All of the lakes which have been discussed in the preceding 

 two paragraphs are quite different from Western Lake Erie in their 

 hydrographic features, and it is not surprising that they should differ 

 from it in the make up of their bottom faunas. However, there is no 

 better agreement in the case of Lake Winnebago, which is similar to 

 Western Lake Erie in its large surface area and meager depth. Here, 

 also, the mayflies occupy a subordinate position relative to a number 

 of other groups (Baker, 192U) . 



Weight of Organisms 



Since the data on bottom organisms in the Island Section were 

 all taken from stations on mud bottom, they form an imperfect basis 

 for the determination of abundance for the Island Section as a unit. 

 But so large a part of the Island Section has a mud bottom that it 

 seemed worth while to make an approximate determination of the weight 

 per unit area of the standing crop in summer. It was possible to make 

 determinations of the weight of Hexagenia only. Four groups of 25 in- 

 dividuals, selected at random, were dried at 65*-*C. for a period of 

 2U hours, weighed, then ignited in an electric furnace, and weighed 

 again. The mean dry weight of an individual Hexagenia larva was found 

 to be 10,9 milligrams, and the mean weight of organic matter, as in- 

 dicated by the loss on ignition, to be 8,6 milligrams. 



If we apply these figures to the mean number of Hexagenia at 

 the seven stations in 1929, we find that the dry weight of the crop was 

 30,8 kilograms per hectare (?7.5 pounds per acre), and the weight of 

 organic matter 2U.3 kilograms per hectare (21.7 pounds per acre). For 

 1930, the dry weight of the Hexagenia crop was 55.6 kilograms per 

 hectare (1|9.6 pounds per acre), and the weight of organic matter l4.3»8 

 kilograms per hectare (39.1 pounds per acre). From the results ob- 

 tained by other workers, it has been estimated that the addition of the 

 dry weight of the associated organisms would increase the 1929 crop 

 about 2 kilograms per hectare, and the 1930 crop a bout one-half that 

 amount. The average weight of Hexagenia for both 1929 and 1930 is as 

 follows: dry weight, Ii3.2 kilograms per hectare (38.5 pounds per acre), 

 and weight of organic matter, 3li.l kilograms per hectare (30. li pounds 

 per acre) . 



It is recognized that the foregoing figures represent the 

 sub-littoral zone only imperfectly. But, admitting the deficiencies in 

 the data^ it seems worth while to compare them with data from similar 

 zones in certain other lakes. To facilitate the comparison. Table 86 

 was assembled, showing, besides the dry weight of Hexagenia per unit of 

 area in the area under consideration, the dry weight per unit of area 

 for all organisms in parts of Lakes Wawasee, Mendota, Green, Simcoe, 

 and Nipigon. An attempt was made to s elect data from a zone in each lake 



266 



