matter. The records of Stations 207, 208, and 209, not shown in 

 tables, also yielded no evidence of pollution. 



Pollution offshore 



Table 9$ shows the results obtained at ten stations located 

 on a straight line from a point near Toledo Beach (Station 116) to a 

 point near the mouth of Detroit River (Station 126) . The line has a north- 

 east-southwest trend and lies off the shore at a distance varying from 

 1 l/U to 3 1/2 miles. All of the stations except the regular ones (117 and 

 126) are represented by single samples. These meager data indicate that 

 the Tubificidae were confined to the area off the mouth of River Raisin and 

 to the station nearest the mouth of Detroit River, Based on these data, 

 the line limiting the zone of light pollution would be drawn between 

 Stations 116F and 116D on the southwest, and between Stations 117 and 118 

 on the northeast. A discussion of conditions in the vicinity of the 

 mouth of Detroit River appears in a later section of this paper. 



Summary stattsment regarding pollution in the 

 River Raisin Section 



A study of the bottom organisms at a large number of stations in 

 the vicinity of River Raisin was carried on in the summer of 1930 to 

 supplement the small amount of data obtained at one station in 1929. The 

 conclusions may be summarized briefly as follows. The zone of heavy pollu- 

 tion was small in extent, reaching no farhter than 3/8 mile out into the 

 lake, and probably noc more than<l/8 mile north and south from the mouth 

 of the river. It is estimated th^t the outer limit of the zone of moderate 

 pollution extended 3/U mile out into the lake, 3/8 mile along the shore 

 toward the south, and 5/8 mile along the shore toward the north. The 

 outer limit of the zone of light pollution extended approximately 2 3/U 

 miles out into the lake, 2 l/2 miles along the shore toward the south, and 

 1 mile toward the north. At a distance of 2 miles lakeward from the 

 river's mouth (Station 117) the northward and southward extension of this 

 zone was about the same as it was near the shore. The extent of the 

 three zones is shown graphically in Figure 23, Because of the small 

 space available, only the zone of light pollution was labelled on the map. 

 The zone of heavy pollution is enclosed by the elliptical line at the 

 mouth of the river, and the zone of moderate pollution lies between this 

 line and the zone of light pollution. The areas are: heavy pollution, 

 0,3 square kilometer (0.1 square mile); moderate pollution, 2,1 square 

 kilometers (0.8 square mile); light pollution, 30.3 square kilometers 



289 



