284 



the water, and contributes by its decay to the nutrition of 

 aquatic organisms; rise and fall of temperature are more pro- 

 found here in shoal surface waters; light pervades more com- 

 pletely; and currents are less rapid. It is in many respects a 

 less stable region than the central waters which it bounds, and 

 it may, indeed, be regarded as a separate unit of environment, 

 in contrast with mid-lake or channel waters. 



The effect of the shore-line upon the distribution of the 

 plankton in the lake has not entered into the data referred to 

 in the previous section, for in the investigations of both Ap- 

 stein ('96) and Reighard ('94) aloug-shore collections were not 

 made, and, moreover, the shore-line is less important relatively 

 in the lake as compared with the stream. For example, the 

 absolute development of the shore-line in Lake St. Clair — de- 

 termined by the method of Seligo ('90) (=shore-line divided by 

 square root of area) is given by Reighard ('94) as 9.23. In the 

 Illinois River at high water, from Utica to the mouth it is ap- 

 proximately 17.1, and at low water 78.3, omitting all the con- 

 necting lakes and bayous, computing the area on the basis of 

 the average of the low-water widths given on page 110, and ig- 

 noring sinuosities exceeding that of the channel. The relative 

 development (absolute development divided by absolute devel- 

 opment in a circle in which r~l) in Lake St. Clair is 2.607, 

 in the Illinois River at highest water, 4.83, and at low water, 

 22.1. These figures serve to show in a general way the exceed- 

 ing importance of the shore-line in the environment of the po- 

 tamoplankton. Owing to the great sinuosities of the shore- 

 line as rising waters invade the bottom-land, these figures are 

 probably very much smaller than actual measurement would 

 make them. It is probable that the relative shore develop- 

 ment in the Illinois is ten times that of Lake St. Clair, and hf- 

 teei\ times that of most lakes. 



Added to the diversifying action and predominance of the 

 shore-line in the river, there is the tendency of its tributary 

 waters, especially of the smaller lateral feeders, to follow their 

 shore for some distance. The absence of great sinuosity in the 



