PEOGRESS IN BIOLOGICAL INQUIRIES^ 1931 523 



Erosion silt in suspension. — ^A special apparatus making use of 

 photoelectric cells was devised for the measurement of the amount of 

 material in suspension in the various waters studied. By means of 

 this instrument readings could be made both rapidly and very accu- 

 rately, so that a large amount of data of this sort was assembled 

 during the past year. These studies show that the erosion silt in 

 suspension materially effects the light transmission into the river 

 water, thus materially changing the river as a habitat for certain 

 microorganisms and other animals even before the silt has settled to 

 the bottom. The elimination of light by the silt suspension amounts 

 to over 90 per cent in the first 25 millimeters of water, and silt sus- 

 pension was found to be selective against certain parts of the spec- 

 trum. The greatest penetration through the silt suspension was 

 made by the orange-red rays, although only a small amount of this 

 light was transmitted. The alteration of light penetration by the 

 suspended silt in river water, particularly in the larger bodies of 

 water which are impounded behind the power dams on the Missis- 

 sippi, produced changes in the plankton, thus altering the basic unit 

 in the food chain of the aquatic fauna. Sudden and abrupt changes 

 in the plankton count followed sudden rises in the amount of sus- 

 pended matter present in these waters. 



The erosion silt in the water also produces definite changes in the 

 heat conduction and heat radiation of the water; and since the dis- 

 solved oxygen varies in inverse direction with the temperature, these 

 changes in the heating and cooling of the water attendant on the 

 presence of erosion silt have important bearing on the respiration of 

 the river and therefore on the available oxygen for the animals living 

 therein. 



Erosion silt and oxygen demand. — Because of the colloidal nature 

 of erosion silt, it carries down with it particles of organic material 

 in various stages of decomposition. Decomposing carcasses of ani- 

 mals and masses of rotting vegetation are also readily covered by 

 the silt deposits and are thus cut off from free access with the water 

 of the stream. Under these conditions the rate and character of the 

 decomposition of the organic material carried down by the silt or 

 buried under it is altered and enormous quantities of gas produced 

 in some places. This condition creates a high oxygen demand in the 

 top layers of the deposited silt, resulting in complete change of the 

 bottom fauna, reducing it to those few forms capable of withstand- 

 ing conditions of low oxygen and water polluted by products of 

 decomposition. This oxygen demand is not only evident in the 

 quieter portions of the rivers but also shows in the main streams 

 themseh'Cs. At the junction of the Ohio and Mississippi Kivers 

 below Birds Point, for example, the oxygen demand of the Missis- 

 sippi River water loaded with polluted silt from the St. Louis dis- 

 trict was sufficient to reduce the entire volume of Ohio River water, 

 which had a much higher oxygen content than the M^sissippi, to the 

 Mississippi River level in less than 12 miles. 



Erosion silt and bacteria. — Comparative bacteriological studies on 

 the erosion silt and organic deposits at the bottom of the river as 

 compared with bacterial counts in the main stream itself show 

 definite differences in the character of the bacterial flora in the silt 

 layers, and in many places an excessively high bacterial count, due 



