or- 

 ganic nitrogen, and when carbonic acid is excluded from the water 

 in which they live, they can digest diluted volatile fatty acids, amido- 

 acids, urea, peptone, and other substances. Karsten^ found that 

 certain diatoms not normally saprophytic could be made so by the 

 proper cultivation in nutrient media. With this information on the 

 food habits of diatoms, it is not difficult to see why the nutritive 

 conditions in a lake like Vadnais are unusually favorable for these 

 forms during the circulation period following several months of 

 stagnation. The following theory for the explanation of the occur- 

 rence of maximum growths of diatoms during the circulation period 

 following stagnation in deep lakes is offered by Whipple^ and is 

 well worth our attention here : With the decay of organic matter 

 in the bottom, during the stagnation period, the water near the bot- 

 tom undergoes great changes. Oxygen is exhausted, ammonia' and 

 other compounds, both organic and inorganic, increase and are dis- 

 solved in the water. With the upward currents during circulation, 

 not only is this water, rich in food materials, carried upward where 

 conditions for diatom growth are more favorable; but the spores, 

 and the diatoms themselves, — which in their inactive condition, at 

 least, are heavier than water and have lain dormant in the mud of 

 the bottom, where light is too weak, and other conditions are un- 

 favorable for growth, — are also carried upward where light and air 



