BIOLOGICAL SURVEY OP THE UPPER MISSISSIPPI RIVER 145 



if this sewage is the primary factor involved in the depletion of dissolved oxygen in 

 the upper Mississippi River, the oxygen content at stations 11 and 14 should show 

 a distinct increase above that of station No. 9. The data show that this increase 

 does occur. The decrease in the dissolved-oxygen supply of the Mississippi waters 

 in the section described above is unquestionably due to the pollution of the river. 



2. The data in Table 4 show that the heavy rains in the fall quickly increase 

 the oxygen content of the polluted waters and eventually restore the normal supply 

 even in the most polluted areas. (See data for September 27 to 30, Table 4.) 



BOTTOM FAUNA 

 RELATIONSHIP OF BOTTOM FAUNA AND POLLUTION 



Next to a thorough chemical study, a study of the bottom-dwelling organisms 

 is perhaps the best criterion to be used in determining whether or not a given body 

 of water is polluted. We know with a fair degree of accuracy what kind of organ- 

 isms are indicative of grossly polluted waters and what organisms can be expected 

 only in fairly clean waters. Of course, numbers of individuals play quite as im- 

 portant a part here as does the species or kind of animals. The presence of a few 

 specimens of the worms Limnodrilus and Tubifex, or of the mollusk Musculium 

 transversum, or of the red midge Chironomus plumosus is not significant. Studies 

 in the Illinois River, however, have shown that the presence of a large number of 

 these organisms is indicative of pollution. Again, it has been shown that the presence 

 of the larvse of the caddis fly and the sand fly and the nymph of the May fly may 

 be taken as evidence that the water inhabited by them is not polluted to any con- 

 siderable extent. These facts are based on many years of study of a biological and 

 a chemical nature by Forbes, Richardson, Shelford, Thompson, and others. 



METHODS 



Samples for the study of the bottom-dwelling forms were taken by means of a 

 small Ekman dredge. An attempt was made to obtain on each trip at least two 

 samples at each field station — one near the shore and the other in the channel. At 

 times it was impossible to get a sample in the channel because of the nature of the 

 bottom and the strength of the current. The small Ekman dredge works best in 

 soft mud or in places where a great deal of organic debris has settled out. On hard, 

 gravelly bottom it is not very effective; it is too light to go into the bottom to any 

 appreciable extent and, also, the gravel gets behind the jaws and prevents them from 

 closing. A record was kept of the number of hauls taken at each station. Then, 

 either the entire sample or a known portion of it was preserved in 4 per cent formal- 

 dehyde. 



In the laboratory, the samples were placed in a strainer having a bottom of 

 bolting cloth of 50 meshes per square centimeter. In this strainer the samples were 

 washed with tap water until they were free from mud. The coarse sand and organic 

 debris, as well as the macroscopic animals, were retained in the strainer. After the 

 removal of the mud the sample was placed in a dish and the animals were separated 

 and counted. From the number of each kind of animal obtained from the sample, 

 the area of the dredge, and the number of hauls made for the sample, the number 

 of animals per square yard was calculated. In some cases the volume of the sample 



