REED ON ECOLOGY OF A GLACIAL LAKE. 43 



THE ECOLOGY OF A GLACIAL LAKE. 



HOWARD S. REED. 



About three miles west of Ann Arbor there is ;i group of small 

 lakes and ponds which, on account of their richness of life, are favorite 

 collecting grounds for biological students. In the autumn of 190(1 

 I undertook a study of the eecdogical relations of the flora around one 

 of these lakes, of which study a preliminary report is here furnished. 

 The lake which I studied is the remnant of a lake which in recent 

 geological times had four or live times the area of the present lake, 

 and in its deepest portions was forty or fifty feet deep; at that time 

 it probably had an outlet to the southwest. The lake extended ten 

 or fifteen rods farther east than it does at present; from the south- 

 east a wide cape extended into the lake; west and southwest of this 

 cape the lake extended back twenty or twenty-five rods farther than 

 it does now, covering several acres of ground, the former western 

 shore was approximately parallel to the present one, but two or three 

 rods farther back. A shallow bay or arm extended off to the north- 

 west wiiich is still traceable by the extension of zones in that direc- 

 tion. The northern shore was steep and the water probably extended 

 only a few feet back of the present zone of willows. 



This former extension of the lake has had an important influence 

 upon the character of the present vegetation. AVhile it prevailed, it of 

 course drowned out all terrestrial plants, then, as the surface of the lake 

 Avas gradually lowered, the aquatic and semi-aquatic forms had the 

 first chance to get a foothold and become established on the land which 

 was thus uncovered. This was especially the case on the east, west, 

 and southwest shores of the lake, where, on account of the gentle 

 slope of the bottom, there was always a wide strip of shallow water 

 in which aquatic and semi-aquatic plants could get a strong foot- 

 hold long- before it was suitable for terrestrial forms to occupy. 



At a certain stage in the recent geological history of the region, as 

 the surface of the lake fell, the large lake was divided into two, — the 

 present lake and another similar, smaller lake four or five rods south of 

 it, which has now^ almost disappeared. 



At the present lake there are six fairly well defined, concentric zones 

 of vegetation. 



I. A zone of Chorofear occupying the central. de<'per portion of the 

 Like. The chaiacteristic plant of this zone is Chard, sp. 



II. A zone of Potaniot/rfoiis characterized by Pofninof/ffon htcoifi, L. 



III. A zone of Xj/iiipJira from ten to thirty feet wide just outside 

 of the shore line in watei' varying .from six inches to two feet in 

 depth, its characteiisHc plants are Xtiphar adrrna, Ait. f., Pofamogeton 

 iwtaiifi. L., and Chara. 



