114 BULLETIN OF THE BUREAU OF FISHERIES. 



SUSPENDED MATTER. 



The solids carried in suspension by water consist of mineral and organic substances. 

 The particles of mineral matter brought in b}' surface drainage or derived from bottcm 

 and shores, apart from that which is in solution, range in size from coarse to very minute. 

 The carrying power of the water varies with the sixth power of the velocity, although 

 in the case of the most minutely divided substances other factors than rate of flow 

 come into play. 



Mussels are affected in various ways by the matter in suspension. It has been 

 reported that some mussels stop feeding when the water is excessively turbid, as after 

 a storm. In this way they would avoid taking into their stomachs large amounts of 

 indigestible mineral. They have, however, the power of ejecting undesirable matter; 

 this may enable them to continue feeding even though the water is moderately turbid 

 In streams like the Mississippi, mussels could hardly survive without feeding during 

 the long periods of turbidity that prevail. Excessive precipitation of silt may smother 

 or even bury the mussel (Headlee and Simonton, 1904, p. 176). The turbidity of water 

 over deeper beds materially restricts the amount of light reaching the mussel, and it is 

 possible that this has an untoward effect. Data regarding the turbidity of several 

 streams are given in Table 9, page 116. The turbidity of representative mussel-producing 

 streams varies from 37 to 188, except that the Des Moines River at Keosauqua has a 

 turbidity rating of 542 — a striking exception. The Missouri and Red Rivers (non- 

 productive) and portions of the Mississippi River which do not yield commercial mussels 

 have turbidity ratings from 556 to 1,931. 



Organic materials, both living and dead, are abundantly suspended in most natural 

 waters, and form a large part of the food of mussels. (See p. 91.) The living bodies 

 are the microscopic plants and animals which make up what is called the plankton. 

 The dead organic materials are the remains or fragments of plants and animals in a 

 state of decomposition, and such also form a part of the food supply. 



Some of the plankton originates in the lake or stream in which the mussels are 

 living. Another and perhaps the greater part is brought in by the tributary streams. 

 Similar statements may be made regarding the dead organic matter, with the addition 

 that some of this may be brought in by surface drainage from the bordering lands. 



MINERALS IN SOLUTION. 



To what extent mussels derive the mineral matter necessary for the sustenance of 

 life and the formation of shells directly from the water or through the solid food con- 

 sumed can not be said, but even that part which is derived from solid food must have 

 been obtained by the smaller organism from the water or the soil. Churchill (191 5 and 

 1916), from experiments conducted at the Fairport Station, has shown that fresh-water 

 mussels possess the ability to make use of nutriment which is in solution in the water. 

 While he demonstrated this for such nutritive substances as fat, protein, and starch, 

 there are yet wanting, as he has pointed out, analyses of the natural water in which 

 mussels live to prove that such organic substances are present in the waters in quantities 

 sufficient to play an important part in the nutrition of mussels. There are, however, 

 abundant analyses to prove the presence of dissolved minerals. 



The requirements of mussels in mineral food may be ascertained by analysis of 

 the soft bodies and shells. Such analysis shows that while the shell is about 95 per cent 



