110 



PROBLEMS OF LAKE BIOLOGY 



most extensive use of these factors has been 

 made by Steenis (1932, 1933, 1934, 1935). 

 Several hundred lakes were investi«iated by 

 him in a survey of the economic value of the 

 laro'er aquatic plants to fish, game, and 

 summer resorts in Wisconsin. The tables 

 presented in Steenis' papers show conclu- 

 sively that a marked correlation exists be- 

 tween the vegetation and soft, medium hard, 

 and hard waters. Additional work by the 

 writer on more than 100 lakes in adjoining 

 territory agrees with the findings of Steenis. 

 The classification of waters that was used 

 in these studies is one that has been em- 

 ployed for a number of years by the AVis- 

 consin Geological and Natural History Sur- 

 vey, and is given below: 



Soft — 0-10 parts per million of bound 



carbon dioxide. 

 Medium — 10-30 parts per million of bound 



carbon dioxide. 

 Hard — over 30 parts per million of bound 

 carbon dioxide. 



Gray-green. 

 Soft or fluid. 



Steenis has followed and extended an 

 ecological classification of the hj'drophytes 

 begun by Fassett (1930). This classifica- 

 tion is similar to Warming's (1925) 

 "growth forms" and for practical value in 

 classifying the lakes of AVisconsin it is very 

 useful. It is not Avithout the fault, how- 

 ever, of being somewhat empirical. When a 

 study is made of Steenis' charts it is evident 

 that the "growth forms" of the aquatic 

 plants overlap several of his lake types, and 

 when viewed in their entire distribution 

 they merge into a complicated pattern of 

 dynamic succession. 

 ■^ A study of the vegetation in 102 lakes of 

 northern Wisconsin for its relation to the 

 pH of the waters has been made by the 

 writer, and the greatest number of species 

 was found in those lakes having slightly 

 more than the pH of 7.0. An examination 

 of the plant species showed them to be in- 

 habitants of either acid or alkaline waters 

 and a few that grow in both environments. 

 Therefore, the greatest number of species 

 appears to occur where the first two overlap 

 in their tolerance range. 



Light 



The necessity of light for the growth of 

 aquatic plants is probably one of the most 

 important factors in vertical distribution. 

 The number of published studies dealing 

 with the relation of light to aquatic plant 

 distribution is veiy few, and all show the 

 need for further intense work in this field. 

 Pearsall (1920, 1933, 1934) working with 

 the iodide-sulphuric acid method in the 

 lakes of the Windermere District of En- 

 gland has shown the lower limit of vegeta- 

 tion growth to be at approximately the 

 depth of 2 per cent of full daylight. In 

 1920 this was at 6.5 meters while in 1932, 

 due to the change in the transparency of the 

 Avater, caused by the greater abundance of 

 phytoplankton, the lower limit of rooted 

 vegetation was 4.3 meters. 



Juday (1934), using a photoelectric 

 method, records the occurrence of two spe- 

 cies of mosses and one species of hepaticae 

 growing at depths of 18 to 20 meters in 

 Ciystal Lake, Vilas County, Wisconsin. At 

 18 meters, only one to 4 per cent of the 

 solar energy penetrates at midday. 



Wilson (1935), also using photoelectric 

 observations, studied the vertical distribu- 

 tion of vegetation in three Vilas County, 

 Wisconsin, lakes and found that the maxi- 

 mum vegetation limit was reached at the 

 depths where approximately 4 per cent of 

 total sunlight penetrated. This was found 

 to vary from 7 meters in Muskellunge Lake 

 to 3 meters in Little John Lake. The latter 

 lake possessed an abundant crop of phyto- 

 plankton, which made the water very turbid. 



Temperature 



The affect of moderately low tempera- 

 tures upon the rooted aquatics does not 

 appear to be one of the very important 

 ecological factors in their ultimate growth 

 and distribution. This is probably because 

 the vertical distribution of the rooted 

 plants generally does not extend to depths 

 where the waters are very cold. They ap- 

 pear to be restricted in this distribution, by 

 the scarcity of sunlight, long before the low 

 temperatures of the lake waters are reached. 



