Biological Survey — Oswego Watershed 153 



Phytoplankton. Cyanophyceae. — The blue-green algae were well 

 represented in the net plankton by two common genera Anabaena 

 and Microcystis. Anabaena was found at all times and was especi- 

 ally abundant from 0-6 meters deep. It reached its greatest num- 

 bers during July and August. Three species, Anabaena circinalis, 

 A. flos-aquae, and A. Lemmermanni were observed but no attempt 

 was made to count the species separately. Microcystis was found at 

 all depths every time samples were taken; it seemed to be most 

 abundant, however, at or near the surface. Traces of Gloeotrichia 

 were found near the surface in August, and Oscillatoria was found 

 only once, near the surface, on July 12. 



Chlorophyceae. — Members of the green algae at no time formed 

 any dominant part of the phytoplankton. Staurastrum was the 

 most abundant genus ; in addition very small numbers of Actina- 

 strum, Dictyosphaerium and Pediastrum were found. 



Bacillariae. — Diatoms formed the greater part of the phyto- 

 plankton at all depths, especially during August and September. 

 Asterionella and Stephanodiscus were present at all times, the 

 former being the most common. It reached it maximum numbers in 

 August and then declined. Fragilaria and Tab ell aria were practi- 

 cally absent in June and early July but during August formed the 

 predominating organisms of the plankton. Melosira occurred 

 only sparingly in the net plankton during the latter part of the 

 season. 



Nannoplankton. Cyanophyceae. — Microcystis was found in 

 every sample, becoming very abundant at all depths during July 

 and, after a decline, increasing again in September. Coclosphaer- 

 ium was found in large numbers (about 1000 to 6000 per liter) 

 in nearly all samples. No constant variation was noted due to 

 depth or season. Aphanocapsa and Chroococcus were found in 

 smaller numbers in seA T eral samples taken at various depths and at 

 different times. 



Chlorophyceae. — Eudorina Oocystis and Characium were the 

 most common green algae found. These were found mostly in the 

 earlier samples, being practically absent from the later samples. 

 Other genera which were found only occasionally or in small num- 

 bers include Dictyosphaerium, Cosmarinm, Crucigenia, Pandor- 

 ina, Quadrigula, Sphaerocystis and Scenedesmus. 



Bacillariae. — Cyclotella was the only diatom found in the 

 nannoplankton that was not found in the net' plankton. It ap- 

 peared at all depths in the earlier samples. The figures given for 

 Asterionella, Fragilaria, Tabellaria and Melosira are probably too 

 high because the counts represent not whole colonies but frag- 

 ments of colonies which were broken during the process of centri- 

 fuging. These samples were strained through the plankton net 

 after centrifuging. 



Since the nannoplankton samples could not be examined at the 

 time they were taken, it was necessary to preserve them. ■ It is 

 probable that some of the more delicate organisms, especially 

 smaller protozoa, may have been destroyed beyond recognition and 

 were therefore not observed when the counts were made. 



