240 BULLETIN OF THE BUREAU OF FISHERIES. 



No determinations of the weight of the rotifers were made for the Finger Lakes, 

 but such results have been obtained for three species from Wisconsin lakes, namely, 

 Asplanchna brighhi'eUii, Brachionus pala, and ConochUus volvox. The weight of these 

 forms has been used as a basis for estimating the weight of the various rotifers in the 

 plankton catches from the three Finger Lakes, N. Y., and from Green Lake, Wis. The 

 computations are based on the relative volumes of the different forms, so that they 

 are to be regarded as estimates and not the results of actual weighings. These estimates 

 are shown in Table 20. 



Cayuga Lake had the largest amount of rotifer material both in 1910 and in 1918, 

 with III kg./km.^ (i pound per acre) in the former year and 145 kg. (1.3 pounds) 

 in the latter year. It had 4X times as much as Seneca Lake in 1910 and about 3X 

 times as much in 1918; it had 12 times as much as Canandaigua Lake in 1910 and 

 about 52 times as much in 1918. Green Lake had just half as much rotifer material 

 as Cayuga Lake in 1918. 



In the rotifers that have been weighed the ash averaged about 7.4 per cent of the 

 dry weight, ranging from a minimum of a little less than 6 per cent to a maximum 

 of a little more than 9 per cent. Thus between 90 and 95 per cent of the dry weight 

 of these rotifers may be regarded as organic matter, but what proportion of this is 

 indigestible has not been determined. Also it has been found that from 90 to 94 per 

 cent of the living rotifer consists of water, so that the weight of the live organisms 

 would be somewhat more than 10 times as large as the iigures given in the table. 



The relative importance of the Crustacea and the rotifers as sources of organic 

 matter which will serve as food for other organisms is shown in Table 20. In Canan- 

 daigua Lake, which had a very small rotifer population, the ratio of the organic matter 

 in the rotifers to that in the Crustacea was i : 256 in 1910 and i : 292 in 1918. Owing to 

 the very much larger rotifer population in Cayuga Lake the ratio there was i :i3 in 

 1910 and about i .g in 1918. In Seneca Lake these ratios were about i :7o each year. 

 In Green Lake the Crustacea contributed about 49 times as much dry organic matter 

 as the rotifers in 19 18. 



The dry weight of the Crustacea and rotifers combined amounted to 2,588 kg./km.' 

 (23 pounds per acre) in Canandaigua Lake in 19 10; this was the maximtun quantity 

 found in the three Finger Lakes in that year. The minimum amount was noted for 

 Cayuga Lake, namely, 1,945 kg. (17.3 pounds). (See Table 20, p. 248.) 



In 1918 the maximum for these two groups of organisms was found in Seneca Lake 

 and it amounted to 3,267 kg. of dry matter per square kilometer (29.1 pounds per 

 acre). Canandaigua Lake possessed the minimum amoimt for this year, namely, about 

 852 kg. (7.6 pounds). This was only about one-third as much as this lake 3delded 

 in 1910. 



In Green Lake these two groups of plankton animals yielded about 3,458 kg. of 

 dry matter per square kilometer of surface (31.6 poimds per acre) which was about 

 10 per cent larger than the amount in Seneca Lake in 1918. 



No attempt was made to determine the weight of the algae in the net plankton, 

 but as the catches appeared imder the microscope by far the greater portion of the mate- 

 rial consisted of rotifers and Crustacea, probably three-quarters of it or more. Adding 

 25 or even 50 per cent to the above figures would still leave a relatively small amount 



