THE YAN YEAN RESERVOIR, VICTORIA. 41 
as true plankton-forms. Such are :— "P leurotenium mamillatum, Micrasterias 
Hardyi, Cosmarium Нат, Staurastrum mucronatum var. delicatulum, 
Melosira granulata, Peridinium Volzii var. australe, Dinobryon cylindricum 
and its var. divergens. To these may be added Staurastrum longiradiatum, 
Volvow «тген, and Eudorina elegans. 
А few species, such as Cosmarium реризянт, C. achondroides, C. contractum 
and its var. ellipsoideum, are equally common in both plankton and benthos, 
thriving both in the comparative shelter of the weedy margins and in the 
more disturbed limnetie region of the lake. Staurastrum victoriense was 
present in small quantity in the plankton all the year round, but in the 
littoral region it only occurred from April to July, reaching а maximum in 
the latter month and disappearing immediately afterwards. 
Quite a number of species oceur in the drainage area, and even in the 
atehment basin, which have not been found either in the plankton or the 
littoral region of the Yan Yean. In the case of certain species this is rather 
remarkable, as they occur in quantity in small pools within a few yards of 
the reservoir yet not in any part of the lake itself. Of these species, Radio- 
filum ` conjunetivin, Pleuroianium truncatum, Cosmarium amplum, and 
Staurastrum zonatum ате the most noteworthy, and they afford a direct 
contrast to the 23 species exclusively confined to the plankton. 
In comparison with the phytoplankton of the Yan Yean Reservoir, that of 
the Toorourong Reservoir and of the Wallaby and Silver Creek Weirs is 
almost insignificant. Compared with the 104 species constituting the 
phytoplankton of the Yan Yean, 24 occurred in the Toorourong, 18 in 
Wallaby Creek Weir, and only 11 in Silver Creek Weir. Most of the 
plankton of these upper dams consists of decaying organic matter and fine 
sand, and the meagreness of the phytoplankton is due to the constant 
agitation and continual renewal of the water *. Most of the constituents 
are Diatoms, and the number of species gradually decreases as the dis- 
turbance of the water becomes greater owing to the diminished size of the 
reservoirs. The few species of Chlorophyceæ were principally Desmids, but 
they were of quite a distinct character from those found in the Yan Yean. 
The most noticeable feature among the Diatoms was the large size attained 
by the frustules of Vunheurckia rhomboides. 
The algal investigation of the upper dams shows clearly that the Yan Yean 
Reservoir does not receive any of its characteristic algal constituents from 
these sources, even though six-sevenths of its total water-supply comes along 
the aqueduct from the Toorourong, having been derived from the large 
drainage area tapped by these upper reservoirs. We have, therefore, to turn 
next to the Yan Yean catchment basin for information concerning the 
* Cf. also Н. L. Shantz, “Prelim. Report on the Biol. Study of the Lakes of the Pike’s 
Peak Region,” Trans. Amer. Mier. Soc., March 1907, p. 91, 
