236 BULLETIN OF THE BUREAU OF FISHERIES. 



15 cm.^ tubes of water. The sedimentation was usually completed in about six minutes. 

 The material was then transferred to a counting cell with a long pipette and the organ- 

 isms were enumerated with a compound microscope having a 16 mm. objective and a 

 No. 8 ocular. Many of these organisms, more especially the minute flagellates, are de- 

 stroyed by the various preserving agents, so that it is necessary to have the living material 

 for these enumerations; such counts must be made, therefore, as soon as possible after 

 the samples of water are obtained. 



The results obtained in the various enumerations are shown in Tables 17, 18, and 21. 

 The figures indicate the number of individuals per cubic meter of water at the different 

 depths. For purposes of comparison the results obtained for net plankton on Canandai- 

 gua, Cayuga, and Seneca Lakes in 19 10 are shown in Table 18. Observations were made 

 on the net plankton and nannoplankton of Green Lake, Wis., in 1918, soon after these 

 were made on the Finger Lakes, and these have been included in Tables 1 7 and 2 1 for 



comparative purposes also. 



NET PLANKTON. 



Phytoplankton. — ^Table 17 shows that the green and blue-green algae were scarce in 

 the three Finger Lakes at the time of the observations in 1918. Only three forms were 

 present, namely, Anabaena, Microcystis, aim Staurastrum. In Canandaigua Lake a 

 relatively small number of colonies of Microcystis was found in the upper 10 m. and 

 Staurastrum was noted in the 10 to 40 m. stratum. In Cayuga Lake Anabaena was 

 obtained in the upper 5 m. and Microcystis in the upper 10 m. In Seneca Lake this group 

 was represented only by a few colonies of Microcystis in the 10 to 15 m. stratum. A 

 comparison with Table 18 shows that fewer forms were present in 1918 than in 19 10 and 

 also that the number of individuals was much smaller in the former year. The two 

 sets of catches on Canandaigua Lake present the most marked difference in this respect. 



The net catches from Green Lake, Wis., contained a much larger algal population 

 than the Finger Lakes, owing to the presence of a large number of filaments of Oscil- 

 latoria. This form was unusually well represented in the upper 15 m., a maximum of 

 nearly two million filaments oer cubic meter of water being found in the o to 5 m. 

 stratum. 



In the Finger Lakes the most abundant diatom, both in 1910 and 1918, was Aster- 

 ionella, while Fragilaria was second in importance both years. In Canandaigua Lake 

 the diatom population was substantially the same in these two years, while in Cayuga 

 Lake the number was much larger in the former year. In Seneca Lake, on the other 

 hand, the number was larger in 19 18 than in 19 10. 



In Green Lake Asterionella was the only diatom noted, a few individuals of this 

 form being present in two catches. 



Zooplankton. — Uroglena was fairly abundant in the upper 30 m. of Canandaigua 

 Lake and a few colonies of Epistylis were noted in the 5 to 10 m. stratum. In 1910 

 Ceratium was the most abundant protozoan in this lake; but it was not found in 1918. 



In Cayuga Lake Actinosphaerium and Ceratium were about equally numerous in 

 1918, both being most abundant in the upper 15 m. The former was not found in 1910. 

 and the latter was much more abundant in this year than in 19 18, the number reaching 

 more than a million and a half per cubic meter in the o to ^ m. stratum. Dinobryon 

 was not as abundant in 1918 as in 1910 and Mallomonas was not noted in the former 

 year. 



