415 



makes its way through a now abandoned channel to the lake 

 and thence out to the river through the slough. Below this 

 level, the current of the gentle run-off of the great tract of adja- 

 cent impounded backwaters with which this lake has then but 

 a slight connection is the only movement in the area. 



The surrounding bottoms are heavily wooded for a narrow 

 margin along the lake, though the forest gives way to cultivated 

 fields on both sides within a short distance. Its bottoms and 

 shores are of a rich black alluvium, which in low-water seasons 

 such as 1895 becomes the soil of a cultivated field. 



The vegetation of this area is unique among our plankton 

 stations in its character and relation to the plankton. In 1894 

 there was little vegetation present, and whatever aquatic 

 growth had gained a foothold was eradicated by the dry au- 

 tumn and by the cultivation of the soil in 1895. In 1896-1899 

 the occupation of the lake by water was more continuous, and 

 Potamogetons, Naias, and even Nelumbo, gained a slight foothold 

 along the margins. The principal vegetation was a dense mat 

 of filamentous green algse, such as Spirogijra and Zggnema, 

 which covered the margins for a considerable distance into the 

 lake. During the heated term of midsummer a dense felt of 

 Oscillaria covered the bottom of the lake everywhere at times. 

 These algae were present during most of the summer, though 

 most abundant in spring, and by their continuous and prompt 

 decay they release into the lake waters a volume of nitrogenous 

 and other substances which are utilized by the phytoplankton. 

 The cumulative action of the longer-lived aquatic phanerogams 

 in withdrawing from the lake large stores of food which are 

 again released in the ensuing autumn or spring by the decay of 

 the season's growth, is thus quite absent from this body of wa- 

 ter. The rapidly growing and rapidly decaying algse permit a 

 repeated flux of nitrogenous and other substances utilized by 

 the plankton as food in the course of a single season. This fac- 

 tor, combined with the complete impounding function of this 

 lake below river levels of 8 to 9 feet and the absence of tributary 

 and spring water, is, I believe, the secret of the unusual plank- 

 ton production in this area. 



